Merge branch '475-new-sharing-client' into 'master'

New Forum Sharing Client

This is very similar to how the private group invitations work and I am sure there's still some tiny bugs that I didn't catch.

All existing integration tests either pass or have been modified to pass.

Once this has been merged, the code should be usable for blog sharing as well.

Closes #475

See merge request !467

briar-android/src/main/java/org/briarproject/briar/android/contact/ConversationActivity.java

@@ -1020,13 +1020,13 @@ public class ConversationActivity extends BriarActivity
 	@DatabaseExecutor
 	private void respondToForumRequest(SessionId id, boolean accept)
 			throws DbException {
-		forumSharingManager.respondToInvitation(id, accept);
+		forumSharingManager.respondToInvitation(contactId, id, accept);
 	}
 
 	@DatabaseExecutor
 	private void respondToBlogRequest(SessionId id, boolean accept)
 			throws DbException {
-		blogSharingManager.respondToInvitation(id, accept);
+		blogSharingManager.respondToInvitation(contactId, id, accept);
 	}
 
 	@DatabaseExecutor

briar-android/src/main/java/org/briarproject/briar/android/contact/ConversationItem.java

@@ -170,8 +170,7 @@ abstract class ConversationItem {
 			} else if (ir instanceof GroupInvitationRequest) {
 				text = ctx.getString(
 						R.string.groups_invitations_invitation_sent,
-						contactName,
-						((GroupInvitationRequest) ir).getGroupName());
+						contactName, ir.getShareable().getName());
 			} else {
 				throw new IllegalArgumentException("Unknown InvitationRequest");
 			}
@@ -194,8 +193,7 @@ abstract class ConversationItem {
 			} else if (ir instanceof GroupInvitationRequest) {
 				text = ctx.getString(
 						R.string.groups_invitations_invitation_received,
-						contactName,
-						((GroupInvitationRequest) ir).getGroupName());
+						contactName, ir.getShareable().getName());
 				type = GROUP;
 			} else {
 				throw new IllegalArgumentException("Unknown InvitationRequest");
@@ -203,7 +201,7 @@ abstract class ConversationItem {
 			return new ConversationRequestItem(ir.getId(),
 					ir.getGroupId(), type, ir.getSessionId(), text,
 					ir.getMessage(), ir.getTimestamp(), ir.isRead(),
-					ir.getInvitedGroupId(), !ir.isAvailable(),
+					ir.getShareable().getId(), !ir.isAvailable(),
 					ir.canBeOpened());
 		}
 	}

briar-android/src/main/java/org/briarproject/briar/android/sharing/ShareBlogControllerImpl.java

@@ -10,9 +10,11 @@ import org.briarproject.bramble.api.db.NoSuchGroupException;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.briar.android.contactselection.ContactSelectorControllerImpl;
 import org.briarproject.briar.android.controller.handler.ExceptionHandler;
 import org.briarproject.briar.api.blog.BlogSharingManager;
+import org.briarproject.briar.api.messaging.ConversationManager;
 
 import java.util.Collection;
 import java.util.concurrent.Executor;
@@ -22,6 +24,7 @@ import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
 import static java.util.logging.Level.WARNING;
+import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 
 @Immutable
 @NotNullByDefault
@@ -31,14 +34,19 @@ class ShareBlogControllerImpl extends ContactSelectorControllerImpl
 	private final static Logger LOG =
 			Logger.getLogger(ShareBlogControllerImpl.class.getName());
 
+	private final ConversationManager conversationManager;
 	private final BlogSharingManager blogSharingManager;
+	private final Clock clock;
 
 	@Inject
 	ShareBlogControllerImpl(@DatabaseExecutor Executor dbExecutor,
 			LifecycleManager lifecycleManager, ContactManager contactManager,
-			BlogSharingManager blogSharingManager) {
+			ConversationManager conversationManager,
+			BlogSharingManager blogSharingManager, Clock clock) {
 		super(dbExecutor, lifecycleManager, contactManager);
+		this.conversationManager = conversationManager;
 		this.blogSharingManager = blogSharingManager;
+		this.clock = clock;
 	}
 
 	@Override
@@ -48,15 +56,19 @@ class ShareBlogControllerImpl extends ContactSelectorControllerImpl
 
 	@Override
 	public void share(final GroupId g, final Collection<ContactId> contacts,
-			final String msg,
+			final String message,
 			final ExceptionHandler<DbException> handler) {
 		runOnDbThread(new Runnable() {
 			@Override
 			public void run() {
 				try {
+					String msg = isNullOrEmpty(message) ? null : message;
 					for (ContactId c : contacts) {
 						try {
-							blogSharingManager.sendInvitation(g, c, msg);
+							long time = Math.max(clock.currentTimeMillis(),
+									conversationManager.getGroupCount(c)
+											.getLatestMsgTime() + 1);
+							blogSharingManager.sendInvitation(g, c, msg, time);
 						} catch (NoSuchContactException | NoSuchGroupException e) {
 							if (LOG.isLoggable(WARNING))
 								LOG.log(WARNING, e.toString(), e);

briar-android/src/main/java/org/briarproject/briar/android/sharing/ShareForumControllerImpl.java

@@ -10,9 +10,11 @@ import org.briarproject.bramble.api.db.NoSuchGroupException;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.briar.android.contactselection.ContactSelectorControllerImpl;
 import org.briarproject.briar.android.controller.handler.ExceptionHandler;
 import org.briarproject.briar.api.forum.ForumSharingManager;
+import org.briarproject.briar.api.messaging.ConversationManager;
 
 import java.util.Collection;
 import java.util.concurrent.Executor;
@@ -22,6 +24,7 @@ import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
 import static java.util.logging.Level.WARNING;
+import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 
 @Immutable
 @NotNullByDefault
@@ -31,14 +34,19 @@ class ShareForumControllerImpl extends ContactSelectorControllerImpl
 	private final static Logger LOG =
 			Logger.getLogger(ShareForumControllerImpl.class.getName());
 
+	private final ConversationManager conversationManager;
 	private final ForumSharingManager forumSharingManager;
+	private final Clock clock;
 
 	@Inject
 	ShareForumControllerImpl(@DatabaseExecutor Executor dbExecutor,
 			LifecycleManager lifecycleManager, ContactManager contactManager,
-			ForumSharingManager forumSharingManager) {
+			ConversationManager conversationManager,
+			ForumSharingManager forumSharingManager, Clock clock) {
 		super(dbExecutor, lifecycleManager, contactManager);
+		this.conversationManager = conversationManager;
 		this.forumSharingManager = forumSharingManager;
+		this.clock = clock;
 	}
 
 	@Override
@@ -48,15 +56,19 @@ class ShareForumControllerImpl extends ContactSelectorControllerImpl
 
 	@Override
 	public void share(final GroupId g, final Collection<ContactId> contacts,
-			final String msg,
+			final String message,
 			final ExceptionHandler<DbException> handler) {
 		runOnDbThread(new Runnable() {
 			@Override
 			public void run() {
 				try {
+					String msg = isNullOrEmpty(message) ? null : message;
 					for (ContactId c : contacts) {
 						try {
-							forumSharingManager.sendInvitation(g, c, msg);
+							long time = Math.max(clock.currentTimeMillis(),
+									conversationManager.getGroupCount(c)
+											.getLatestMsgTime() + 1);
+							forumSharingManager.sendInvitation(g, c, msg, time);
 						} catch (NoSuchContactException | NoSuchGroupException e) {
 							if (LOG.isLoggable(WARNING))
 								LOG.log(WARNING, e.toString(), e);

briar-api/src/main/java/org/briarproject/briar/api/blog/BlogInvitationRequest.java

@@ -1,7 +1,10 @@
 package org.briarproject.briar.api.blog;
 
 import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.identity.AuthorId;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.Group;
 import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.briar.api.client.SessionId;
@@ -10,7 +13,7 @@ import org.briarproject.briar.api.sharing.InvitationRequest;
 import javax.annotation.Nullable;
 
 @NotNullByDefault
-public class BlogInvitationRequest extends InvitationRequest {
+public class BlogInvitationRequest extends InvitationRequest<Blog> {
 
 	private final String blogAuthorName;
 
@@ -19,9 +22,12 @@ public class BlogInvitationRequest extends InvitationRequest {
 			@Nullable String message, GroupId blogId,
 			boolean available, boolean canBeOpened, long time,
 			boolean local, boolean sent, boolean seen, boolean read) {
-
-		super(id, sessionId, groupId, contactId, message, blogId, available,
-				canBeOpened, time, local, sent, seen, read);
+		// TODO pass a proper blog here when redoing the BlogSharingManager
+		super(id, groupId, time, local, sent, seen, read, sessionId,
+				new Blog(new Group(blogId, BlogManager.CLIENT_ID, new byte[0]),
+						new Author(new AuthorId(new byte[AuthorId.LENGTH]),
+								blogAuthorName, new byte[0])), contactId,
+				message, available, canBeOpened);
 		this.blogAuthorName = blogAuthorName;
 	}
 

briar-api/src/main/java/org/briarproject/briar/api/blog/BlogInvitationResponse.java

@@ -14,9 +14,8 @@ public class BlogInvitationResponse extends InvitationResponse {
 			GroupId groupId, ContactId contactId, GroupId blogId,
 			boolean accept, long time, boolean local, boolean sent,
 			boolean seen, boolean read) {
-
-		super(id, sessionId, groupId, contactId, blogId, accept, time, local,
-				sent, seen, read);
+		super(id, groupId, time, local, sent, seen, read, sessionId, blogId,
+				contactId, accept);
 	}
 
 }

briar-api/src/main/java/org/briarproject/briar/api/forum/ForumInvitationRequest.java

@@ -12,23 +12,18 @@ import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-public class ForumInvitationRequest extends InvitationRequest {
+public class ForumInvitationRequest extends InvitationRequest<Forum> {
 
-	private final String forumName;
-
-	public ForumInvitationRequest(MessageId id, SessionId sessionId,
-			GroupId groupId, ContactId contactId, GroupId forumId,
-			String forumName, @Nullable String message, boolean available,
-			boolean canBeOpened, long time, boolean local, boolean sent,
-			boolean seen, boolean read) {
-
-		super(id, sessionId, groupId, contactId, message, forumId, available,
-				canBeOpened, time, local, sent, seen, read);
-		this.forumName = forumName;
+	public ForumInvitationRequest(MessageId id, GroupId groupId, long time,
+			boolean local, boolean sent, boolean seen, boolean read,
+			SessionId sessionId, Forum forum, ContactId contactId,
+			@Nullable String message, boolean available, boolean canBeOpened) {
+		super(id, groupId, time, local, sent, seen, read, sessionId, forum,
+				contactId, message, available, canBeOpened);
 	}
 
 	public String getForumName() {
-		return forumName;
+		return getShareable().getName();
 	}
 
 }

briar-api/src/main/java/org/briarproject/briar/api/forum/ForumInvitationResponse.java

@@ -13,13 +13,12 @@ import javax.annotation.concurrent.Immutable;
 @NotNullByDefault
 public class ForumInvitationResponse extends InvitationResponse {
 
-	public ForumInvitationResponse(MessageId id, SessionId sessionId,
-			GroupId groupId, ContactId contactId, GroupId forumId,
-			boolean accept, long time, boolean local, boolean sent,
-			boolean seen, boolean read) {
-
-		super(id, sessionId, groupId, contactId, forumId, accept, time, local,
-				sent, seen, read);
+	public ForumInvitationResponse(MessageId id, GroupId groupId, long time,
+			boolean local, boolean sent, boolean seen, boolean read,
+			SessionId sessionId, GroupId forumId, ContactId contactId,
+			boolean accept) {
+		super(id, groupId, time, local, sent, seen, read, sessionId, forumId,
+				contactId, accept);
 	}
 
 }

briar-api/src/main/java/org/briarproject/briar/api/forum/ForumManager.java

@@ -27,6 +27,11 @@ public interface ForumManager {
 	 */
 	Forum addForum(String name) throws DbException;
 
+	/**
+	 * Subscribes to a forum within the given {@link Transaction}.
+	 */
+	void addForum(Transaction txn, Forum f) throws DbException;
+
 	/**
 	 * Unsubscribes from a forum.
 	 */

briar-api/src/main/java/org/briarproject/briar/api/forum/event/ForumInvitationResponseReceivedEvent.java

@@ -1,21 +1,20 @@
 package org.briarproject.briar.api.forum.event;
 
 import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.briar.api.forum.ForumInvitationResponse;
 import org.briarproject.briar.api.sharing.event.InvitationResponseReceivedEvent;
 
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
 public class ForumInvitationResponseReceivedEvent extends
 		InvitationResponseReceivedEvent {
 
-	private final String forumName;
-
-	public ForumInvitationResponseReceivedEvent(String forumName,
-			ContactId contactId, ForumInvitationResponse response) {
+	public ForumInvitationResponseReceivedEvent(ContactId contactId,
+			ForumInvitationResponse response) {
 		super(contactId, response);
-		this.forumName = forumName;
 	}
 
-	public String getForumName() {
-		return forumName;
-	}
 }

briar-api/src/main/java/org/briarproject/briar/api/privategroup/invitation/GroupInvitationRequest.java

@@ -1,11 +1,11 @@
 package org.briarproject.briar.api.privategroup.invitation;
 
 import org.briarproject.bramble.api.contact.ContactId;
-import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.briar.api.client.SessionId;
+import org.briarproject.briar.api.privategroup.PrivateGroup;
 import org.briarproject.briar.api.sharing.InvitationRequest;
 
 import javax.annotation.Nullable;
@@ -13,28 +13,14 @@ import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-public class GroupInvitationRequest extends InvitationRequest {
+public class GroupInvitationRequest extends InvitationRequest<PrivateGroup> {
 
-	private final String groupName;
-	private final Author creator;
-
-	public GroupInvitationRequest(MessageId id, SessionId sessionId,
-			GroupId groupId, ContactId contactId, @Nullable String message,
-			GroupId privateGroupId, String groupName, Author creator,
-			boolean available, boolean canBeOpened, long time,
-			boolean local, boolean sent, boolean seen, boolean read) {
-		super(id, sessionId, groupId, contactId, message, privateGroupId,
-				available, canBeOpened, time, local, sent, seen, read);
-		this.groupName = groupName;
-		this.creator = creator;
-	}
-
-	public String getGroupName() {
-		return groupName;
-	}
-
-	public Author getCreator() {
-		return creator;
+	public GroupInvitationRequest(MessageId id, GroupId groupId, long time,
+			boolean local, boolean sent, boolean seen, boolean read,
+			SessionId sessionId, PrivateGroup shareable, ContactId contactId,
+			@Nullable String message, boolean available, boolean canBeOpened) {
+		super(id, groupId, time, local, sent, seen, read, sessionId, shareable,
+				contactId, message, available, canBeOpened);
 	}
 
 }

briar-api/src/main/java/org/briarproject/briar/api/privategroup/invitation/GroupInvitationResponse.java

@@ -13,11 +13,12 @@ import javax.annotation.concurrent.Immutable;
 @NotNullByDefault
 public class GroupInvitationResponse extends InvitationResponse {
 
-	public GroupInvitationResponse(MessageId id, SessionId sessionId,
-			GroupId groupId, ContactId contactId, GroupId privateGroupId,
-			boolean accept, long time, boolean local, boolean sent,
-			boolean seen, boolean read) {
-		super(id, sessionId, groupId, contactId, privateGroupId, accept, time,
-				local, sent, seen, read);
+	public GroupInvitationResponse(MessageId id, GroupId groupId, long time,
+			boolean local, boolean sent, boolean seen, boolean read,
+			SessionId sessionId, GroupId shareableId, ContactId contactId,
+			boolean accept) {
+		super(id, groupId, time, local, sent, seen, read, sessionId,
+				shareableId, contactId, accept);
 	}
+
 }

briar-api/src/main/java/org/briarproject/briar/api/sharing/InvitationFactory.java

@@ -4,6 +4,7 @@ import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.data.BdfDictionary;
 import org.briarproject.bramble.api.sync.GroupId;
 
+@Deprecated
 public interface InvitationFactory<I extends SharingMessage.Invitation> {
 
 	I build(GroupId groupId, BdfDictionary d) throws FormatException;

briar-api/src/main/java/org/briarproject/briar/api/sharing/InvitationMessage.java

@@ -7,25 +7,22 @@ import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.briar.api.client.BaseMessageHeader;
 import org.briarproject.briar.api.client.SessionId;
 
-import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-public abstract class InvitationMessage extends BaseMessageHeader {
+public class InvitationMessage extends BaseMessageHeader {
 
 	private final SessionId sessionId;
 	private final ContactId contactId;
-	private final GroupId invitedGroupId;
 
-	public InvitationMessage(MessageId id, SessionId sessionId, GroupId groupId,
-			ContactId contactId, GroupId invitedGroupId, long time,
-			boolean local, boolean sent, boolean seen, boolean read) {
+	public InvitationMessage(MessageId id, GroupId groupId, long time,
+			boolean local, boolean sent, boolean seen, boolean read,
+			SessionId sessionId, ContactId contactId) {
 
 		super(id, groupId, time, local, read, sent, seen);
 		this.sessionId = sessionId;
 		this.contactId = contactId;
-		this.invitedGroupId = invitedGroupId;
 	}
 
 	public SessionId getSessionId() {
@@ -36,9 +33,4 @@ public abstract class InvitationMessage extends BaseMessageHeader {
 		return contactId;
 	}
 
-	@Nullable
-	public GroupId getInvitedGroupId() {
-		return invitedGroupId;
-	}
-
 }

briar-api/src/main/java/org/briarproject/briar/api/sharing/InvitationRequest.java

@@ -11,20 +11,19 @@ import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-public abstract class InvitationRequest extends InvitationMessage {
+public class InvitationRequest<S extends Shareable> extends InvitationMessage {
 
+	private final S shareable;
 	@Nullable
 	private final String message;
 	private final boolean available, canBeOpened;
 
-	public InvitationRequest(MessageId id, SessionId sessionId, GroupId groupId,
-			ContactId contactId, @Nullable String message,
-			GroupId invitedGroupId, boolean available,
-			boolean canBeOpened, long time, boolean local, boolean sent,
-			boolean seen, boolean read) {
-		super(id, sessionId, groupId, contactId, invitedGroupId, time, local,
-				sent, seen, read);
-		if (available && canBeOpened) throw new IllegalArgumentException();
+	public InvitationRequest(MessageId id, GroupId groupId, long time,
+			boolean local, boolean sent, boolean seen, boolean read,
+			SessionId sessionId, S shareable, ContactId contactId,
+			@Nullable String message, boolean available, boolean canBeOpened) {
+		super(id, groupId, time, local, sent, seen, read, sessionId, contactId);
+		this.shareable = shareable;
 		this.message = message;
 		this.available = available;
 		this.canBeOpened = canBeOpened;
@@ -43,4 +42,8 @@ public abstract class InvitationRequest extends InvitationMessage {
 		return canBeOpened;
 	}
 
+	public S getShareable() {
+		return shareable;
+	}
+
 }

briar-api/src/main/java/org/briarproject/briar/api/sharing/InvitationResponse.java

@@ -10,21 +10,26 @@ import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
-public abstract class InvitationResponse extends InvitationMessage {
+public class InvitationResponse extends InvitationMessage {
 
+	private final GroupId shareableId;
 	private final boolean accept;
 
-	public InvitationResponse(MessageId id, SessionId sessionId,
-			GroupId groupId, ContactId contactId,
-			GroupId invitedGroupId, boolean accept, long time,
-			boolean local, boolean sent, boolean seen, boolean read) {
-
-		super(id, sessionId, groupId, contactId, invitedGroupId, time, local,
-				sent, seen, read);
+	public InvitationResponse(MessageId id, GroupId groupId,
+			long time, boolean local, boolean sent, boolean seen,
+			boolean read, SessionId sessionId, GroupId shareableId,
+			ContactId contactId, boolean accept) {
+		super(id, groupId, time, local, sent, seen, read, sessionId, contactId);
+		this.shareableId = shareableId;
 		this.accept = accept;
 	}
 
 	public boolean wasAccepted() {
 		return accept;
 	}
+
+	public GroupId getShareableId() {
+		return shareableId;
+	}
+
 }

briar-api/src/main/java/org/briarproject/briar/api/sharing/SharingConstants.java

@@ -15,34 +15,63 @@ public interface SharingConstants {
 	 */
 	int MAX_INVITATION_MESSAGE_LENGTH = MAX_MESSAGE_BODY_LENGTH - 1024;
 
+	@Deprecated
 	String CONTACT_ID = "contactId";
+	@Deprecated
 	String GROUP_ID = "groupId";
+	@Deprecated
 	String TO_BE_SHARED_BY_US = "toBeSharedByUs";
+	@Deprecated
 	String SHARED_BY_US = "sharedByUs";
+	@Deprecated
 	String SHARED_WITH_US = "sharedWithUs";
+	@Deprecated
 	String TYPE = "type";
+	@Deprecated
 	String SESSION_ID = "sessionId";
+	@Deprecated
 	String STORAGE_ID = "storageId";
+	@Deprecated
 	String STATE = "state";
+	@Deprecated
 	String LOCAL = "local";
+	@Deprecated
 	String TIME = "time";
+	@Deprecated
 	String IS_SHARER = "isSharer";
+	@Deprecated
 	String SHAREABLE_ID = "shareableId";
+	@Deprecated
 	String INVITATION_MSG = "invitationMsg";
+	@Deprecated
 	String INVITATION_ID = "invitationId";
+	@Deprecated
 	String RESPONSE_ID = "responseId";
+	@Deprecated
 	int SHARE_MSG_TYPE_INVITATION = 1;
+	@Deprecated
 	int SHARE_MSG_TYPE_ACCEPT = 2;
+	@Deprecated
 	int SHARE_MSG_TYPE_DECLINE = 3;
+	@Deprecated
 	int SHARE_MSG_TYPE_LEAVE = 4;
+	@Deprecated
 	int SHARE_MSG_TYPE_ABORT = 5;
+	@Deprecated
 	int TASK_ADD_SHAREABLE_TO_LIST_SHARED_WITH_US = 0;
+	@Deprecated
 	int TASK_REMOVE_SHAREABLE_FROM_LIST_SHARED_WITH_US = 1;
+	@Deprecated
 	int TASK_ADD_SHARED_SHAREABLE = 2;
+	@Deprecated
 	int TASK_ADD_SHAREABLE_TO_LIST_TO_BE_SHARED_BY_US = 3;
+	@Deprecated
 	int TASK_REMOVE_SHAREABLE_FROM_LIST_TO_BE_SHARED_BY_US = 4;
+	@Deprecated
 	int TASK_SHARE_SHAREABLE = 5;
+	@Deprecated
 	int TASK_UNSHARE_SHAREABLE_SHARED_BY_US = 6;
+	@Deprecated
 	int TASK_UNSHARE_SHAREABLE_SHARED_WITH_US = 7;
 
 }

briar-api/src/main/java/org/briarproject/briar/api/sharing/SharingManager.java

@@ -20,8 +20,8 @@ public interface SharingManager<S extends Shareable>
 	 * Sends an invitation to share the given group with the given contact
 	 * and sends an optional message along with it.
 	 */
-	void sendInvitation(GroupId groupId, ContactId contactId,
-			@Nullable String message) throws DbException;
+	void sendInvitation(GroupId shareableId, ContactId contactId,
+			@Nullable String message, long timestamp) throws DbException;
 
 	/**
 	 * Responds to a pending group invitation
@@ -32,7 +32,7 @@ public interface SharingManager<S extends Shareable>
 	/**
 	 * Responds to a pending group invitation
 	 */
-	void respondToInvitation(SessionId id, boolean accept)
+	void respondToInvitation(ContactId c, SessionId id, boolean accept)
 			throws DbException;
 
 	/**

briar-api/src/main/java/org/briarproject/briar/api/sharing/SharingMessage.java

@@ -21,6 +21,7 @@ import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE
 import static org.briarproject.briar.api.sharing.SharingConstants.TIME;
 import static org.briarproject.briar.api.sharing.SharingConstants.TYPE;
 
+@Deprecated
 @NotNullByDefault
 public interface SharingMessage {
 

briar-core/src/main/java/org/briarproject/briar/forum/ForumManagerImpl.java

@@ -108,6 +108,11 @@ class ForumManagerImpl extends BdfIncomingMessageHook implements ForumManager {
 		return f;
 	}
 
+	@Override
+	public void addForum(Transaction txn, Forum f) throws DbException {
+		db.addGroup(txn, f.getGroup());
+	}
+
 	@Override
 	public void removeForum(Forum f) throws DbException {
 		Transaction txn = db.startTransaction(false);

briar-core/src/main/java/org/briarproject/briar/privategroup/invitation/AbstractProtocolEngine.java

@@ -4,7 +4,6 @@ import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.data.BdfDictionary;
-import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
@@ -177,7 +176,7 @@ abstract class AbstractProtocolEngine<S extends Session>
 
 	void subscribeToPrivateGroup(Transaction txn, MessageId inviteId)
 			throws DbException, FormatException {
-		InviteMessage invite = getInviteMessage(txn, inviteId);
+		InviteMessage invite = messageParser.getInviteMessage(txn, inviteId);
 		PrivateGroup privateGroup = privateGroupFactory.createPrivateGroup(
 				invite.getGroupName(), invite.getCreator(), invite.getSalt());
 		long timestamp =
@@ -197,14 +196,6 @@ abstract class AbstractProtocolEngine<S extends Session>
 						session.getInviteTimestamp()) + 1);
 	}
 
-	private InviteMessage getInviteMessage(Transaction txn, MessageId m)
-			throws DbException, FormatException {
-		Message message = clientHelper.getMessage(txn, m);
-		if (message == null) throw new DbException();
-		BdfList body = clientHelper.toList(message);
-		return messageParser.parseInviteMessage(message, body);
-	}
-
 	private void sendMessage(Transaction txn, Message m, MessageType type,
 			GroupId privateGroupId, boolean visibleInConversation)
 			throws DbException {

briar-core/src/main/java/org/briarproject/briar/privategroup/invitation/CreatorProtocolEngine.java

@@ -254,8 +254,8 @@ class CreatorProtocolEngine extends AbstractProtocolEngine<CreatorSession> {
 	private GroupInvitationResponse createInvitationResponse(
 			GroupInvitationMessage m, ContactId c, boolean accept) {
 		SessionId sessionId = new SessionId(m.getPrivateGroupId().getBytes());
-		return new GroupInvitationResponse(m.getId(), sessionId,
-				m.getContactGroupId(), c, m.getPrivateGroupId(), accept,
-				m.getTimestamp(), false, false, true, false);
+		return new GroupInvitationResponse(m.getId(), m.getContactGroupId(),
+				m.getTimestamp(), false, false, true, false, sessionId,
+				m.getPrivateGroupId(), c, accept);
 	}
 }

briar-core/src/main/java/org/briarproject/briar/privategroup/invitation/GroupInvitationManagerImpl.java

@@ -71,7 +71,6 @@ class GroupInvitationManagerImpl extends ConversationClientImpl
 	private final ProtocolEngine<CreatorSession> creatorEngine;
 	private final ProtocolEngine<InviteeSession> inviteeEngine;
 	private final ProtocolEngine<PeerSession> peerEngine;
-	private final Group localGroup;
 
 	@Inject
 	GroupInvitationManagerImpl(DatabaseComponent db,
@@ -93,12 +92,10 @@ class GroupInvitationManagerImpl extends ConversationClientImpl
 		creatorEngine = engineFactory.createCreatorEngine();
 		inviteeEngine = engineFactory.createInviteeEngine();
 		peerEngine = engineFactory.createPeerEngine();
-		localGroup = contactGroupFactory.createLocalGroup(CLIENT_ID);
 	}
 
 	@Override
 	public void createLocalState(Transaction txn) throws DbException {
-		db.addGroup(txn, localGroup);
 		// Ensure we've set things up for any pre-existing contacts
 		for (Contact c : db.getContacts(txn)) addingContact(txn, c);
 	}
@@ -404,22 +401,15 @@ class GroupInvitationManagerImpl extends ConversationClientImpl
 			throws DbException, FormatException {
 		SessionId sessionId = getSessionId(meta.getPrivateGroupId());
 		// Look up the invite message to get the details of the private group
-		InviteMessage invite = getInviteMessage(txn, m);
+		InviteMessage invite = messageParser.getInviteMessage(txn, m);
+		PrivateGroup pg = privateGroupFactory
+				.createPrivateGroup(invite.getGroupName(), invite.getCreator(),
+						invite.getSalt());
 		boolean canBeOpened = db.containsGroup(txn, invite.getPrivateGroupId());
-		return new GroupInvitationRequest(m, sessionId, contactGroupId, c,
-				invite.getMessage(), invite.getPrivateGroupId(),
-				invite.getGroupName(), invite.getCreator(),
-				meta.isAvailableToAnswer(), canBeOpened, meta.getTimestamp(),
-				meta.isLocal(), status.isSent(), status.isSeen(),
-				meta.isRead());
-	}
-
-	private InviteMessage getInviteMessage(Transaction txn, MessageId m)
-			throws DbException, FormatException {
-		Message message = clientHelper.getMessage(txn, m);
-		if (message == null) throw new DbException();
-		BdfList body = clientHelper.toList(message);
-		return messageParser.parseInviteMessage(message, body);
+		return new GroupInvitationRequest(m, contactGroupId,
+				meta.getTimestamp(), meta.isLocal(), status.isSent(),
+				status.isSeen(), meta.isRead(), sessionId, pg, c,
+				invite.getMessage(), meta.isAvailableToAnswer(), canBeOpened);
 	}
 
 	private GroupInvitationResponse parseInvitationResponse(ContactId c,
@@ -427,10 +417,10 @@ class GroupInvitationManagerImpl extends ConversationClientImpl
 			MessageStatus status, boolean accept)
 			throws DbException, FormatException {
 		SessionId sessionId = getSessionId(meta.getPrivateGroupId());
-		return new GroupInvitationResponse(m, sessionId, contactGroupId, c,
-				meta.getPrivateGroupId(), accept, meta.getTimestamp(),
-				meta.isLocal(), status.isSent(), status.isSeen(),
-				meta.isRead());
+		return new GroupInvitationResponse(m, contactGroupId,
+				meta.getTimestamp(), meta.isLocal(), status.isSent(),
+				status.isSeen(), meta.isRead(), sessionId,
+				meta.getPrivateGroupId(), c, accept);
 	}
 
 	@Override
@@ -481,7 +471,7 @@ class GroupInvitationManagerImpl extends ConversationClientImpl
 
 	private GroupInvitationItem parseGroupInvitationItem(Transaction txn,
 			Contact c, MessageId m) throws DbException, FormatException {
-		InviteMessage invite = getInviteMessage(txn, m);
+		InviteMessage invite = messageParser.getInviteMessage(txn, m);
 		PrivateGroup privateGroup = privateGroupFactory.createPrivateGroup(
 				invite.getGroupName(), invite.getCreator(), invite.getSalt());
 		return new GroupInvitationItem(privateGroup, c);

briar-core/src/main/java/org/briarproject/briar/privategroup/invitation/InviteeProtocolEngine.java

@@ -234,8 +234,9 @@ class InviteeProtocolEngine extends AbstractProtocolEngine<InviteeSession> {
 		// Broadcast an event
 		PrivateGroup privateGroup = privateGroupFactory.createPrivateGroup(
 				m.getGroupName(), m.getCreator(), m.getSalt());
-		txn.attach(new GroupInvitationRequestReceivedEvent(privateGroup,
-				contactId, createInvitationRequest(m, contactId)));
+		txn.attach(
+				new GroupInvitationRequestReceivedEvent(privateGroup, contactId,
+						createInvitationRequest(m, privateGroup, contactId)));
 		// Move to the INVITED state
 		return new InviteeSession(s.getContactGroupId(), s.getPrivateGroupId(),
 				s.getLastLocalMessageId(), m.getId(), s.getLocalTimestamp(),
@@ -317,11 +318,11 @@ class InviteeProtocolEngine extends AbstractProtocolEngine<InviteeSession> {
 	}
 
 	private GroupInvitationRequest createInvitationRequest(InviteMessage m,
-			ContactId c) {
+			PrivateGroup pg, ContactId c) {
 		SessionId sessionId = new SessionId(m.getPrivateGroupId().getBytes());
-		return new GroupInvitationRequest(m.getId(), sessionId,
-				m.getContactGroupId(), c, m.getMessage(), m.getPrivateGroupId(),
-				m.getGroupName(), m.getCreator(), true, false, m.getTimestamp(),
-				false, false, true, false);
+		return new GroupInvitationRequest(m.getId(), m.getContactGroupId(),
+				m.getTimestamp(), false, false, true, false, sessionId, pg, c,
+				m.getMessage(), true, false);
 	}
+
 }

briar-core/src/main/java/org/briarproject/briar/privategroup/invitation/MessageParser.java

@@ -3,9 +3,12 @@ package org.briarproject.briar.privategroup.invitation;
 import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.data.BdfDictionary;
 import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
 
 @NotNullByDefault
 interface MessageParser {
@@ -18,6 +21,9 @@ interface MessageParser {
 
 	MessageMetadata parseMetadata(BdfDictionary meta) throws FormatException;
 
+	InviteMessage getInviteMessage(Transaction txn, MessageId m)
+			throws DbException, FormatException;
+
 	InviteMessage parseInviteMessage(Message m, BdfList body)
 			throws FormatException;
 

briar-core/src/main/java/org/briarproject/briar/privategroup/invitation/MessageParserImpl.java

@@ -1,9 +1,12 @@
 package org.briarproject.briar.privategroup.invitation;
 
 import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.data.BdfDictionary;
 import org.briarproject.bramble.api.data.BdfEntry;
 import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.identity.AuthorFactory;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
@@ -31,12 +34,14 @@ class MessageParserImpl implements MessageParser {
 
 	private final AuthorFactory authorFactory;
 	private final PrivateGroupFactory privateGroupFactory;
+	private final ClientHelper clientHelper;
 
 	@Inject
 	MessageParserImpl(AuthorFactory authorFactory,
-			PrivateGroupFactory privateGroupFactory) {
+			PrivateGroupFactory privateGroupFactory, ClientHelper clientHelper) {
 		this.authorFactory = authorFactory;
 		this.privateGroupFactory = privateGroupFactory;
+		this.clientHelper = clientHelper;
 	}
 
 	@Override
@@ -78,6 +83,15 @@ class MessageParserImpl implements MessageParser {
 				visible, available);
 	}
 
+	@Override
+	public InviteMessage getInviteMessage(Transaction txn, MessageId m)
+			throws DbException, FormatException {
+		Message message = clientHelper.getMessage(txn, m);
+		if (message == null) throw new DbException();
+		BdfList body = clientHelper.toList(message);
+		return parseInviteMessage(message, body);
+	}
+
 	@Override
 	public InviteMessage parseInviteMessage(Message m, BdfList body)
 			throws FormatException {

briar-core/src/main/java/org/briarproject/briar/sharing/AbortMessage.java

@@ -0,0 +1,19 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class AbortMessage extends SharingMessage {
+
+	AbortMessage(MessageId id, GroupId contactGroupId, GroupId shareableId,
+			long timestamp, @Nullable MessageId previousMessageId) {
+		super(id, contactGroupId, shareableId, timestamp, previousMessageId);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/AcceptMessage.java

@@ -0,0 +1,19 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class AcceptMessage extends SharingMessage {
+
+	AcceptMessage(MessageId id, @Nullable MessageId previousMessageId,
+			GroupId contactGroupId, GroupId shareableId, long timestamp) {
+		super(id, contactGroupId, shareableId, timestamp, previousMessageId);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/BlogSharingManagerImpl.java

@@ -52,7 +52,7 @@ import static org.briarproject.briar.api.sharing.SharingConstants.RESPONSE_ID;
 @Immutable
 @NotNullByDefault
 class BlogSharingManagerImpl extends
-		SharingManagerImpl<Blog, BlogInvitation, BlogInviteeSessionState, BlogSharerSessionState, BlogInvitationRequestReceivedEvent, BlogInvitationResponseReceivedEvent>
+		OldSharingManagerImpl<Blog, BlogInvitation, BlogInviteeSessionState, BlogSharerSessionState, BlogInvitationRequestReceivedEvent, BlogInvitationResponseReceivedEvent>
 		implements BlogSharingManager, RemoveBlogHook {
 
 	private final ContactManager contactManager;
@@ -159,7 +159,7 @@ class BlogSharingManagerImpl extends
 	}
 
 	@Override
-	protected InvitationFactory<BlogInvitation, BlogSharerSessionState> getIFactory() {
+	protected OldInvitationFactory<BlogInvitation, BlogSharerSessionState> getIFactory() {
 		return iFactory;
 	}
 
@@ -251,7 +251,7 @@ class BlogSharingManagerImpl extends
 	}
 
 	private static class IFactory implements
-			InvitationFactory<BlogInvitation, BlogSharerSessionState> {
+			OldInvitationFactory<BlogInvitation, BlogSharerSessionState> {
 		@Override
 		public BlogInvitation build(GroupId groupId, BdfDictionary d)
 				throws FormatException {

briar-core/src/main/java/org/briarproject/briar/sharing/DeclineMessage.java

@@ -0,0 +1,20 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class DeclineMessage extends SharingMessage {
+
+	DeclineMessage(MessageId id, GroupId contactGroupId,
+			GroupId shareableId, long timestamp,
+			@Nullable MessageId previousMessageId) {
+		super(id, contactGroupId, shareableId, timestamp, previousMessageId);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/ForumInvitationFactoryImpl.java

@@ -0,0 +1,39 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.briar.api.client.SessionId;
+import org.briarproject.briar.api.forum.Forum;
+import org.briarproject.briar.api.forum.ForumInvitationRequest;
+import org.briarproject.briar.api.forum.ForumInvitationResponse;
+
+import javax.inject.Inject;
+
+public class ForumInvitationFactoryImpl implements InvitationFactory<Forum> {
+
+	@Inject
+	ForumInvitationFactoryImpl() {
+	}
+
+	@Override
+	public ForumInvitationRequest createInvitationRequest(boolean local,
+			boolean sent, boolean seen, boolean read, InviteMessage<Forum> m,
+			ContactId c, boolean available, boolean canBeOpened) {
+		SessionId sessionId = new SessionId(m.getShareableId().getBytes());
+		return new ForumInvitationRequest(m.getId(), m.getContactGroupId(),
+				m.getTimestamp(), local, sent, seen, read, sessionId,
+				m.getShareable(), c, m.getMessage(), available, canBeOpened);
+	}
+
+	@Override
+	public ForumInvitationResponse createInvitationResponse(MessageId id,
+			GroupId contactGroupId, long time, boolean local, boolean sent,
+			boolean seen, boolean read, GroupId shareableId,
+			ContactId contactId, boolean accept) {
+		SessionId sessionId = new SessionId(shareableId.getBytes());
+		return new ForumInvitationResponse(id, contactGroupId, time, local,
+				sent, seen, read, sessionId, shareableId, contactId, accept);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/ForumInviteeSessionState.java

@@ -1,47 +0,0 @@
-package org.briarproject.briar.sharing;
-
-import org.briarproject.bramble.api.contact.ContactId;
-import org.briarproject.bramble.api.data.BdfDictionary;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.sync.GroupId;
-import org.briarproject.bramble.api.sync.MessageId;
-import org.briarproject.briar.api.client.SessionId;
-
-import javax.annotation.concurrent.NotThreadSafe;
-
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_NAME;
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT;
-
-@NotThreadSafe
-@NotNullByDefault
-class ForumInviteeSessionState extends InviteeSessionState {
-
-	private final String forumName;
-	private final byte[] forumSalt;
-
-	ForumInviteeSessionState(SessionId sessionId, MessageId storageId,
-			GroupId groupId, State state, ContactId contactId, GroupId forumId,
-			String forumName, byte[] forumSalt, MessageId invitationId) {
-		super(sessionId, storageId, groupId, state, contactId, forumId,
-				invitationId);
-
-		this.forumName = forumName;
-		this.forumSalt = forumSalt;
-	}
-
-	@Override
-	public BdfDictionary toBdfDictionary() {
-		BdfDictionary d = super.toBdfDictionary();
-		d.put(FORUM_NAME, getForumName());
-		d.put(FORUM_SALT, getForumSalt());
-		return d;
-	}
-
-	String getForumName() {
-		return forumName;
-	}
-
-	byte[] getForumSalt() {
-		return forumSalt;
-	}
-}

briar-core/src/main/java/org/briarproject/briar/sharing/ForumMessageParserImpl.java

@@ -0,0 +1,34 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.forum.Forum;
+import org.briarproject.briar.api.forum.ForumFactory;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+@Immutable
+@NotNullByDefault
+class ForumMessageParserImpl extends MessageParserImpl<Forum> {
+
+	private final ForumFactory forumFactory;
+
+	@Inject
+	ForumMessageParserImpl(ClientHelper clientHelper,
+			ForumFactory forumFactory) {
+		super(clientHelper);
+		this.forumFactory = forumFactory;
+	}
+
+	@Override
+	protected Forum createShareable(BdfList descriptor)
+			throws FormatException {
+		String name = descriptor.getString(0);
+		byte[] salt = descriptor.getRaw(1);
+		return forumFactory.createForum(name, salt);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/ForumProtocolEngineImpl.java

@@ -0,0 +1,93 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.ClientId;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.briar.api.client.MessageTracker;
+import org.briarproject.briar.api.forum.Forum;
+import org.briarproject.briar.api.forum.ForumInvitationRequest;
+import org.briarproject.briar.api.forum.ForumInvitationResponse;
+import org.briarproject.briar.api.forum.ForumManager;
+import org.briarproject.briar.api.forum.ForumSharingManager;
+import org.briarproject.briar.api.forum.event.ForumInvitationRequestReceivedEvent;
+import org.briarproject.briar.api.forum.event.ForumInvitationResponseReceivedEvent;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+@Immutable
+@NotNullByDefault
+class ForumProtocolEngineImpl extends ProtocolEngineImpl<Forum> {
+
+	private final ForumManager forumManager;
+	private final InvitationFactory<Forum> invitationFactory;
+
+	@Inject
+	ForumProtocolEngineImpl(DatabaseComponent db,
+			ClientHelper clientHelper, MessageEncoder messageEncoder,
+			MessageParser<Forum> messageParser, MessageTracker messageTracker,
+			Clock clock, ForumManager forumManager,
+			InvitationFactory<Forum> invitationFactory) {
+		super(db, clientHelper, messageEncoder, messageParser, messageTracker,
+				clock);
+		this.forumManager = forumManager;
+		this.invitationFactory = invitationFactory;
+	}
+
+	@Override
+	Event getInvitationRequestReceivedEvent(InviteMessage<Forum> m,
+			ContactId contactId, boolean available, boolean canBeOpened) {
+		ForumInvitationRequest request =
+				(ForumInvitationRequest) invitationFactory
+						.createInvitationRequest(false, false, true, false, m,
+								contactId, available, canBeOpened);
+		return new ForumInvitationRequestReceivedEvent(m.getShareable(),
+				contactId, request);
+	}
+
+	@Override
+	Event getInvitationResponseReceivedEvent(AcceptMessage m,
+			ContactId contactId) {
+		ForumInvitationResponse response =
+				(ForumInvitationResponse) invitationFactory
+						.createInvitationResponse(m.getId(),
+								m.getContactGroupId(), m.getTimestamp(), false,
+								false, true, false, m.getShareableId(),
+								contactId, true);
+		return new ForumInvitationResponseReceivedEvent(contactId, response);
+	}
+
+	@Override
+	Event getInvitationResponseReceivedEvent(DeclineMessage m,
+			ContactId contactId) {
+		ForumInvitationResponse response =
+				(ForumInvitationResponse) invitationFactory
+						.createInvitationResponse(m.getId(),
+								m.getContactGroupId(), m.getTimestamp(), false,
+								false, true, false, m.getShareableId(),
+								contactId, true);
+		return new ForumInvitationResponseReceivedEvent(contactId, response);
+	}
+
+	@Override
+	protected ClientId getClientId() {
+		return ForumSharingManager.CLIENT_ID;
+	}
+
+	@Override
+	protected void addShareable(Transaction txn, MessageId inviteId)
+			throws DbException, FormatException {
+		InviteMessage<Forum> invite =
+				messageParser.getInviteMessage(txn, inviteId);
+		forumManager.addForum(txn, invite.getShareable());
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/ForumSharerSessionState.java

@@ -1,49 +0,0 @@
-package org.briarproject.briar.sharing;
-
-import org.briarproject.bramble.api.contact.ContactId;
-import org.briarproject.bramble.api.data.BdfDictionary;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.sync.GroupId;
-import org.briarproject.bramble.api.sync.MessageId;
-import org.briarproject.briar.api.client.SessionId;
-
-import javax.annotation.Nullable;
-import javax.annotation.concurrent.NotThreadSafe;
-
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_NAME;
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT;
-
-@NotThreadSafe
-@NotNullByDefault
-class ForumSharerSessionState extends SharerSessionState {
-
-	private final String forumName;
-	private final byte[] forumSalt;
-
-	ForumSharerSessionState(SessionId sessionId, MessageId storageId,
-			GroupId groupId, State state, ContactId contactId, GroupId forumId,
-			String forumName, byte[] forumSalt,
-			@Nullable MessageId responseId) {
-		super(sessionId, storageId, groupId, state, contactId, forumId,
-				responseId);
-
-		this.forumName = forumName;
-		this.forumSalt = forumSalt;
-	}
-
-	@Override
-	public BdfDictionary toBdfDictionary() {
-		BdfDictionary d = super.toBdfDictionary();
-		d.put(FORUM_NAME, getForumName());
-		d.put(FORUM_SALT, getForumSalt());
-		return d;
-	}
-
-	String getForumName() {
-		return forumName;
-	}
-
-	byte[] getForumSalt() {
-		return forumSalt;
-	}
-}

briar-core/src/main/java/org/briarproject/briar/sharing/ForumSharingManagerImpl.java

@@ -1,78 +1,35 @@
 package org.briarproject.briar.sharing;
 
-import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.client.ContactGroupFactory;
-import org.briarproject.bramble.api.contact.ContactId;
-import org.briarproject.bramble.api.data.BdfDictionary;
-import org.briarproject.bramble.api.data.BdfList;
-import org.briarproject.bramble.api.data.MetadataEncoder;
 import org.briarproject.bramble.api.data.MetadataParser;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.ClientId;
-import org.briarproject.bramble.api.sync.GroupId;
-import org.briarproject.bramble.api.sync.MessageId;
-import org.briarproject.bramble.api.system.Clock;
-import org.briarproject.briar.api.client.MessageQueueManager;
 import org.briarproject.briar.api.client.MessageTracker;
-import org.briarproject.briar.api.client.SessionId;
 import org.briarproject.briar.api.forum.Forum;
-import org.briarproject.briar.api.forum.ForumFactory;
-import org.briarproject.briar.api.forum.ForumInvitationRequest;
-import org.briarproject.briar.api.forum.ForumInvitationResponse;
-import org.briarproject.briar.api.forum.ForumManager;
 import org.briarproject.briar.api.forum.ForumManager.RemoveForumHook;
 import org.briarproject.briar.api.forum.ForumSharingManager;
-import org.briarproject.briar.api.forum.ForumSharingMessage.ForumInvitation;
-import org.briarproject.briar.api.forum.event.ForumInvitationRequestReceivedEvent;
-import org.briarproject.briar.api.forum.event.ForumInvitationResponseReceivedEvent;
-import org.briarproject.briar.api.sharing.InvitationMessage;
 
-import java.security.SecureRandom;
-
-import javax.annotation.Nullable;
 import javax.inject.Inject;
 
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_NAME;
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT;
-import static org.briarproject.briar.api.sharing.SharingConstants.INVITATION_ID;
-import static org.briarproject.briar.api.sharing.SharingConstants.RESPONSE_ID;
-
 @NotNullByDefault
-class ForumSharingManagerImpl extends
-		SharingManagerImpl<Forum, ForumInvitation, ForumInviteeSessionState, ForumSharerSessionState, ForumInvitationRequestReceivedEvent, ForumInvitationResponseReceivedEvent>
+class ForumSharingManagerImpl extends SharingManagerImpl<Forum>
 		implements ForumSharingManager, RemoveForumHook {
 
-	private final SFactory sFactory;
-	private final IFactory iFactory;
-	private final ISFactory isFactory;
-	private final SSFactory ssFactory;
-	private final IRFactory irFactory;
-	private final IRRFactory irrFactory;
-
 	@Inject
-	ForumSharingManagerImpl(ClientHelper clientHelper,
-			Clock clock, DatabaseComponent db,
-			ForumFactory forumFactory,
-			ForumManager forumManager,
-			MessageQueueManager messageQueueManager,
-			MetadataEncoder metadataEncoder,
-			MetadataParser metadataParser,
+	ForumSharingManagerImpl(DatabaseComponent db, ClientHelper clientHelper,
+			MetadataParser metadataParser, MessageParser<Forum> messageParser,
+			SessionEncoder sessionEncoder, SessionParser sessionParser,
+			MessageTracker messageTracker,
 			ContactGroupFactory contactGroupFactory,
-			SecureRandom random, MessageTracker messageTracker) {
-		super(db, messageQueueManager, clientHelper, metadataParser,
-				metadataEncoder, random, contactGroupFactory, messageTracker,
-				clock);
-
-		sFactory = new SFactory(forumFactory, forumManager);
-		iFactory = new IFactory();
-		isFactory = new ISFactory();
-		ssFactory = new SSFactory();
-		irFactory = new IRFactory(sFactory);
-		irrFactory = new IRRFactory();
+			ProtocolEngine<Forum> engine,
+			InvitationFactory<Forum> invitationFactory) {
+		super(db, clientHelper, metadataParser, messageParser, sessionEncoder,
+				sessionParser, messageTracker, contactGroupFactory, engine,
+				invitationFactory);
 	}
 
 	@Override
@@ -80,221 +37,9 @@ class ForumSharingManagerImpl extends
 		return CLIENT_ID;
 	}
 
-	@Override
-	protected InvitationMessage createInvitationRequest(MessageId id,
-			ForumInvitation msg, ContactId contactId, GroupId forumId,
-			boolean available, boolean canBeOpened, long time,
-			boolean local, boolean sent, boolean seen, boolean read) {
-		return new ForumInvitationRequest(id, msg.getSessionId(),
-				msg.getGroupId(), contactId, forumId, msg.getForumName(),
-				msg.getMessage(), available, canBeOpened, time, local, sent,
-				seen, read);
-	}
-
-	@Override
-	protected InvitationMessage createInvitationResponse(MessageId id,
-			SessionId sessionId, GroupId groupId, ContactId contactId,
-			GroupId forumId, boolean accept, long time, boolean local,
-			boolean sent, boolean seen, boolean read) {
-		return new ForumInvitationResponse(id, sessionId, groupId, contactId,
-				forumId, accept, time, local, sent, seen, read);
-	}
-
-	@Override
-	protected ShareableFactory<Forum, ForumInvitation, ForumInviteeSessionState, ForumSharerSessionState> getSFactory() {
-		return sFactory;
-	}
-
-	@Override
-	protected InvitationFactory<ForumInvitation, ForumSharerSessionState> getIFactory() {
-		return iFactory;
-	}
-
-	@Override
-	protected InviteeSessionStateFactory<Forum, ForumInviteeSessionState> getISFactory() {
-		return isFactory;
-	}
-
-	@Override
-	protected SharerSessionStateFactory<Forum, ForumSharerSessionState> getSSFactory() {
-		return ssFactory;
-	}
-
-	@Override
-	protected InvitationReceivedEventFactory<ForumInviteeSessionState, ForumInvitationRequestReceivedEvent> getIRFactory() {
-		return irFactory;
-	}
-
-	@Override
-	protected InvitationResponseReceivedEventFactory<ForumSharerSessionState, ForumInvitationResponseReceivedEvent> getIRRFactory() {
-		return irrFactory;
-	}
-
 	@Override
 	public void removingForum(Transaction txn, Forum f) throws DbException {
 		removingShareable(txn, f);
 	}
 
-	private static class SFactory implements
-			ShareableFactory<Forum, ForumInvitation, ForumInviteeSessionState, ForumSharerSessionState> {
-
-		private final ForumFactory forumFactory;
-		private final ForumManager forumManager;
-
-		private SFactory(ForumFactory forumFactory, ForumManager forumManager) {
-			this.forumFactory = forumFactory;
-			this.forumManager = forumManager;
-		}
-
-		@Override
-		public BdfList encode(Forum f) {
-			return BdfList.of(f.getName(), f.getSalt());
-		}
-
-		@Override
-		public Forum get(Transaction txn, GroupId groupId)
-				throws DbException {
-			return forumManager.getForum(txn, groupId);
-		}
-
-		@Override
-		public Forum parse(BdfList shareable) throws FormatException {
-			return forumFactory
-					.createForum(shareable.getString(0), shareable.getRaw(1));
-		}
-
-		@Override
-		public Forum parse(ForumInvitation msg) {
-			return forumFactory
-					.createForum(msg.getForumName(), msg.getForumSalt());
-		}
-
-		@Override
-		public Forum parse(ForumInviteeSessionState state) {
-			return forumFactory
-					.createForum(state.getForumName(), state.getForumSalt());
-		}
-
-		@Override
-		public Forum parse(ForumSharerSessionState state) {
-			return forumFactory
-					.createForum(state.getForumName(), state.getForumSalt());
-		}
-	}
-
-	private static class IFactory implements
-			InvitationFactory<ForumInvitation, ForumSharerSessionState> {
-		@Override
-		public ForumInvitation build(GroupId groupId, BdfDictionary d)
-				throws FormatException {
-			return ForumInvitation.from(groupId, d);
-		}
-
-		@Override
-		public ForumInvitation build(ForumSharerSessionState localState,
-				long time) {
-			return new ForumInvitation(localState.getContactGroupId(),
-					localState.getSessionId(), localState.getForumName(),
-					localState.getForumSalt(), time, localState.getMessage());
-		}
-	}
-
-	private static class ISFactory implements
-			InviteeSessionStateFactory<Forum, ForumInviteeSessionState> {
-		@Override
-		public ForumInviteeSessionState build(SessionId sessionId,
-				MessageId storageId, GroupId groupId,
-				InviteeSessionState.State state, ContactId contactId,
-				GroupId forumId, BdfDictionary d) throws FormatException {
-			String forumName = d.getString(FORUM_NAME);
-			byte[] forumSalt = d.getRaw(FORUM_SALT);
-			MessageId invitationId = new MessageId(d.getRaw(INVITATION_ID));
-			return new ForumInviteeSessionState(sessionId, storageId,
-					groupId, state, contactId, forumId, forumName, forumSalt,
-					invitationId);
-		}
-
-		@Override
-		public ForumInviteeSessionState build(SessionId sessionId,
-				MessageId storageId, GroupId groupId,
-				InviteeSessionState.State state, ContactId contactId,
-				Forum forum, MessageId invitationId) {
-			return new ForumInviteeSessionState(sessionId, storageId,
-					groupId, state, contactId, forum.getId(), forum.getName(),
-					forum.getSalt(), invitationId);
-		}
-	}
-
-	private static class SSFactory implements
-			SharerSessionStateFactory<Forum, ForumSharerSessionState> {
-		@Override
-		public ForumSharerSessionState build(SessionId sessionId,
-				MessageId storageId, GroupId groupId,
-				SharerSessionState.State state, ContactId contactId,
-				GroupId forumId, BdfDictionary d) throws FormatException {
-			String forumName = d.getString(FORUM_NAME);
-			byte[] forumSalt = d.getRaw(FORUM_SALT);
-			MessageId responseId = null;
-			byte[] responseIdBytes = d.getOptionalRaw(RESPONSE_ID);
-			if (responseIdBytes != null)
-				responseId = new MessageId(responseIdBytes);
-			return new ForumSharerSessionState(sessionId, storageId,
-					groupId, state, contactId, forumId, forumName, forumSalt,
-					responseId);
-		}
-
-		@Override
-		public ForumSharerSessionState build(SessionId sessionId,
-				MessageId storageId, GroupId groupId,
-				SharerSessionState.State state, ContactId contactId,
-				Forum forum) {
-			return new ForumSharerSessionState(sessionId, storageId,
-					groupId, state, contactId, forum.getId(), forum.getName(),
-					forum.getSalt(), null);
-		}
-	}
-
-	private static class IRFactory implements
-			InvitationReceivedEventFactory<ForumInviteeSessionState, ForumInvitationRequestReceivedEvent> {
-
-		private final SFactory sFactory;
-
-		private IRFactory(SFactory sFactory) {
-			this.sFactory = sFactory;
-		}
-
-		@Override
-		public ForumInvitationRequestReceivedEvent build(
-				ForumInviteeSessionState localState, long time,
-				@Nullable String msg) {
-			Forum forum = sFactory.parse(localState);
-			ContactId contactId = localState.getContactId();
-			ForumInvitationRequest request = new ForumInvitationRequest(
-					localState.getInvitationId(), localState.getSessionId(),
-					localState.getContactGroupId(), contactId,
-					localState.getShareableId(), forum.getName(), msg, true,
-					false, time, false, false, false, false);
-			return new ForumInvitationRequestReceivedEvent(forum, contactId,
-					request);
-		}
-	}
-
-	private static class IRRFactory implements
-			InvitationResponseReceivedEventFactory<ForumSharerSessionState, ForumInvitationResponseReceivedEvent> {
-		@Override
-		public ForumInvitationResponseReceivedEvent build(
-				ForumSharerSessionState localState, boolean accept, long time) {
-			String name = localState.getForumName();
-			ContactId c = localState.getContactId();
-			MessageId responseId = localState.getResponseId();
-			if (responseId == null)
-				throw new IllegalStateException("No responseId");
-			ForumInvitationResponse response = new ForumInvitationResponse(
-					responseId, localState.getSessionId(),
-					localState.getContactGroupId(), localState.getContactId(),
-					localState.getShareableId(), accept, time, false, false,
-					false, false);
-			return new ForumInvitationResponseReceivedEvent(name, c, response);
-		}
-	}
 }

briar-core/src/main/java/org/briarproject/briar/sharing/ForumSharingValidator.java

@@ -1,89 +1,47 @@
 package org.briarproject.briar.sharing;
 
 import org.briarproject.bramble.api.FormatException;
-import org.briarproject.bramble.api.client.BdfMessageContext;
 import org.briarproject.bramble.api.client.ClientHelper;
-import org.briarproject.bramble.api.data.BdfDictionary;
 import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.data.MetadataEncoder;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.sync.Group;
-import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.system.Clock;
-import org.briarproject.briar.api.client.SessionId;
-import org.briarproject.briar.client.BdfQueueMessageValidator;
+import org.briarproject.briar.api.forum.Forum;
+import org.briarproject.briar.api.forum.ForumFactory;
 
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
 import static org.briarproject.bramble.util.ValidationUtils.checkLength;
 import static org.briarproject.bramble.util.ValidationUtils.checkSize;
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_NAME;
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT;
 import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT_LENGTH;
 import static org.briarproject.briar.api.forum.ForumConstants.MAX_FORUM_NAME_LENGTH;
-import static org.briarproject.briar.api.sharing.SharingConstants.INVITATION_MSG;
-import static org.briarproject.briar.api.sharing.SharingConstants.LOCAL;
-import static org.briarproject.briar.api.sharing.SharingConstants.MAX_INVITATION_MESSAGE_LENGTH;
-import static org.briarproject.briar.api.sharing.SharingConstants.SESSION_ID;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_ABORT;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_ACCEPT;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_DECLINE;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_INVITATION;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_LEAVE;
-import static org.briarproject.briar.api.sharing.SharingConstants.TIME;
-import static org.briarproject.briar.api.sharing.SharingConstants.TYPE;
 
 @Immutable
 @NotNullByDefault
-class ForumSharingValidator extends BdfQueueMessageValidator {
+class ForumSharingValidator extends SharingValidator {
+
+	private final ForumFactory forumFactory;
 
 	@Inject
-	ForumSharingValidator(ClientHelper clientHelper,
-			MetadataEncoder metadataEncoder, Clock clock) {
-		super(clientHelper, metadataEncoder, clock);
+	ForumSharingValidator(MessageEncoder messageEncoder,
+			ClientHelper clientHelper, MetadataEncoder metadataEncoder,
+			Clock clock, ForumFactory forumFactory) {
+		super(messageEncoder, clientHelper, metadataEncoder, clock);
+		this.forumFactory = forumFactory;
 	}
 
 	@Override
-	protected BdfMessageContext validateMessage(Message m, Group g,
-			BdfList body) throws FormatException {
-
-		BdfDictionary d = new BdfDictionary();
-		long type = body.getLong(0);
-		byte[] id = body.getRaw(1);
-		checkLength(id, SessionId.LENGTH);
-
-		if (type == SHARE_MSG_TYPE_INVITATION) {
-			checkSize(body, 4, 5);
-
-			String name = body.getString(2);
-			checkLength(name, 1, MAX_FORUM_NAME_LENGTH);
-
-			byte[] salt = body.getRaw(3);
-			checkLength(salt, FORUM_SALT_LENGTH);
-
-			d.put(FORUM_NAME, name);
-			d.put(FORUM_SALT, salt);
-
-			if (body.size() > 4) {
-				String msg = body.getString(4);
-				checkLength(msg, 0, MAX_INVITATION_MESSAGE_LENGTH);
-				d.put(INVITATION_MSG, msg);
-			}
-		} else {
-			checkSize(body, 2);
-			if (type != SHARE_MSG_TYPE_ACCEPT &&
-					type != SHARE_MSG_TYPE_DECLINE &&
-					type != SHARE_MSG_TYPE_LEAVE &&
-					type != SHARE_MSG_TYPE_ABORT) {
-				throw new FormatException();
-			}
-		}
-		// Return the metadata
-		d.put(TYPE, type);
-		d.put(SESSION_ID, id);
-		d.put(LOCAL, false);
-		d.put(TIME, m.getTimestamp());
-		return new BdfMessageContext(d);
+	protected GroupId validateDescriptor(BdfList descriptor)
+			throws FormatException {
+		checkSize(descriptor, 2);
+		String name = descriptor.getString(0);
+		checkLength(name, 1, MAX_FORUM_NAME_LENGTH);
+		byte[] salt = descriptor.getRaw(1);
+		checkLength(salt, FORUM_SALT_LENGTH);
+		Forum forum = forumFactory.createForum(name, salt);
+		return forum.getId();
 	}
+
 }

briar-core/src/main/java/org/briarproject/briar/sharing/InvitationFactory.java

@@ -1,11 +1,21 @@
 package org.briarproject.briar.sharing;
 
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.briar.api.sharing.SharingMessage;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.briar.api.sharing.InvitationRequest;
+import org.briarproject.briar.api.sharing.InvitationResponse;
+import org.briarproject.briar.api.sharing.Shareable;
 
-@NotNullByDefault
-interface InvitationFactory<I extends SharingMessage.Invitation, SS extends SharerSessionState>
-		extends org.briarproject.briar.api.sharing.InvitationFactory<I> {
+public interface InvitationFactory<S extends Shareable> {
+
+	InvitationRequest<S> createInvitationRequest(boolean local, boolean sent,
+			boolean seen, boolean read, InviteMessage<S> m, ContactId c,
+			boolean available, boolean canBeOpened);
+
+	InvitationResponse createInvitationResponse(MessageId id,
+			GroupId contactGroupId, long time, boolean local, boolean sent,
+			boolean seen, boolean read, GroupId shareableId,
+			ContactId contactId, boolean accept);
 
-	I build(SS localState, long time);
 }

briar-core/src/main/java/org/briarproject/briar/sharing/InvitationReceivedEventFactory.java

@@ -5,6 +5,7 @@ import org.briarproject.briar.api.sharing.event.InvitationRequestReceivedEvent;
 
 import javax.annotation.Nullable;
 
+@Deprecated
 @NotNullByDefault
 interface InvitationReceivedEventFactory<IS extends InviteeSessionState, IR extends InvitationRequestReceivedEvent> {
 

briar-core/src/main/java/org/briarproject/briar/sharing/InvitationResponseReceivedEventFactory.java

@@ -3,6 +3,7 @@ package org.briarproject.briar.sharing;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.briar.api.sharing.event.InvitationResponseReceivedEvent;
 
+@Deprecated
 @NotNullByDefault
 interface InvitationResponseReceivedEventFactory<SS extends SharerSessionState, IRR extends InvitationResponseReceivedEvent> {
 

briar-core/src/main/java/org/briarproject/briar/sharing/InviteMessage.java

@@ -0,0 +1,39 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.briar.api.sharing.Shareable;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class InviteMessage<S extends Shareable> extends SharingMessage {
+
+	private final S shareable;
+	@Nullable
+	private final String message;
+
+	InviteMessage(MessageId id, @Nullable MessageId previousMessageId,
+			GroupId contactGroupId, S shareable, @Nullable String message,
+			long timestamp) {
+		super(id, contactGroupId, shareable.getId(), timestamp,
+				previousMessageId);
+		if (message != null && message.equals(""))
+			throw new IllegalArgumentException();
+		this.shareable = shareable;
+		this.message = message;
+	}
+
+	public S getShareable() {
+		return shareable;
+	}
+
+	@Nullable
+	public String getMessage() {
+		return message;
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/InviteeEngine.java

@@ -28,6 +28,7 @@ import static org.briarproject.briar.api.sharing.SharingConstants.TASK_UNSHARE_S
 import static org.briarproject.briar.api.sharing.SharingMessage.BaseMessage;
 import static org.briarproject.briar.api.sharing.SharingMessage.SimpleMessage;
 
+@Deprecated
 @Immutable
 @NotNullByDefault
 class InviteeEngine<IS extends InviteeSessionState, IR extends InvitationRequestReceivedEvent>

briar-core/src/main/java/org/briarproject/briar/sharing/InviteeSessionState.java

@@ -23,6 +23,7 @@ import static org.briarproject.briar.sharing.InviteeSessionState.Action.LOCAL_LE
 import static org.briarproject.briar.sharing.InviteeSessionState.Action.REMOTE_INVITATION;
 import static org.briarproject.briar.sharing.InviteeSessionState.Action.REMOTE_LEAVE;
 
+@Deprecated
 @NotThreadSafe
 @NotNullByDefault
 public abstract class InviteeSessionState extends SharingSessionState {

briar-core/src/main/java/org/briarproject/briar/sharing/InviteeSessionStateFactory.java

@@ -8,6 +8,7 @@ import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.briar.api.client.SessionId;
 import org.briarproject.briar.api.sharing.Shareable;
 
+@Deprecated
 interface InviteeSessionStateFactory<S extends Shareable, IS extends InviteeSessionState> {
 
 	IS build(SessionId sessionId, MessageId storageId, GroupId groupId,

briar-core/src/main/java/org/briarproject/briar/sharing/LeaveMessage.java

@@ -0,0 +1,20 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class LeaveMessage extends SharingMessage {
+
+	LeaveMessage(MessageId id, GroupId contactGroupId,
+			GroupId shareableId, long timestamp,
+			@Nullable MessageId previousMessageId) {
+		super(id, contactGroupId, shareableId, timestamp, previousMessageId);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/MessageEncoder.java

@@ -0,0 +1,39 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+interface MessageEncoder {
+
+	BdfDictionary encodeMetadata(MessageType type, GroupId shareableId,
+			long timestamp, boolean local, boolean read, boolean visible,
+			boolean available);
+
+	void setVisibleInUi(BdfDictionary meta, boolean visible);
+
+	void setAvailableToAnswer(BdfDictionary meta, boolean available);
+
+	Message encodeInviteMessage(GroupId contactGroupId, long timestamp,
+			@Nullable MessageId previousMessageId, BdfList descriptor,
+			@Nullable String message);
+
+	Message encodeAcceptMessage(GroupId contactGroupId, GroupId shareableId,
+			long timestamp, @Nullable MessageId previousMessageId);
+
+	Message encodeDeclineMessage(GroupId contactGroupId, GroupId shareableId,
+			long timestamp, @Nullable MessageId previousMessageId);
+
+	Message encodeLeaveMessage(GroupId contactGroupId, GroupId shareableId,
+			long timestamp, @Nullable MessageId previousMessageId);
+
+	Message encodeAbortMessage(GroupId contactGroupId, GroupId shareableId,
+			long timestamp, @Nullable MessageId previousMessageId);
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/MessageEncoderImpl.java

@@ -0,0 +1,137 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageFactory;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.briar.sharing.MessageType.ABORT;
+import static org.briarproject.briar.sharing.MessageType.ACCEPT;
+import static org.briarproject.briar.sharing.MessageType.DECLINE;
+import static org.briarproject.briar.sharing.MessageType.INVITE;
+import static org.briarproject.briar.sharing.MessageType.LEAVE;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_AVAILABLE_TO_ANSWER;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_LOCAL;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_SHAREABLE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_READ;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_TIMESTAMP;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_VISIBLE_IN_UI;
+
+@Immutable
+@NotNullByDefault
+class MessageEncoderImpl implements MessageEncoder {
+
+	private final ClientHelper clientHelper;
+	private final MessageFactory messageFactory;
+
+	@Inject
+	MessageEncoderImpl(ClientHelper clientHelper,
+			MessageFactory messageFactory) {
+		this.clientHelper = clientHelper;
+		this.messageFactory = messageFactory;
+	}
+
+	@Override
+	public BdfDictionary encodeMetadata(MessageType type,
+			GroupId shareableId, long timestamp, boolean local, boolean read,
+			boolean visible, boolean available) {
+		BdfDictionary meta = new BdfDictionary();
+		meta.put(MSG_KEY_MESSAGE_TYPE, type.getValue());
+		meta.put(MSG_KEY_SHAREABLE_ID, shareableId);
+		meta.put(MSG_KEY_TIMESTAMP, timestamp);
+		meta.put(MSG_KEY_LOCAL, local);
+		meta.put(MSG_KEY_READ, read);
+		meta.put(MSG_KEY_VISIBLE_IN_UI, visible);
+		meta.put(MSG_KEY_AVAILABLE_TO_ANSWER, available);
+		return meta;
+	}
+
+	@Override
+	public void setVisibleInUi(BdfDictionary meta, boolean visible) {
+		meta.put(MSG_KEY_VISIBLE_IN_UI, visible);
+	}
+
+	@Override
+	public void setAvailableToAnswer(BdfDictionary meta, boolean available) {
+		meta.put(MSG_KEY_AVAILABLE_TO_ANSWER, available);
+	}
+
+	@Override
+	public Message encodeInviteMessage(GroupId contactGroupId, long timestamp,
+			@Nullable MessageId previousMessageId, BdfList descriptor,
+			@Nullable String message) {
+		if (message != null && message.equals(""))
+			throw new IllegalArgumentException();
+		BdfList body = BdfList.of(
+				INVITE.getValue(),
+				previousMessageId,
+				descriptor,
+				message
+		);
+		try {
+			return messageFactory.createMessage(contactGroupId, timestamp,
+					clientHelper.toByteArray(body));
+		} catch (FormatException e) {
+			throw new AssertionError(e);
+		}
+	}
+
+	@Override
+	public Message encodeAcceptMessage(GroupId contactGroupId,
+			GroupId shareableId, long timestamp,
+			@Nullable MessageId previousMessageId) {
+		return encodeMessage(ACCEPT, contactGroupId, shareableId, timestamp,
+				previousMessageId);
+	}
+
+	@Override
+	public Message encodeDeclineMessage(GroupId contactGroupId,
+			GroupId shareableId, long timestamp,
+			@Nullable MessageId previousMessageId) {
+		return encodeMessage(DECLINE, contactGroupId, shareableId, timestamp,
+				previousMessageId);
+	}
+
+	@Override
+	public Message encodeLeaveMessage(GroupId contactGroupId,
+			GroupId shareableId, long timestamp,
+			@Nullable MessageId previousMessageId) {
+		return encodeMessage(LEAVE, contactGroupId, shareableId, timestamp,
+				previousMessageId);
+	}
+
+	@Override
+	public Message encodeAbortMessage(GroupId contactGroupId,
+			GroupId shareableId, long timestamp,
+			@Nullable MessageId previousMessageId) {
+		return encodeMessage(ABORT, contactGroupId, shareableId, timestamp,
+				previousMessageId);
+	}
+
+	private Message encodeMessage(MessageType type, GroupId contactGroupId,
+			GroupId shareableId, long timestamp,
+			@Nullable MessageId previousMessageId) {
+		BdfList body = BdfList.of(
+				type.getValue(),
+				shareableId,
+				previousMessageId
+		);
+		try {
+			return messageFactory.createMessage(contactGroupId, timestamp,
+					clientHelper.toByteArray(body));
+		} catch (FormatException e) {
+			throw new AssertionError(e);
+		}
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/MessageMetadata.java

@@ -0,0 +1,56 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class MessageMetadata {
+
+	private final MessageType type;
+	private final GroupId shareableId;
+	private final long timestamp;
+	private final boolean local, read, visible, available;
+
+	MessageMetadata(MessageType type, GroupId shareableId, long timestamp,
+			boolean local, boolean read, boolean visible, boolean available) {
+		this.shareableId = shareableId;
+		this.type = type;
+		this.timestamp = timestamp;
+		this.local = local;
+		this.read = read;
+		this.visible = visible;
+		this.available = available;
+	}
+
+	MessageType getMessageType() {
+		return type;
+	}
+
+	GroupId getShareableId() {
+		return shareableId;
+	}
+
+	long getTimestamp() {
+		return timestamp;
+	}
+
+	boolean isLocal() {
+		return local;
+	}
+
+	boolean isRead() {
+		return read;
+	}
+
+	boolean isVisibleInConversation() {
+		return visible;
+	}
+
+	boolean isAvailableToAnswer() {
+		return available;
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/MessageParser.java

@@ -0,0 +1,43 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.briar.api.sharing.Shareable;
+
+@NotNullByDefault
+interface MessageParser<S extends Shareable> {
+
+	BdfDictionary getMessagesVisibleInUiQuery();
+
+	BdfDictionary getInvitesAvailableToAnswerQuery();
+
+	BdfDictionary getInvitesAvailableToAnswerQuery(GroupId shareableId);
+
+	MessageMetadata parseMetadata(BdfDictionary meta) throws FormatException;
+
+	InviteMessage<S> getInviteMessage(Transaction txn, MessageId m)
+			throws DbException, FormatException;
+
+	InviteMessage<S> parseInviteMessage(Message m, BdfList body)
+			throws FormatException;
+
+	AcceptMessage parseAcceptMessage(Message m, BdfList body)
+			throws FormatException;
+
+	DeclineMessage parseDeclineMessage(Message m, BdfList body)
+			throws FormatException;
+
+	LeaveMessage parseLeaveMessage(Message m, BdfList body)
+			throws FormatException;
+
+	AbortMessage parseAbortMessage(Message m, BdfList body)
+			throws FormatException;
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/MessageParserImpl.java

@@ -0,0 +1,139 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.briar.api.sharing.Shareable;
+
+import javax.annotation.concurrent.Immutable;
+
+import static org.briarproject.briar.sharing.MessageType.INVITE;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_AVAILABLE_TO_ANSWER;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_LOCAL;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_READ;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_SHAREABLE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_TIMESTAMP;
+import static org.briarproject.briar.sharing.SharingConstants.MSG_KEY_VISIBLE_IN_UI;
+
+@Immutable
+@NotNullByDefault
+abstract class MessageParserImpl<S extends Shareable>
+		implements MessageParser<S> {
+
+	private final ClientHelper clientHelper;
+
+	MessageParserImpl(ClientHelper clientHelper) {
+		this.clientHelper = clientHelper;
+	}
+
+	@Override
+	public BdfDictionary getMessagesVisibleInUiQuery() {
+		return BdfDictionary.of(new BdfEntry(MSG_KEY_VISIBLE_IN_UI, true));
+	}
+
+	@Override
+	public BdfDictionary getInvitesAvailableToAnswerQuery() {
+		return BdfDictionary.of(
+				new BdfEntry(MSG_KEY_AVAILABLE_TO_ANSWER, true),
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, INVITE.getValue())
+		);
+	}
+
+	@Override
+	public BdfDictionary getInvitesAvailableToAnswerQuery(GroupId shareableId) {
+		return BdfDictionary.of(
+				new BdfEntry(MSG_KEY_AVAILABLE_TO_ANSWER, true),
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, INVITE.getValue()),
+				new BdfEntry(MSG_KEY_SHAREABLE_ID, shareableId)
+		);
+	}
+
+	@Override
+	public MessageMetadata parseMetadata(BdfDictionary meta)
+			throws FormatException {
+		MessageType type = MessageType
+				.fromValue(meta.getLong(MSG_KEY_MESSAGE_TYPE).intValue());
+		GroupId shareableId = new GroupId(meta.getRaw(MSG_KEY_SHAREABLE_ID));
+		long timestamp = meta.getLong(MSG_KEY_TIMESTAMP);
+		boolean local = meta.getBoolean(MSG_KEY_LOCAL);
+		boolean read = meta.getBoolean(MSG_KEY_READ, false);
+		boolean visible = meta.getBoolean(MSG_KEY_VISIBLE_IN_UI, false);
+		boolean available = meta.getBoolean(MSG_KEY_AVAILABLE_TO_ANSWER, false);
+		return new MessageMetadata(type, shareableId, timestamp, local, read,
+				visible, available);
+	}
+
+	@Override
+	public InviteMessage<S> getInviteMessage(Transaction txn, MessageId m)
+			throws DbException, FormatException {
+		Message message = clientHelper.getMessage(txn, m);
+		if (message == null) throw new DbException();
+		BdfList body = clientHelper.toList(message);
+		return parseInviteMessage(message, body);
+	}
+
+	@Override
+	public InviteMessage<S> parseInviteMessage(Message m, BdfList body)
+			throws FormatException {
+		byte[] b = body.getOptionalRaw(1);
+		MessageId previousMessageId = (b == null ? null : new MessageId(b));
+		BdfList descriptor = body.getList(2);
+		S shareable = createShareable(descriptor);
+		String message = body.getOptionalString(3);
+		return new InviteMessage<S>(m.getId(), previousMessageId,
+				m.getGroupId(), shareable, message, m.getTimestamp());
+	}
+
+	protected abstract S createShareable(BdfList descriptor)
+			throws FormatException;
+
+	@Override
+	public AcceptMessage parseAcceptMessage(Message m, BdfList body)
+			throws FormatException {
+		GroupId shareableId = new GroupId(body.getRaw(1));
+		byte[] b = body.getOptionalRaw(2);
+		MessageId previousMessageId = (b == null ? null : new MessageId(b));
+		return new AcceptMessage(m.getId(), previousMessageId, m.getGroupId(),
+				shareableId, m.getTimestamp());
+	}
+
+	@Override
+	public DeclineMessage parseDeclineMessage(Message m, BdfList body)
+			throws FormatException {
+		GroupId shareableId = new GroupId(body.getRaw(1));
+		byte[] b = body.getOptionalRaw(2);
+		MessageId previousMessageId = (b == null ? null : new MessageId(b));
+		return new DeclineMessage(m.getId(), m.getGroupId(), shareableId,
+				m.getTimestamp(), previousMessageId);
+	}
+
+	@Override
+	public LeaveMessage parseLeaveMessage(Message m, BdfList body)
+			throws FormatException {
+		GroupId shareableId = new GroupId(body.getRaw(1));
+		byte[] b = body.getOptionalRaw(2);
+		MessageId previousMessageId = (b == null ? null : new MessageId(b));
+		return new LeaveMessage(m.getId(), m.getGroupId(), shareableId,
+				m.getTimestamp(), previousMessageId);
+	}
+
+	@Override
+	public AbortMessage parseAbortMessage(Message m, BdfList body)
+			throws FormatException {
+		GroupId shareableId = new GroupId(body.getRaw(1));
+		byte[] b = body.getOptionalRaw(2);
+		MessageId previousMessageId = (b == null ? null : new MessageId(b));
+		return new AbortMessage(m.getId(), m.getGroupId(), shareableId,
+				m.getTimestamp(), previousMessageId);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/MessageType.java

@@ -0,0 +1,29 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+enum MessageType {
+
+	INVITE(0), ACCEPT(1), DECLINE(2), LEAVE(3), ABORT(4);
+
+	private final int value;
+
+	MessageType(int value) {
+		this.value = value;
+	}
+
+	int getValue() {
+		return value;
+	}
+
+	static MessageType fromValue(int value) throws FormatException {
+		for (MessageType m : values()) if (m.value == value) return m;
+		throw new FormatException();
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/OldInvitationFactory.java

@@ -0,0 +1,12 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.sharing.SharingMessage;
+
+@Deprecated
+@NotNullByDefault
+interface OldInvitationFactory<I extends SharingMessage.Invitation, SS extends SharerSessionState>
+		extends org.briarproject.briar.api.sharing.InvitationFactory<I> {
+
+	I build(SS localState, long time);
+}

briar-core/src/main/java/org/briarproject/briar/sharing/ProtocolEngine.java

@@ -0,0 +1,39 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.sharing.Shareable;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+interface ProtocolEngine<S extends Shareable> {
+
+	Session onInviteAction(Transaction txn, Session session,
+			@Nullable String message, long timestamp) throws DbException;
+
+	Session onAcceptAction(Transaction txn, Session session) throws DbException;
+
+	Session onDeclineAction(Transaction txn, Session session)
+			throws DbException;
+
+	Session onLeaveAction(Transaction txn, Session session) throws DbException;
+
+	Session onInviteMessage(Transaction txn, Session session,
+			InviteMessage<S> m) throws DbException, FormatException;
+
+	Session onAcceptMessage(Transaction txn, Session session, AcceptMessage m)
+			throws DbException, FormatException;
+
+	Session onDeclineMessage(Transaction txn, Session session, DeclineMessage m)
+			throws DbException, FormatException;
+
+	Session onLeaveMessage(Transaction txn, Session session, LeaveMessage m)
+			throws DbException, FormatException;
+
+	Session onAbortMessage(Transaction txn, Session session, AbortMessage m)
+			throws DbException, FormatException;
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/ProtocolEngineImpl.java

@@ -0,0 +1,619 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.ClientId;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Group.Visibility;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.briar.api.client.MessageTracker;
+import org.briarproject.briar.api.client.ProtocolStateException;
+import org.briarproject.briar.api.sharing.Shareable;
+
+import java.util.Map;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
+import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
+import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
+import static org.briarproject.briar.sharing.MessageType.ABORT;
+import static org.briarproject.briar.sharing.MessageType.ACCEPT;
+import static org.briarproject.briar.sharing.MessageType.DECLINE;
+import static org.briarproject.briar.sharing.MessageType.INVITE;
+import static org.briarproject.briar.sharing.MessageType.LEAVE;
+import static org.briarproject.briar.sharing.SharingConstants.GROUP_KEY_CONTACT_ID;
+import static org.briarproject.briar.sharing.State.ERROR;
+import static org.briarproject.briar.sharing.State.LOCAL_INVITED;
+import static org.briarproject.briar.sharing.State.LOCAL_LEFT;
+import static org.briarproject.briar.sharing.State.REMOTE_HANGING;
+import static org.briarproject.briar.sharing.State.REMOTE_INVITED;
+import static org.briarproject.briar.sharing.State.REMOTE_LEFT;
+import static org.briarproject.briar.sharing.State.SHARING;
+import static org.briarproject.briar.sharing.State.START;
+
+@Immutable
+@NotNullByDefault
+abstract class ProtocolEngineImpl<S extends Shareable>
+		implements ProtocolEngine<S> {
+
+	protected final DatabaseComponent db;
+	protected final ClientHelper clientHelper;
+	protected final MessageParser<S> messageParser;
+
+	private final MessageEncoder messageEncoder;
+	private final MessageTracker messageTracker;
+	private final Clock clock;
+
+	ProtocolEngineImpl(DatabaseComponent db, ClientHelper clientHelper,
+			MessageEncoder messageEncoder, MessageParser<S> messageParser,
+			MessageTracker messageTracker, Clock clock) {
+		this.db = db;
+		this.clientHelper = clientHelper;
+		this.messageEncoder = messageEncoder;
+		this.messageParser = messageParser;
+		this.messageTracker = messageTracker;
+		this.clock = clock;
+	}
+
+	@Override
+	public Session onInviteAction(Transaction txn, Session s,
+			@Nullable String message, long timestamp) throws DbException {
+		switch (s.getState()) {
+			case START:
+			case REMOTE_LEFT:
+				return onLocalInvite(txn, s, message, timestamp);
+			case LOCAL_INVITED:
+			case REMOTE_INVITED:
+			case SHARING:
+			case LOCAL_LEFT:
+			case REMOTE_HANGING:
+			case ERROR:
+				throw new ProtocolStateException(); // Invalid in these states
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onLocalInvite(Transaction txn, Session s,
+			@Nullable String message, long timestamp) throws DbException {
+		// Send an INVITE message
+		Message sent = sendInviteMessage(txn, s, message, timestamp);
+		// Track the message
+		messageTracker.trackOutgoingMessage(txn, sent);
+		// Make the shareable visible to the contact
+		try {
+			setShareableVisibility(txn, s, VISIBLE);
+		} catch (FormatException e) {
+			throw new DbException(e); // Invalid group metadata
+		}
+		// Move to the REMOTE_INVITED state
+		return new Session(REMOTE_INVITED, s.getContactGroupId(),
+				s.getShareableId(), sent.getId(), s.getLastRemoteMessageId(),
+				sent.getTimestamp(), s.getInviteTimestamp());
+	}
+
+	private Message sendInviteMessage(Transaction txn, Session s,
+			@Nullable String message, long timestamp) throws DbException {
+		Group g = db.getGroup(txn, s.getShareableId());
+		BdfList descriptor;
+		try {
+			descriptor = clientHelper.toList(g.getDescriptor());
+		} catch (FormatException e) {
+			throw new DbException(e); // Invalid group descriptor
+		}
+		long localTimestamp = Math.max(timestamp, getLocalTimestamp(s));
+		Message m = messageEncoder
+				.encodeInviteMessage(s.getContactGroupId(), localTimestamp,
+						s.getLastLocalMessageId(), descriptor, message);
+		sendMessage(txn, m, INVITE, s.getShareableId(), true);
+		return m;
+	}
+
+	@Override
+	public Session onAcceptAction(Transaction txn, Session s)
+			throws DbException {
+		switch (s.getState()) {
+			case LOCAL_INVITED:
+				return onLocalAccept(txn, s);
+			case START:
+			case REMOTE_INVITED:
+			case SHARING:
+			case LOCAL_LEFT:
+			case REMOTE_LEFT:
+			case REMOTE_HANGING:
+			case ERROR:
+				throw new ProtocolStateException(); // Invalid in these states
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onLocalAccept(Transaction txn, Session s)
+			throws DbException {
+		// Mark the invite message unavailable to answer
+		MessageId inviteId = s.getLastRemoteMessageId();
+		if (inviteId == null) throw new IllegalStateException();
+		markMessageAvailableToAnswer(txn, inviteId, false);
+		// Send a ACCEPT message
+		Message sent = sendAcceptMessage(txn, s);
+		// Track the message
+		messageTracker.trackOutgoingMessage(txn, sent);
+		try {
+			// Add and subscribe to the shareable
+			addShareable(txn, inviteId);
+			// Share the shareable with the contact
+			setShareableVisibility(txn, s, SHARED);
+		} catch (FormatException e) {
+			throw new DbException(e); // Invalid group metadata
+		}
+		// Move to the SHARING state
+		return new Session(SHARING, s.getContactGroupId(), s.getShareableId(),
+				sent.getId(), s.getLastRemoteMessageId(), sent.getTimestamp(),
+				s.getInviteTimestamp());
+	}
+
+	protected abstract void addShareable(Transaction txn, MessageId inviteId)
+			throws DbException, FormatException;
+
+	private Message sendAcceptMessage(Transaction txn, Session session)
+			throws DbException {
+		Message m = messageEncoder.encodeAcceptMessage(
+				session.getContactGroupId(), session.getShareableId(),
+				getLocalTimestamp(session), session.getLastLocalMessageId());
+		sendMessage(txn, m, ACCEPT, session.getShareableId(), true);
+		return m;
+	}
+
+	@Override
+	public Session onDeclineAction(Transaction txn, Session s)
+			throws DbException {
+		switch (s.getState()) {
+			case LOCAL_INVITED:
+				return onLocalDecline(txn, s);
+			case START:
+			case REMOTE_INVITED:
+			case SHARING:
+			case LOCAL_LEFT:
+			case REMOTE_LEFT:
+			case REMOTE_HANGING:
+			case ERROR:
+				throw new ProtocolStateException(); // Invalid in these states
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onLocalDecline(Transaction txn, Session s)
+			throws DbException {
+		// Mark the invite message unavailable to answer
+		MessageId inviteId = s.getLastRemoteMessageId();
+		if (inviteId == null) throw new IllegalStateException();
+		markMessageAvailableToAnswer(txn, inviteId, false);
+		// Send a DECLINE message
+		Message sent = sendDeclineMessage(txn, s);
+		// Track the message
+		messageTracker.trackOutgoingMessage(txn, sent);
+		// Move to the START state
+		return new Session(START, s.getContactGroupId(), s.getShareableId(),
+				sent.getId(), s.getLastRemoteMessageId(), sent.getTimestamp(),
+				s.getInviteTimestamp());
+	}
+
+	private Message sendDeclineMessage(Transaction txn, Session session)
+			throws DbException {
+		Message m = messageEncoder.encodeDeclineMessage(
+				session.getContactGroupId(), session.getShareableId(),
+				getLocalTimestamp(session), session.getLastLocalMessageId());
+		sendMessage(txn, m, DECLINE, session.getShareableId(), true);
+		return m;
+	}
+
+	@Override
+	public Session onLeaveAction(Transaction txn, Session s)
+			throws DbException {
+		switch (s.getState()) {
+			case REMOTE_INVITED:
+				return onLocalLeave(txn, s, REMOTE_HANGING);
+			case SHARING:
+				return onLocalLeave(txn, s, LOCAL_LEFT);
+			case REMOTE_LEFT:
+				return onLocalLeave(txn, s, START);
+			case START:
+			case LOCAL_INVITED:
+			case LOCAL_LEFT:
+			case REMOTE_HANGING:
+			case ERROR:
+				return s; // Ignored in this state
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onLocalLeave(Transaction txn, Session s, State nextState)
+			throws DbException {
+		try {
+			// Stop sharing the shareable (not actually needed in REMOTE_LEFT)
+			setShareableVisibility(txn, s, INVISIBLE);
+		} catch (FormatException e) {
+			throw new DbException(e); // Invalid group metadata
+		}
+		// Send a LEAVE message
+		Message sent = sendLeaveMessage(txn, s);
+		// Move to the next state
+		return new Session(nextState, s.getContactGroupId(), s.getShareableId(),
+				sent.getId(), s.getLastRemoteMessageId(), sent.getTimestamp(),
+				s.getInviteTimestamp());
+	}
+
+	private Message sendLeaveMessage(Transaction txn, Session session)
+			throws DbException {
+		Message m = messageEncoder.encodeLeaveMessage(
+				session.getContactGroupId(), session.getShareableId(),
+				getLocalTimestamp(session), session.getLastLocalMessageId());
+		sendMessage(txn, m, LEAVE, session.getShareableId(), false);
+		return m;
+	}
+
+	@Override
+	public Session onInviteMessage(Transaction txn, Session s,
+			InviteMessage<S> m) throws DbException, FormatException {
+		switch (s.getState()) {
+			case START:
+			case LOCAL_LEFT:
+				return onRemoteInvite(txn, s, m, true, LOCAL_INVITED);
+			case REMOTE_INVITED:
+				return onRemoteInviteWhenInvited(txn, s, m);
+			case REMOTE_HANGING:
+				return onRemoteInvite(txn, s, m, false, LOCAL_LEFT);
+			case LOCAL_INVITED:
+			case SHARING:
+			case REMOTE_LEFT:
+				return abort(txn, s); // Invalid in these states
+			case ERROR:
+				return s; // Ignored in this state
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onRemoteInvite(Transaction txn, Session s,
+			InviteMessage<S> m, boolean available, State nextState)
+			throws DbException, FormatException {
+		// The timestamp must be higher than the last invite message, if any
+		if (m.getTimestamp() <= s.getInviteTimestamp()) return abort(txn, s);
+		// The dependency, if any, must be the last remote message
+		if (!isValidDependency(s, m.getPreviousMessageId()))
+			return abort(txn, s);
+		// Mark the invite message visible in the UI and (un)available to answer
+		markMessageVisibleInUi(txn, m.getId(), true);
+		markMessageAvailableToAnswer(txn, m.getId(), available);
+		// Track the message
+		messageTracker.trackMessage(txn, m.getContactGroupId(),
+				m.getTimestamp(), false);
+		// Broadcast an event
+		ContactId contactId = getContactId(txn, s.getContactGroupId());
+		txn.attach(getInvitationRequestReceivedEvent(m, contactId, available,
+				false));
+		// Move to the next state
+		return new Session(nextState, s.getContactGroupId(), s.getShareableId(),
+				s.getLastLocalMessageId(), m.getId(), s.getLocalTimestamp(),
+				m.getTimestamp());
+	}
+
+	private Session onRemoteInviteWhenInvited(Transaction txn, Session s,
+			InviteMessage<S> m) throws DbException, FormatException {
+		// The timestamp must be higher than the last invite message, if any
+		if (m.getTimestamp() <= s.getInviteTimestamp()) return abort(txn, s);
+		// The dependency, if any, must be the last remote message
+		if (!isValidDependency(s, m.getPreviousMessageId()))
+			return abort(txn, s);
+		// Mark the invite message visible in the UI and unavailable to answer
+		markMessageVisibleInUi(txn, m.getId(), true);
+		markMessageAvailableToAnswer(txn, m.getId(), false);
+		// Track the message
+		messageTracker.trackMessage(txn, m.getContactGroupId(),
+				m.getTimestamp(), false);
+		// Share the shareable with the contact
+		setShareableVisibility(txn, s, SHARED);
+		// Broadcast an event
+		ContactId contactId = getContactId(txn, s.getContactGroupId());
+		txn.attach(
+				getInvitationRequestReceivedEvent(m, contactId, false, true));
+		// Move to the next state
+		return new Session(SHARING, s.getContactGroupId(), s.getShareableId(),
+				s.getLastLocalMessageId(), m.getId(), s.getLocalTimestamp(),
+				m.getTimestamp());
+	}
+
+	abstract Event getInvitationRequestReceivedEvent(InviteMessage<S> m,
+			ContactId contactId, boolean available, boolean canBeOpened);
+
+	@Override
+	public Session onAcceptMessage(Transaction txn, Session s,
+			AcceptMessage m) throws DbException, FormatException {
+		switch (s.getState()) {
+			case REMOTE_INVITED:
+				return onRemoteAcceptWhenInvited(txn, s, m);
+			case REMOTE_HANGING:
+				return onRemoteAccept(txn, s, m, LOCAL_LEFT);
+			case START:
+			case LOCAL_INVITED:
+			case SHARING:
+			case LOCAL_LEFT:
+			case REMOTE_LEFT:
+				return abort(txn, s); // Invalid in these states
+			case ERROR:
+				return s; // Ignored in this state
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onRemoteAccept(Transaction txn, Session s, AcceptMessage m,
+			State nextState) throws DbException, FormatException {
+		// The timestamp must be higher than the last invite message
+		if (m.getTimestamp() <= s.getInviteTimestamp()) return abort(txn, s);
+		// The dependency, if any, must be the last remote message
+		if (!isValidDependency(s, m.getPreviousMessageId()))
+			return abort(txn, s);
+		// Mark the response visible in the UI
+		markMessageVisibleInUi(txn, m.getId(), true);
+		// Track the message
+		messageTracker.trackMessage(txn, m.getContactGroupId(),
+				m.getTimestamp(), false);
+		// Broadcast an event
+		ContactId contactId = getContactId(txn, m.getContactGroupId());
+		txn.attach(getInvitationResponseReceivedEvent(m, contactId));
+		// Move to the next state
+		return new Session(nextState, s.getContactGroupId(), s.getShareableId(),
+				s.getLastLocalMessageId(), m.getId(), s.getLocalTimestamp(),
+				s.getInviteTimestamp());
+	}
+
+	private Session onRemoteAcceptWhenInvited(Transaction txn, Session s,
+			AcceptMessage m) throws DbException, FormatException {
+		// Perform normal remote accept validation and operation
+		Session session = onRemoteAccept(txn, s, m, SHARING);
+		// Share the shareable with the contact
+		if (session.getState() != ERROR)
+			setShareableVisibility(txn, s, SHARED);
+		return session;
+	}
+
+	abstract Event getInvitationResponseReceivedEvent(AcceptMessage m,
+			ContactId contactId);
+
+	@Override
+	public Session onDeclineMessage(Transaction txn, Session s,
+			DeclineMessage m) throws DbException, FormatException {
+		switch (s.getState()) {
+			case REMOTE_INVITED:
+			case REMOTE_HANGING:
+				return onRemoteDecline(txn, s, m);
+			case START:
+			case LOCAL_INVITED:
+			case SHARING:
+			case LOCAL_LEFT:
+			case REMOTE_LEFT:
+				return abort(txn, s); // Invalid in these states
+			case ERROR:
+				return s; // Ignored in this state
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onRemoteDecline(Transaction txn, Session s,
+			DeclineMessage m) throws DbException, FormatException {
+		// The timestamp must be higher than the last invite message
+		if (m.getTimestamp() <= s.getInviteTimestamp()) return abort(txn, s);
+		// The dependency, if any, must be the last remote message
+		if (!isValidDependency(s, m.getPreviousMessageId()))
+			return abort(txn, s);
+		// Mark the response visible in the UI
+		markMessageVisibleInUi(txn, m.getId(), true);
+		// Track the message
+		messageTracker.trackMessage(txn, m.getContactGroupId(),
+				m.getTimestamp(), false);
+		// Make the shareable invisible (not actually needed in REMOTE_HANGING)
+		try {
+			setShareableVisibility(txn, s, INVISIBLE);
+		} catch (FormatException e) {
+			throw new DbException(e); // Invalid group metadata
+		}
+		// Broadcast an event
+		ContactId contactId = getContactId(txn, m.getContactGroupId());
+		txn.attach(getInvitationResponseReceivedEvent(m, contactId));
+		// Move to the next state
+		return new Session(START, s.getContactGroupId(), s.getShareableId(),
+				s.getLastLocalMessageId(), m.getId(), s.getLocalTimestamp(),
+				s.getInviteTimestamp());
+	}
+
+	abstract Event getInvitationResponseReceivedEvent(DeclineMessage m,
+			ContactId contactId);
+
+	@Override
+	public Session onLeaveMessage(Transaction txn, Session s,
+			LeaveMessage m) throws DbException, FormatException {
+		switch (s.getState()) {
+			case LOCAL_INVITED:
+				return onRemoteLeaveWhenInvited(txn, s, m, START);
+			case LOCAL_LEFT:
+				return onRemoteLeave(txn, s, m, START);
+			case SHARING:
+				return onRemoteLeaveWhenSharing(txn, s, m);
+			case START:
+			case REMOTE_INVITED:
+			case REMOTE_LEFT:
+			case REMOTE_HANGING:
+				return abort(txn, s); // Invalid in these states
+			case ERROR:
+				return s; // Ignored in this state
+			default:
+				throw new AssertionError();
+		}
+	}
+
+	private Session onRemoteLeaveWhenInvited(Transaction txn, Session s,
+			LeaveMessage m, State nextState)
+			throws DbException, FormatException {
+		// Carry out normal leave validation and operation
+		Session session = onRemoteLeave(txn, s, m, nextState);
+		// Mark any invite messages in the session unavailable to answer
+		if (session.getState() != ERROR)
+			markInvitesUnavailableToAnswer(txn, s);
+		// Move to the next state
+		return session;
+	}
+
+	private Session onRemoteLeave(Transaction txn, Session s,
+			LeaveMessage m, State nextState)
+			throws DbException, FormatException {
+		// The dependency, if any, must be the last remote message
+		if (!isValidDependency(s, m.getPreviousMessageId()))
+			return abort(txn, s);
+		// Move to the next state
+		return new Session(nextState, s.getContactGroupId(), s.getShareableId(),
+				s.getLastLocalMessageId(), m.getId(), s.getLocalTimestamp(),
+				s.getInviteTimestamp());
+	}
+
+	private Session onRemoteLeaveWhenSharing(Transaction txn, Session s,
+			LeaveMessage m) throws DbException, FormatException {
+		// Carry out normal leave validation and operation
+		Session session = onRemoteLeave(txn, s, m, REMOTE_LEFT);
+		// Stop sharing the shareable with the contact
+		if (session.getState() != ERROR)
+			setShareableVisibility(txn, s, INVISIBLE);
+		// Move to the next state
+		return session;
+	}
+
+	@Override
+	public Session onAbortMessage(Transaction txn, Session s, AbortMessage m)
+			throws DbException, FormatException {
+		return abort(txn, s);
+	}
+
+	private Session abort(Transaction txn, Session s)
+			throws DbException, FormatException {
+		// If the session has already been aborted, do nothing
+		if (s.getState() == ERROR) return s;
+		// Mark any invite messages in the session unavailable to answer
+		markInvitesUnavailableToAnswer(txn, s);
+		// If we subscribe, make the shareable invisible to the contact
+		if (isSubscribed(txn, s.getShareableId()))
+			setShareableVisibility(txn, s, INVISIBLE);
+		// Send an ABORT message
+		Message sent = sendAbortMessage(txn, s);
+		// Move to the ERROR state
+		return new Session(ERROR, s.getContactGroupId(), s.getShareableId(),
+				sent.getId(), s.getLastRemoteMessageId(), sent.getTimestamp(),
+				s.getInviteTimestamp());
+	}
+
+	private void markInvitesUnavailableToAnswer(Transaction txn, Session s)
+			throws DbException, FormatException {
+		GroupId shareableId = s.getShareableId();
+		BdfDictionary query =
+				messageParser.getInvitesAvailableToAnswerQuery(shareableId);
+		Map<MessageId, BdfDictionary> results =
+				clientHelper.getMessageMetadataAsDictionary(txn,
+						s.getContactGroupId(), query);
+		for (MessageId m : results.keySet())
+			markMessageAvailableToAnswer(txn, m, false);
+	}
+
+	private boolean isSubscribed(Transaction txn, GroupId g)
+			throws DbException {
+		if (!db.containsGroup(txn, g)) return false;
+		Group group = db.getGroup(txn, g);
+		return group.getClientId().equals(getClientId());
+	}
+
+	protected abstract ClientId getClientId();
+
+	private Message sendAbortMessage(Transaction txn, Session session)
+			throws DbException {
+		Message m = messageEncoder.encodeAbortMessage(
+				session.getContactGroupId(), session.getShareableId(),
+				getLocalTimestamp(session), session.getLastLocalMessageId());
+		sendMessage(txn, m, ABORT, session.getShareableId(), false);
+		return m;
+	}
+
+	private void sendMessage(Transaction txn, Message m, MessageType type,
+			GroupId shareableId, boolean visibleInConversation)
+			throws DbException {
+		BdfDictionary meta = messageEncoder.encodeMetadata(type, shareableId,
+				m.getTimestamp(), true, true, visibleInConversation, false);
+		try {
+			clientHelper.addLocalMessage(txn, m, meta, true);
+		} catch (FormatException e) {
+			throw new AssertionError(e);
+		}
+	}
+
+	private void markMessageAvailableToAnswer(Transaction txn, MessageId m,
+			boolean available) throws DbException {
+		BdfDictionary meta = new BdfDictionary();
+		messageEncoder.setAvailableToAnswer(meta, available);
+		try {
+			clientHelper.mergeMessageMetadata(txn, m, meta);
+		} catch (FormatException e) {
+			throw new AssertionError(e);
+		}
+	}
+
+	private void markMessageVisibleInUi(Transaction txn, MessageId m,
+			boolean visible) throws DbException {
+		BdfDictionary meta = new BdfDictionary();
+		messageEncoder.setVisibleInUi(meta, visible);
+		try {
+			clientHelper.mergeMessageMetadata(txn, m, meta);
+		} catch (FormatException e) {
+			throw new AssertionError(e);
+		}
+	}
+
+	private void setShareableVisibility(Transaction txn, Session session,
+			Visibility v) throws DbException, FormatException {
+		ContactId contactId = getContactId(txn, session.getContactGroupId());
+		db.setGroupVisibility(txn, contactId, session.getShareableId(), v);
+	}
+
+	private ContactId getContactId(Transaction txn, GroupId contactGroupId)
+			throws DbException, FormatException {
+		BdfDictionary meta = clientHelper.getGroupMetadataAsDictionary(txn,
+				contactGroupId);
+		return new ContactId(meta.getLong(GROUP_KEY_CONTACT_ID).intValue());
+	}
+
+	private boolean isValidDependency(Session session,
+			@Nullable MessageId dependency) {
+		MessageId expected = session.getLastRemoteMessageId();
+		if (dependency == null) return expected == null;
+		return expected != null && dependency.equals(expected);
+	}
+
+	private long getLocalTimestamp(Session session) {
+		return Math.max(clock.currentTimeMillis(),
+				Math.max(session.getLocalTimestamp(),
+						session.getInviteTimestamp()) + 1);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/Session.java

@@ -0,0 +1,69 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+import static org.briarproject.briar.sharing.State.START;
+
+@Immutable
+@NotNullByDefault
+class Session {
+
+	private final State state;
+	private final GroupId contactGroupId, shareableId;
+	@Nullable
+	private final MessageId lastLocalMessageId, lastRemoteMessageId;
+	private final long localTimestamp, inviteTimestamp;
+
+	Session(State state, GroupId contactGroupId, GroupId shareableId,
+			@Nullable MessageId lastLocalMessageId,
+			@Nullable MessageId lastRemoteMessageId, long localTimestamp,
+			long inviteTimestamp) {
+		this.state = state;
+		this.contactGroupId = contactGroupId;
+		this.shareableId = shareableId;
+		this.lastLocalMessageId = lastLocalMessageId;
+		this.lastRemoteMessageId = lastRemoteMessageId;
+		this.localTimestamp = localTimestamp;
+		this.inviteTimestamp = inviteTimestamp;
+	}
+
+	Session(GroupId contactGroupId, GroupId shareableId) {
+		this(START, contactGroupId, shareableId, null, null, 0, 0);
+	}
+
+	public State getState() {
+		return state;
+	}
+
+	GroupId getContactGroupId() {
+		return contactGroupId;
+	}
+
+	GroupId getShareableId() {
+		return shareableId;
+	}
+
+	@Nullable
+	MessageId getLastLocalMessageId() {
+		return lastLocalMessageId;
+	}
+
+	@Nullable
+	MessageId getLastRemoteMessageId() {
+		return lastRemoteMessageId;
+	}
+
+	long getLocalTimestamp() {
+		return localTimestamp;
+	}
+
+	long getInviteTimestamp() {
+		return inviteTimestamp;
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/SessionEncoder.java

@@ -0,0 +1,11 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+interface SessionEncoder {
+
+	BdfDictionary encodeSession(Session s);
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/SessionEncoderImpl.java

@@ -0,0 +1,46 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.data.BdfDictionary.NULL_VALUE;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_INVITE_TIMESTAMP;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_LAST_LOCAL_MESSAGE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_LAST_REMOTE_MESSAGE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_LOCAL_TIMESTAMP;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_SESSION_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_SHAREABLE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_STATE;
+
+@Immutable
+@NotNullByDefault
+class SessionEncoderImpl implements SessionEncoder {
+
+	@Inject
+	SessionEncoderImpl() {
+	}
+
+	@Override
+	public BdfDictionary encodeSession(Session s) {
+		BdfDictionary d = new BdfDictionary();
+		d.put(SESSION_KEY_SESSION_ID, s.getShareableId());
+		d.put(SESSION_KEY_SHAREABLE_ID, s.getShareableId());
+		MessageId lastLocalMessageId = s.getLastLocalMessageId();
+		if (lastLocalMessageId == null)
+			d.put(SESSION_KEY_LAST_LOCAL_MESSAGE_ID, NULL_VALUE);
+		else d.put(SESSION_KEY_LAST_LOCAL_MESSAGE_ID, lastLocalMessageId);
+		MessageId lastRemoteMessageId = s.getLastRemoteMessageId();
+		if (lastRemoteMessageId == null)
+			d.put(SESSION_KEY_LAST_REMOTE_MESSAGE_ID, NULL_VALUE);
+		else d.put(SESSION_KEY_LAST_REMOTE_MESSAGE_ID, lastRemoteMessageId);
+		d.put(SESSION_KEY_LOCAL_TIMESTAMP, s.getLocalTimestamp());
+		d.put(SESSION_KEY_INVITE_TIMESTAMP, s.getInviteTimestamp());
+		d.put(SESSION_KEY_STATE, s.getState().getValue());
+		return d;
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/SessionParser.java

@@ -0,0 +1,17 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.briar.api.client.SessionId;
+
+@NotNullByDefault
+interface SessionParser {
+
+	BdfDictionary getSessionQuery(SessionId s);
+
+	Session parseSession(GroupId contactGroupId, BdfDictionary d)
+			throws FormatException;
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/SessionParserImpl.java

@@ -0,0 +1,75 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.briar.api.client.SessionId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_INVITE_TIMESTAMP;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_LAST_LOCAL_MESSAGE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_LAST_REMOTE_MESSAGE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_LOCAL_TIMESTAMP;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_SESSION_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_SHAREABLE_ID;
+import static org.briarproject.briar.sharing.SharingConstants.SESSION_KEY_STATE;
+
+@Immutable
+@NotNullByDefault
+class SessionParserImpl implements SessionParser {
+
+	@Inject
+	SessionParserImpl() {
+	}
+
+	@Override
+	public BdfDictionary getSessionQuery(SessionId s) {
+		return BdfDictionary.of(new BdfEntry(SESSION_KEY_SESSION_ID, s));
+	}
+
+	@Override
+	public Session parseSession(GroupId contactGroupId,
+			BdfDictionary d) throws FormatException {
+		return new Session(State.fromValue(getState(d)), contactGroupId,
+				getShareableId(d), getLastLocalMessageId(d),
+				getLastRemoteMessageId(d), getLocalTimestamp(d),
+				getInviteTimestamp(d));
+	}
+
+	private int getState(BdfDictionary d) throws FormatException {
+		return d.getLong(SESSION_KEY_STATE).intValue();
+	}
+
+	private GroupId getShareableId(BdfDictionary d) throws FormatException {
+		return new GroupId(d.getRaw(SESSION_KEY_SHAREABLE_ID));
+	}
+
+	@Nullable
+	private MessageId getLastLocalMessageId(BdfDictionary d)
+			throws FormatException {
+		byte[] b = d.getOptionalRaw(SESSION_KEY_LAST_LOCAL_MESSAGE_ID);
+		return b == null ? null : new MessageId(b);
+	}
+
+	@Nullable
+	private MessageId getLastRemoteMessageId(BdfDictionary d)
+			throws FormatException {
+		byte[] b = d.getOptionalRaw(SESSION_KEY_LAST_REMOTE_MESSAGE_ID);
+		return b == null ? null : new MessageId(b);
+	}
+
+	private long getLocalTimestamp(BdfDictionary d) throws FormatException {
+		return d.getLong(SESSION_KEY_LOCAL_TIMESTAMP);
+	}
+
+	private long getInviteTimestamp(BdfDictionary d) throws FormatException {
+		return d.getLong(SESSION_KEY_INVITE_TIMESTAMP);
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/ShareableFactory.java

@@ -9,6 +9,7 @@ import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.briar.api.sharing.Shareable;
 import org.briarproject.briar.api.sharing.SharingMessage;
 
+@Deprecated
 @NotNullByDefault
 interface ShareableFactory<S extends Shareable, I extends SharingMessage.Invitation, IS extends InviteeSessionState, SS extends SharerSessionState> {
 

briar-core/src/main/java/org/briarproject/briar/sharing/SharerEngine.java

@@ -29,6 +29,7 @@ import static org.briarproject.briar.api.sharing.SharingMessage.SimpleMessage;
 import static org.briarproject.briar.sharing.SharerSessionState.Action.REMOTE_ACCEPT;
 import static org.briarproject.briar.sharing.SharerSessionState.Action.REMOTE_DECLINE;
 
+@Deprecated
 @Immutable
 @NotNullByDefault
 class SharerEngine<I extends Invitation, SS extends SharerSessionState, IRR extends InvitationResponseReceivedEvent>
@@ -37,12 +38,12 @@ class SharerEngine<I extends Invitation, SS extends SharerSessionState, IRR exte
 	private static final Logger LOG =
 			Logger.getLogger(SharerEngine.class.getName());
 
-	private final InvitationFactory<I, SS> invitationFactory;
+	private final OldInvitationFactory<I, SS> invitationFactory;
 	private final InvitationResponseReceivedEventFactory<SS, IRR>
 			invitationResponseReceivedEventFactory;
 	private final Clock clock;
 
-	SharerEngine(InvitationFactory<I, SS> invitationFactory,
+	SharerEngine(OldInvitationFactory<I, SS> invitationFactory,
 			InvitationResponseReceivedEventFactory<SS, IRR> invitationResponseReceivedEventFactory,
 			Clock clock) {
 		this.invitationFactory = invitationFactory;

briar-core/src/main/java/org/briarproject/briar/sharing/SharerSessionState.java

@@ -23,6 +23,7 @@ import static org.briarproject.briar.sharing.SharerSessionState.Action.REMOTE_AC
 import static org.briarproject.briar.sharing.SharerSessionState.Action.REMOTE_DECLINE;
 import static org.briarproject.briar.sharing.SharerSessionState.Action.REMOTE_LEAVE;
 
+@Deprecated
 @NotThreadSafe
 @NotNullByDefault
 public abstract class SharerSessionState extends SharingSessionState {

briar-core/src/main/java/org/briarproject/briar/sharing/SharerSessionStateFactory.java

@@ -9,6 +9,7 @@ import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.briar.api.client.SessionId;
 import org.briarproject.briar.api.sharing.Shareable;
 
+@Deprecated
 @NotNullByDefault
 interface SharerSessionStateFactory<S extends Shareable, SS extends SharerSessionState> {
 

briar-core/src/main/java/org/briarproject/briar/sharing/SharingConstants.java

@@ -0,0 +1,28 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.briar.client.MessageTrackerConstants;
+
+interface SharingConstants {
+
+	// Group metadata keys
+	String GROUP_KEY_CONTACT_ID = "contactId";
+
+	// Message metadata keys
+	String MSG_KEY_MESSAGE_TYPE = "messageType";
+	String MSG_KEY_SHAREABLE_ID = "shareableId";
+	String MSG_KEY_TIMESTAMP = "timestamp";
+	String MSG_KEY_READ = MessageTrackerConstants.MSG_KEY_READ;
+	String MSG_KEY_LOCAL = "local";
+	String MSG_KEY_VISIBLE_IN_UI = "visibleInUi";
+	String MSG_KEY_AVAILABLE_TO_ANSWER = "availableToAnswer";
+
+	// Session keys
+	String SESSION_KEY_STATE = "state";
+	String SESSION_KEY_SESSION_ID = "sessionId";
+	String SESSION_KEY_SHAREABLE_ID = "shareableId";
+	String SESSION_KEY_LAST_LOCAL_MESSAGE_ID = "lastLocalMessageId";
+	String SESSION_KEY_LAST_REMOTE_MESSAGE_ID = "lastRemoteMessageId";
+	String SESSION_KEY_LOCAL_TIMESTAMP = "localTimestamp";
+	String SESSION_KEY_INVITE_TIMESTAMP = "inviteTimestamp";
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/SharingMessage.java

@@ -0,0 +1,50 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.MessageId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+abstract class SharingMessage {
+
+	private final MessageId id;
+	private final GroupId contactGroupId, shareableId;
+	private final long timestamp;
+	@Nullable
+	private final MessageId previousMessageId;
+
+	SharingMessage(MessageId id, GroupId contactGroupId, GroupId shareableId,
+			long timestamp, @Nullable MessageId previousMessageId) {
+		this.id = id;
+		this.previousMessageId = previousMessageId;
+		this.contactGroupId = contactGroupId;
+		this.shareableId = shareableId;
+		this.timestamp = timestamp;
+	}
+
+	MessageId getId() {
+		return id;
+	}
+
+	GroupId getContactGroupId() {
+		return contactGroupId;
+	}
+
+	GroupId getShareableId() {
+		return shareableId;
+	}
+
+	long getTimestamp() {
+		return timestamp;
+	}
+
+	@Nullable
+	public MessageId getPreviousMessageId() {
+		return previousMessageId;
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/SharingModule.java

@@ -4,10 +4,13 @@ import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.contact.ContactManager;
 import org.briarproject.bramble.api.data.MetadataEncoder;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
+import org.briarproject.bramble.api.sync.ValidationManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.briar.api.blog.BlogManager;
 import org.briarproject.briar.api.blog.BlogSharingManager;
 import org.briarproject.briar.api.client.MessageQueueManager;
+import org.briarproject.briar.api.forum.Forum;
+import org.briarproject.briar.api.forum.ForumFactory;
 import org.briarproject.briar.api.forum.ForumManager;
 import org.briarproject.briar.api.forum.ForumSharingManager;
 import org.briarproject.briar.api.messaging.ConversationManager;
@@ -68,14 +71,15 @@ public class SharingModule {
 	@Provides
 	@Singleton
 	ForumSharingValidator provideForumSharingValidator(
-			MessageQueueManager messageQueueManager, ClientHelper clientHelper,
-			MetadataEncoder metadataEncoder, Clock clock) {
-
+			ValidationManager validationManager, MessageEncoder messageEncoder,
+			ClientHelper clientHelper, MetadataEncoder metadataEncoder,
+			Clock clock, ForumFactory forumFactory) {
 		ForumSharingValidator validator =
-				new ForumSharingValidator(clientHelper, metadataEncoder, clock);
-		messageQueueManager.registerMessageValidator(
-				ForumSharingManager.CLIENT_ID, validator);
-
+				new ForumSharingValidator(messageEncoder, clientHelper,
+						metadataEncoder, clock, forumFactory);
+		validationManager
+				.registerMessageValidator(ForumSharingManager.CLIENT_ID,
+						validator);
 		return validator;
 	}
 
@@ -83,14 +87,14 @@ public class SharingModule {
 	@Singleton
 	ForumSharingManager provideForumSharingManager(
 			LifecycleManager lifecycleManager, ContactManager contactManager,
-			MessageQueueManager messageQueueManager,
+			ValidationManager validationManager,
 			ConversationManager conversationManager, ForumManager forumManager,
 			ForumSharingManagerImpl forumSharingManager) {
 
 		lifecycleManager.registerClient(forumSharingManager);
 		contactManager.registerAddContactHook(forumSharingManager);
 		contactManager.registerRemoveContactHook(forumSharingManager);
-		messageQueueManager.registerIncomingMessageHook(
+		validationManager.registerIncomingMessageHook(
 				ForumSharingManager.CLIENT_ID, forumSharingManager);
 		conversationManager.registerConversationClient(forumSharingManager);
 		forumManager.registerRemoveForumHook(forumSharingManager);
@@ -98,4 +102,37 @@ public class SharingModule {
 		return forumSharingManager;
 	}
 
+	@Provides
+	MessageEncoder provideMessageEncoder(MessageEncoderImpl messageEncoder) {
+		return messageEncoder;
+	}
+
+	@Provides
+	MessageParser<Forum> provideForumMessageParser(
+			ForumMessageParserImpl forumMessageParser) {
+		return forumMessageParser;
+	}
+
+	@Provides
+	SessionEncoder provideSessionEncoder(SessionEncoderImpl sessionEncoder) {
+		return sessionEncoder;
+	}
+
+	@Provides
+	SessionParser provideSessionParser(SessionParserImpl sessionParser) {
+		return sessionParser;
+	}
+
+	@Provides
+	ProtocolEngine<Forum> provideForumProtocolEngine(
+			ForumProtocolEngineImpl forumProtocolEngine) {
+		return forumProtocolEngine;
+	}
+
+	@Provides
+	InvitationFactory<Forum> provideForumInvitationFactory(
+			ForumInvitationFactoryImpl forumInvitationFactory) {
+		return forumInvitationFactory;
+	}
+
 }

briar-core/src/main/java/org/briarproject/briar/sharing/SharingSessionState.java

@@ -18,6 +18,7 @@ import static org.briarproject.briar.api.sharing.SharingConstants.SHAREABLE_ID;
 import static org.briarproject.briar.api.sharing.SharingConstants.STATE;
 import static org.briarproject.briar.api.sharing.SharingConstants.STORAGE_ID;
 
+@Deprecated
 @NotThreadSafe
 @NotNullByDefault
 abstract class SharingSessionState {

briar-core/src/main/java/org/briarproject/briar/sharing/SharingValidator.java

@@ -0,0 +1,101 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.UniqueId;
+import org.briarproject.bramble.api.client.BdfMessageContext;
+import org.briarproject.bramble.api.client.BdfMessageValidator;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataEncoder;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.system.Clock;
+
+import java.util.Collections;
+
+import javax.annotation.concurrent.Immutable;
+
+import static org.briarproject.bramble.util.ValidationUtils.checkLength;
+import static org.briarproject.bramble.util.ValidationUtils.checkSize;
+import static org.briarproject.briar.api.sharing.SharingConstants.MAX_INVITATION_MESSAGE_LENGTH;
+import static org.briarproject.briar.sharing.MessageType.INVITE;
+
+@Immutable
+@NotNullByDefault
+abstract class SharingValidator extends BdfMessageValidator {
+
+	private final MessageEncoder messageEncoder;
+
+	SharingValidator(MessageEncoder messageEncoder, ClientHelper clientHelper,
+			MetadataEncoder metadataEncoder, Clock clock) {
+		super(clientHelper, metadataEncoder, clock);
+		this.messageEncoder = messageEncoder;
+	}
+
+	@Override
+	protected BdfMessageContext validateMessage(Message m, Group g,
+			BdfList body) throws FormatException {
+		MessageType type = MessageType.fromValue(body.getLong(0).intValue());
+		switch (type) {
+			case INVITE:
+				return validateInviteMessage(m, body);
+			case ACCEPT:
+			case DECLINE:
+			case LEAVE:
+			case ABORT:
+				return validateNonInviteMessage(type, m, body);
+			default:
+				throw new FormatException();
+		}
+	}
+
+	private BdfMessageContext validateInviteMessage(Message m, BdfList body)
+			throws FormatException {
+		checkSize(body, 4);
+		byte[] previousMessageId = body.getOptionalRaw(1);
+		checkLength(previousMessageId, UniqueId.LENGTH);
+		BdfList descriptor = body.getList(2);
+		GroupId shareableId = validateDescriptor(descriptor);
+		String msg = body.getOptionalString(3);
+		checkLength(msg, 1, MAX_INVITATION_MESSAGE_LENGTH);
+
+		BdfDictionary meta = messageEncoder
+				.encodeMetadata(INVITE, shareableId, m.getTimestamp(), false,
+						false, false, false);
+		if (previousMessageId == null) {
+			return new BdfMessageContext(meta);
+		} else {
+			MessageId dependency = new MessageId(previousMessageId);
+			return new BdfMessageContext(meta,
+					Collections.singletonList(dependency));
+		}
+	}
+
+	protected abstract GroupId validateDescriptor(BdfList descriptor)
+			throws FormatException;
+
+	private BdfMessageContext validateNonInviteMessage(MessageType type,
+			Message m, BdfList body) throws FormatException {
+		checkSize(body, 3);
+		byte[] shareableId = body.getRaw(1);
+		checkLength(shareableId, UniqueId.LENGTH);
+		byte[] previousMessageId = body.getOptionalRaw(2);
+		checkLength(previousMessageId, UniqueId.LENGTH);
+
+		BdfDictionary meta = messageEncoder
+				.encodeMetadata(type, new GroupId(shareableId),
+						m.getTimestamp(), false, false, false, false);
+		if (previousMessageId == null) {
+			return new BdfMessageContext(meta);
+		} else {
+			MessageId dependency = new MessageId(previousMessageId);
+			return new BdfMessageContext(meta,
+					Collections.singletonList(dependency));
+		}
+	}
+
+}

briar-core/src/main/java/org/briarproject/briar/sharing/State.java

@@ -0,0 +1,34 @@
+package org.briarproject.briar.sharing;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+enum State {
+
+	START(0), LOCAL_INVITED(1), REMOTE_INVITED(2), SHARING(3), LOCAL_LEFT(4),
+	REMOTE_LEFT(5),	REMOTE_HANGING(6), ERROR(7);
+
+	private final int value;
+
+	State(int value) {
+		this.value = value;
+	}
+
+	public int getValue() {
+		return value;
+	}
+
+	public boolean canInvite() {
+		return this == START || this == REMOTE_LEFT;
+	}
+
+	static State fromValue(int value) throws FormatException {
+		for (State s : values()) if (s.value == value) return s;
+		throw new FormatException();
+	}
+
+}

briar-core/src/test/java/org/briarproject/briar/forum/ForumManagerTest.java

@@ -45,7 +45,8 @@ public class ForumManagerTest
 		forum0 = forumManager0.addForum("Test Forum");
 		groupId0 = forum0.getId();
 		// share forum
-		forumSharingManager0.sendInvitation(groupId0, contactId1From0, null);
+		forumSharingManager0.sendInvitation(groupId0, contactId1From0, null,
+				clock.currentTimeMillis());
 		sync0To1(1, true);
 		forumSharingManager1.respondToInvitation(forum0, contact0From1, true);
 		sync1To0(1, true);
@@ -189,7 +190,8 @@ public class ForumManagerTest
 		// share a second forum
 		Forum forum1 = forumManager0.addForum("Test Forum1");
 		GroupId g1 = forum1.getId();
-		forumSharingManager0.sendInvitation(g1, contactId1From0, null);
+		forumSharingManager0.sendInvitation(g1, contactId1From0, null,
+				clock.currentTimeMillis());
 		sync0To1(1, true);
 		forumSharingManager1.respondToInvitation(forum1, contact0From1, true);
 		sync1To0(1, true);

briar-core/src/test/java/org/briarproject/briar/privategroup/invitation/GroupInvitationIntegrationTest.java

@@ -12,6 +12,7 @@ import org.briarproject.briar.api.privategroup.invitation.GroupInvitationManager
 import org.briarproject.briar.api.privategroup.invitation.GroupInvitationRequest;
 import org.briarproject.briar.api.privategroup.invitation.GroupInvitationResponse;
 import org.briarproject.briar.api.sharing.InvitationMessage;
+import org.briarproject.briar.api.sharing.InvitationResponse;
 import org.briarproject.briar.test.BriarIntegrationTest;
 import org.briarproject.briar.test.BriarIntegrationTestComponent;
 import org.briarproject.briar.test.DaggerBriarIntegrationTestComponent;
@@ -97,10 +98,10 @@ public class GroupInvitationIntegrationTest
 		GroupInvitationRequest request =
 				(GroupInvitationRequest) messages.iterator().next();
 		assertEquals(msg, request.getMessage());
-		assertEquals(author0, request.getCreator());
+		assertEquals(author0, request.getShareable().getCreator());
 		assertEquals(timestamp, request.getTimestamp());
 		assertEquals(contactId0From1, request.getContactId());
-		assertEquals(privateGroup0.getName(), request.getGroupName());
+		assertEquals(privateGroup0.getName(), request.getShareable().getName());
 		assertFalse(request.isLocal());
 		assertFalse(request.isRead());
 	}
@@ -123,7 +124,7 @@ public class GroupInvitationIntegrationTest
 		for (InvitationMessage m : messages) {
 			if (m instanceof GroupInvitationResponse) {
 				foundResponse = true;
-				GroupInvitationResponse response = (GroupInvitationResponse) m;
+				InvitationResponse response = (GroupInvitationResponse) m;
 				assertEquals(contactId0From1, response.getContactId());
 				assertTrue(response.isLocal());
 				assertFalse(response.wasAccepted());
@@ -140,7 +141,7 @@ public class GroupInvitationIntegrationTest
 		for (InvitationMessage m : messages) {
 			if (m instanceof GroupInvitationResponse) {
 				foundResponse = true;
-				GroupInvitationResponse response = (GroupInvitationResponse) m;
+				InvitationResponse response = (GroupInvitationResponse) m;
 				assertEquals(contactId0From1, response.getContactId());
 				assertFalse(response.isLocal());
 				assertFalse(response.wasAccepted());
@@ -172,7 +173,7 @@ public class GroupInvitationIntegrationTest
 		for (InvitationMessage m : messages) {
 			if (m instanceof GroupInvitationResponse) {
 				foundResponse = true;
-				GroupInvitationResponse response = (GroupInvitationResponse) m;
+				InvitationResponse response = (GroupInvitationResponse) m;
 				assertTrue(response.wasAccepted());
 			}
 		}
@@ -187,7 +188,7 @@ public class GroupInvitationIntegrationTest
 		for (InvitationMessage m : messages) {
 			if (m instanceof GroupInvitationResponse) {
 				foundResponse = true;
-				GroupInvitationResponse response = (GroupInvitationResponse) m;
+				InvitationResponse response = (GroupInvitationResponse) m;
 				assertTrue(response.wasAccepted());
 			}
 		}

briar-core/src/test/java/org/briarproject/briar/privategroup/invitation/GroupInvitationManagerImplTest.java

@@ -90,8 +90,6 @@ public class GroupInvitationManagerImplTest extends BrambleMockTestCase {
 
 	private final GroupInvitationManagerImpl groupInvitationManager;
 
-	private final Group localGroup =
-			new Group(new GroupId(getRandomId()), CLIENT_ID, getRandomBytes(5));
 	private final Transaction txn = new Transaction(null, false);
 	private final ContactId contactId = new ContactId(0);
 	private final Author author =
@@ -141,8 +139,6 @@ public class GroupInvitationManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(inviteeEngine));
 			oneOf(engineFactory).createPeerEngine();
 			will(returnValue(peerEngine));
-			oneOf(contactGroupFactory).createLocalGroup(CLIENT_ID);
-			will(returnValue(localGroup));
 		}});
 		MetadataParser metadataParser = context.mock(MetadataParser.class);
 		MessageTracker messageTracker = context.mock(MessageTracker.class);
@@ -156,7 +152,6 @@ public class GroupInvitationManagerImplTest extends BrambleMockTestCase {
 	@Test
 	public void testCreateLocalState() throws Exception {
 		context.checking(new Expectations() {{
-			oneOf(db).addGroup(txn, localGroup);
 			oneOf(db).getContacts(txn);
 			will(returnValue(Collections.singletonList(contact)));
 		}});
@@ -651,6 +646,9 @@ public class GroupInvitationManagerImplTest extends BrambleMockTestCase {
 				new InviteMessage(message.getId(), contactGroup.getId(),
 						privateGroup.getId(), time1, "name", author,
 						new byte[0], null, new byte[0]);
+		final PrivateGroup pg =
+				new PrivateGroup(privateGroup, invite.getGroupName(),
+						invite.getCreator(), invite.getSalt());
 
 		context.checking(new Expectations() {{
 			oneOf(db).startTransaction(true);
@@ -668,12 +666,11 @@ public class GroupInvitationManagerImplTest extends BrambleMockTestCase {
 			oneOf(messageParser).parseMetadata(meta);
 			will(returnValue(messageMetadata1));
 			oneOf(db).getMessageStatus(txn, contactId, message.getId());
-			oneOf(clientHelper).getMessage(txn, message.getId());
-			will(returnValue(message));
-			oneOf(clientHelper).toList(message);
-			will(returnValue(body));
-			oneOf(messageParser).parseInviteMessage(message, body);
+			oneOf(messageParser).getInviteMessage(txn, message.getId());
 			will(returnValue(invite));
+			oneOf(privateGroupFactory).createPrivateGroup(invite.getGroupName(),
+					invite.getCreator(), invite.getSalt());
+			will(returnValue(pg));
 			oneOf(db).containsGroup(txn, privateGroup.getId());
 			will(returnValue(true));
 			// second message
@@ -742,21 +739,13 @@ public class GroupInvitationManagerImplTest extends BrambleMockTestCase {
 					contactGroup.getId(), query);
 			will(returnValue(results));
 			// message 1
-			oneOf(clientHelper).getMessage(txn, message.getId());
-			will(returnValue(message));
-			oneOf(clientHelper).toList(message);
-			will(returnValue(body));
-			oneOf(messageParser).parseInviteMessage(message, body);
+			oneOf(messageParser).getInviteMessage(txn, message.getId());
 			will(returnValue(inviteMessage1));
 			oneOf(privateGroupFactory).createPrivateGroup(groupName, author,
 					salt);
 			will(returnValue(pg));
 			// message 2
-			oneOf(clientHelper).getMessage(txn, messageId2);
-			will(returnValue(message2));
-			oneOf(clientHelper).toList(message2);
-			will(returnValue(body2));
-			oneOf(messageParser).parseInviteMessage(message2, body2);
+			oneOf(messageParser).getInviteMessage(txn, messageId2);
 			will(returnValue(inviteMessage2));
 			oneOf(privateGroupFactory).createPrivateGroup(groupName, author,
 					salt);

briar-core/src/test/java/org/briarproject/briar/privategroup/invitation/InviteeProtocolEngineTest.java

@@ -144,11 +144,7 @@ public class InviteeProtocolEngineTest extends AbstractProtocolEngineTest {
 		expectSendJoinMessage(properJoinMessage, true);
 		context.checking(new Expectations() {{
 			oneOf(messageTracker).trackOutgoingMessage(txn, message);
-			oneOf(clientHelper).getMessage(txn, lastRemoteMessageId);
-			will(returnValue(inviteMsg));
-			oneOf(clientHelper).toList(inviteMsg);
-			will(returnValue(inviteList));
-			oneOf(messageParser).parseInviteMessage(inviteMsg, inviteList);
+			oneOf(messageParser).getInviteMessage(txn, lastRemoteMessageId);
 			will(returnValue(inviteMessage));
 			oneOf(privateGroupFactory)
 					.createPrivateGroup(inviteMessage.getGroupName(),

briar-core/src/test/java/org/briarproject/briar/sharing/BlogSharingIntegrationTest.java

@@ -107,7 +107,8 @@ public class BlogSharingIntegrationTest
 
 		// create invitation
 		blogSharingManager0
-				.sendInvitation(blog1.getId(), contactId1From0, "Hi!");
+				.sendInvitation(blog1.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync invitation
 		sync0To1(1, false);
@@ -122,7 +123,8 @@ public class BlogSharingIntegrationTest
 
 		// send invitation
 		blogSharingManager0
-				.sendInvitation(blog2.getId(), contactId1From0, "Hi!");
+				.sendInvitation(blog2.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// invitee has own blog and that of the sharer
 		assertEquals(2, blogManager1.getBlogs().size());
@@ -194,7 +196,8 @@ public class BlogSharingIntegrationTest
 
 		// send invitation
 		blogSharingManager0
-				.sendInvitation(blog2.getId(), contactId1From0, null);
+				.sendInvitation(blog2.getId(), contactId1From0, null,
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -251,7 +254,8 @@ public class BlogSharingIntegrationTest
 
 		// send invitation
 		blogSharingManager0
-				.sendInvitation(blog2.getId(), contactId1From0, "Hi!");
+				.sendInvitation(blog2.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -313,7 +317,8 @@ public class BlogSharingIntegrationTest
 
 		// sharer sends invitation for 2's blog to 1
 		blogSharingManager0
-				.sendInvitation(blog2.getId(), contactId1From0, "Hi!");
+				.sendInvitation(blog2.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -349,7 +354,8 @@ public class BlogSharingIntegrationTest
 
 		// send invitation
 		blogSharingManager0
-				.sendInvitation(blog2.getId(), contactId1From0, "Hi!");
+				.sendInvitation(blog2.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -399,7 +405,8 @@ public class BlogSharingIntegrationTest
 
 		// sharer sends invitation for 2's blog to 1
 		blogSharingManager0
-				.sendInvitation(blog2.getId(), contactId1From0, "Hi!");
+				.sendInvitation(blog2.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);

briar-core/src/test/java/org/briarproject/briar/sharing/ForumSharingIntegrationTest.java

@@ -2,19 +2,14 @@ package org.briarproject.briar.sharing;
 
 import net.jodah.concurrentunit.Waiter;
 
-import org.briarproject.bramble.api.Bytes;
 import org.briarproject.bramble.api.contact.Contact;
-import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.db.DbException;
-import org.briarproject.bramble.api.db.Metadata;
 import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.event.EventListener;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.sync.Group;
 import org.briarproject.bramble.test.TestDatabaseModule;
-import org.briarproject.briar.api.client.MessageQueueManager;
-import org.briarproject.briar.api.client.SessionId;
+import org.briarproject.briar.api.client.ProtocolStateException;
 import org.briarproject.briar.api.forum.Forum;
 import org.briarproject.briar.api.forum.ForumInvitationRequest;
 import org.briarproject.briar.api.forum.ForumInvitationResponse;
@@ -39,11 +34,7 @@ import java.util.Collection;
 import java.util.List;
 
 import static junit.framework.Assert.assertNotNull;
-import static org.briarproject.bramble.test.TestUtils.getRandomBytes;
 import static org.briarproject.bramble.test.TestUtils.getRandomString;
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT_LENGTH;
-import static org.briarproject.briar.api.forum.ForumSharingManager.CLIENT_ID;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_INVITATION;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
@@ -115,7 +106,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -171,7 +163,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, null);
+				.sendInvitation(forum0.getId(), contactId1From0, null,
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -227,7 +220,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -245,9 +239,8 @@ public class ForumSharingIntegrationTest
 		assertTrue(forumManager1.getForums().contains(forum0));
 
 		// sharer shares forum with invitee
-		Contact c1 = contactManager0.getContact(contactId1From0);
 		assertTrue(forumSharingManager0.getSharedWith(forum0.getId())
-				.contains(c1));
+				.contains(contact1From0));
 		// invitee gets forum shared by sharer
 		Contact contact0 = contactManager1.getContact(contactId1From0);
 		assertTrue(forumSharingManager1.getSharedWith(forum0.getId())
@@ -265,14 +258,16 @@ public class ForumSharingIntegrationTest
 
 		// sharer no longer shares forum with invitee
 		assertFalse(forumSharingManager0.getSharedWith(forum0.getId())
-				.contains(c1));
+				.contains(contact1From0));
 		// invitee no longer gets forum shared by sharer
 		assertFalse(forumSharingManager1.getSharedWith(forum0.getId())
 				.contains(contact0));
-		// forum can be shared again
-		assertTrue(forumSharingManager0.canBeShared(forum0.getId(), c1));
-		Contact c0 = contactManager1.getContact(contactId0From1);
-		assertTrue(forumSharingManager1.canBeShared(forum0.getId(), c0));
+		// forum can be shared again by sharer
+		assertTrue(forumSharingManager0
+				.canBeShared(forum0.getId(), contact1From0));
+		// invitee that left can not share again
+		assertFalse(forumSharingManager1
+				.canBeShared(forum0.getId(), contact0From1));
 	}
 
 	@Test
@@ -282,7 +277,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, null);
+				.sendInvitation(forum0.getId(), contactId1From0, null,
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -336,7 +332,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, null);
+				.sendInvitation(forum0.getId(), contactId1From0, null,
+						clock.currentTimeMillis());
 
 		// sharer un-subscribes from forum
 		forumManager0.removeForum(forum0);
@@ -360,7 +357,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, null);
+				.sendInvitation(forum0.getId(), contactId1From0, null,
+						clock.currentTimeMillis());
 
 		// sharer un-subscribes from forum
 		forumManager0.removeForum(forum0);
@@ -375,73 +373,15 @@ public class ForumSharingIntegrationTest
 		assertEquals(1, forumManager1.getForums().size());
 	}
 
-	@Test
-	public void testSessionIdReuse() throws Exception {
-		// initialize and let invitee accept all requests
-		listenToEvents(true);
-
-		// send invitation
-		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
-
-		// sync first request message
-		sync0To1(1, true);
-		eventWaiter.await(TIMEOUT, 1);
-		assertTrue(listener1.requestReceived);
-
-		// sync response back
-		sync1To0(1, true);
-		eventWaiter.await(TIMEOUT, 1);
-		assertTrue(listener0.responseReceived);
-
-		// forum was added successfully
-		assertEquals(1, forumManager1.getForums().size());
-
-		// reset event received state
-		listener1.requestReceived = false;
-
-		// get SessionId from invitation
-		List<InvitationMessage> list = new ArrayList<InvitationMessage>(
-				forumSharingManager1
-						.getInvitationMessages(contactId0From1));
-		assertEquals(2, list.size());
-		InvitationMessage msg = list.get(0);
-		SessionId sessionId = msg.getSessionId();
-		assertEquals(sessionId, list.get(1).getSessionId());
-
-		// get all sorts of stuff needed to send a message
-		MessageQueueManager queue = c0.getMessageQueueManager();
-		Contact c1 = contactManager0.getContact(contactId1From0);
-		Group group = contactGroupFactory.createContactGroup(CLIENT_ID, c1);
-		long time = clock.currentTimeMillis();
-		BdfList bodyList =
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId.getBytes(),
-						getRandomString(42), getRandomBytes(FORUM_SALT_LENGTH));
-		byte[] body = clientHelper.toByteArray(bodyList);
-
-		// add the message to the queue
-		Transaction txn = db0.startTransaction(false);
-		try {
-			queue.sendMessage(txn, group, time, body, new Metadata());
-			db0.commitTransaction(txn);
-		} finally {
-			db0.endTransaction(txn);
-		}
-
-		// actually send the message
-		sync0To1(1, false);
-		// make sure there was no new request received
-		assertFalse(listener1.requestReceived);
-	}
-
-	@Test
+	@Test(expected = ProtocolStateException.class)
 	public void testSharingSameForumWithEachOther() throws Exception {
 		// initialize and let invitee accept all requests
 		listenToEvents(true);
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -455,24 +395,12 @@ public class ForumSharingIntegrationTest
 
 		// forum was added successfully
 		assertEquals(1, forumManager1.getForums().size());
-		assertEquals(2,
-				forumSharingManager0.getInvitationMessages(contactId1From0)
-						.size());
 
 		// invitee now shares same forum back
 		forumSharingManager1.sendInvitation(forum0.getId(),
 				contactId0From1,
-				"I am re-sharing this forum with you.");
-
-		// sync re-share invitation
-		sync1To0(1, false);
-
-		// make sure that no new request was received
-		assertFalse(listener0.requestReceived);
-		assertEquals(2,
-				forumSharingManager0.getInvitationMessages(contactId1From0)
-						.size());
-		assertEquals(0, forumSharingManager0.getInvitations().size());
+				"I am re-sharing this forum with you.",
+				clock.currentTimeMillis());
 	}
 
 	@Test
@@ -482,62 +410,48 @@ public class ForumSharingIntegrationTest
 
 		// invitee adds the same forum
 		Transaction txn = db1.startTransaction(false);
-		db1.addGroup(txn, forum0.getGroup());
+		forumManager1.addForum(txn, forum0);
 		db1.commitTransaction(txn);
 		db1.endTransaction(txn);
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// invitee now shares same forum back
 		forumSharingManager1.sendInvitation(forum0.getId(),
-				contactId0From1, "I am re-sharing this forum with you.");
+				contactId0From1, "I am re-sharing this forum with you.",
+				clock.currentTimeMillis());
 
-		// find out who should be Alice, because of random keys
-		Bytes key0 = new Bytes(author0.getPublicKey());
-		Bytes key1 = new Bytes(author1.getPublicKey());
+		// prevent automatic responses
+		respond = false;
 
 		// only now sync first request message
-		boolean alice = key1.compareTo(key0) < 0;
-		if (alice) {
-			sync0To1(1, false);
-			assertFalse(listener1.requestReceived);
-		} else {
-			sync0To1(1, true);
-			eventWaiter.await(TIMEOUT, 1);
-			assertTrue(listener1.requestReceived);
-		}
+		sync0To1(1, true);
+		eventWaiter.await(TIMEOUT, 1);
+		assertTrue(listener1.requestReceived);
 
-		// sync second invitation
-		alice = key0.compareTo(key1) < 0;
-		if (alice) {
-			sync1To0(1, false);
-			assertFalse(listener0.requestReceived);
+		// sync second invitation which counts as accept
+		sync1To0(1, true);
+		eventWaiter.await(TIMEOUT, 1);
+		assertTrue(listener0.requestReceived);
 
-			// sharer did not receive request, but response to own request
-			eventWaiter.await(TIMEOUT, 1);
-			assertTrue(listener0.responseReceived);
+		// both peers should share the forum with each other now
+		assertTrue(forumSharingManager0.getSharedWith(forum0.getId())
+				.contains(contact1From0));
+		assertTrue(forumSharingManager1.getSharedWith(forum0.getId())
+				.contains(contact0From1));
 
-			assertEquals(2, forumSharingManager0
-					.getInvitationMessages(contactId1From0).size());
-			assertEquals(3, forumSharingManager1
-					.getInvitationMessages(contactId0From1).size());
-		} else {
-			sync1To0(1, true);
-			eventWaiter.await(TIMEOUT, 1);
-			assertTrue(listener0.requestReceived);
+		// and both have each other's invitations (and no response)
+		assertEquals(2, forumSharingManager0
+				.getInvitationMessages(contactId1From0).size());
+		assertEquals(2, forumSharingManager1
+				.getInvitationMessages(contactId0From1).size());
 
-			// send response from sharer to invitee and make sure it arrived
-			sync0To1(1, true);
-			eventWaiter.await(TIMEOUT, 1);
-			assertTrue(listener1.responseReceived);
-
-			assertEquals(3, forumSharingManager0
-					.getInvitationMessages(contactId1From0).size());
-			assertEquals(2, forumSharingManager1
-					.getInvitationMessages(contactId0From1).size());
-		}
+		// there are no more open invitations
+		assertTrue(forumSharingManager0.getInvitations().isEmpty());
+		assertTrue(forumSharingManager1.getInvitations().isEmpty());
 	}
 
 	@Test
@@ -547,7 +461,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -564,18 +479,12 @@ public class ForumSharingIntegrationTest
 		assertEquals(1,
 				forumSharingManager0.getSharedWith(forum0.getId()).size());
 
-		// remember SessionId from invitation
-		List<InvitationMessage> list = new ArrayList<InvitationMessage>(
-				forumSharingManager1
-						.getInvitationMessages(contactId0From1));
-		assertEquals(2, list.size());
-		InvitationMessage msg = list.get(0);
-		SessionId sessionId = msg.getSessionId();
-		assertEquals(sessionId, list.get(1).getSessionId());
-
 		// contacts now remove each other
 		removeAllContacts();
 
+		// invitee still has forum
+		assertEquals(1, forumManager1.getForums().size());
+
 		// make sure sharer does share the forum with nobody now
 		assertEquals(0,
 				forumSharingManager0.getSharedWith(forum0.getId()).size());
@@ -584,35 +493,30 @@ public class ForumSharingIntegrationTest
 		addDefaultContacts();
 		addContacts1And2();
 
-		// get all sorts of stuff needed to send a message
-		MessageQueueManager queue = c0.getMessageQueueManager();
-		Contact c1 = contactManager0.getContact(contactId1From0);
-		Group group = contactGroupFactory.createContactGroup(CLIENT_ID, c1);
-		long time = clock.currentTimeMillis();
+		// forum can be shared with contacts again
+		assertTrue(forumSharingManager0
+				.canBeShared(forum0.getId(), contact1From0));
+		assertTrue(forumSharingManager0
+				.canBeShared(forum0.getId(), contact2From0));
+		// send invitation
+		forumSharingManager0
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
-		// construct a new message re-using the old SessionId
-		BdfList bodyList = BdfList.of(SHARE_MSG_TYPE_INVITATION,
-				sessionId.getBytes(),
-				getRandomString(42),
-				getRandomBytes(FORUM_SALT_LENGTH)
-		);
-		byte[] body = clientHelper.toByteArray(bodyList);
-
-		// add the message to the queue
-		Transaction txn = db0.startTransaction(false);
-		try {
-			queue.sendMessage(txn, group, time, body, new Metadata());
-			db0.commitTransaction(txn);
-		} finally {
-			db0.endTransaction(txn);
-		}
-
-		// actually send the message
+		// sync first request message
 		sync0To1(1, true);
 		eventWaiter.await(TIMEOUT, 1);
-		// make sure the new request was received with the same sessionId
-		// as proof that the state got deleted along with contacts
 		assertTrue(listener1.requestReceived);
+
+		// sync response back
+		sync1To0(1, true);
+		eventWaiter.await(TIMEOUT, 1);
+		assertTrue(listener0.responseReceived);
+
+		// forum is still there
+		assertEquals(1, forumManager1.getForums().size());
+		assertEquals(1,
+				forumSharingManager0.getSharedWith(forum0.getId()).size());
 	}
 
 	@Test
@@ -633,14 +537,16 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 		// sync first request message
 		sync0To1(1, true);
 
 		// second sharer sends invitation for same forum
 		assertTrue(contactId1From2 != null);
 		forumSharingManager2
-				.sendInvitation(forum0.getId(), contactId1From2, null);
+				.sendInvitation(forum0.getId(), contactId1From2, null,
+						clock.currentTimeMillis());
 		// sync second request message
 		sync2To1(1, true);
 
@@ -650,13 +556,6 @@ public class ForumSharingIntegrationTest
 		assertEquals(1, forums.size());
 		assertEquals(2, forums.iterator().next().getNewSharers().size());
 		assertEquals(forum0, forums.iterator().next().getShareable());
-		assertEquals(2,
-				forumSharingManager1.getSharedWith(forum0.getId()).size());
-
-		// make sure both sharers actually share the forum
-		Collection<Contact> contacts =
-				forumSharingManager1.getSharedWith(forum0.getId());
-		assertEquals(2, contacts.size());
 
 		// answer second request
 		assertNotNull(contactId2From1);
@@ -675,6 +574,11 @@ public class ForumSharingIntegrationTest
 		sync1To0(1, true);
 		eventWaiter.await(TIMEOUT, 1);
 		assertTrue(listener0.responseReceived);
+
+		// make sure both sharers actually share the forum
+		Collection<Contact> contacts =
+				forumSharingManager1.getSharedWith(forum0.getId());
+		assertEquals(2, contacts.size());
 	}
 
 	@Test
@@ -684,7 +588,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -745,7 +650,8 @@ public class ForumSharingIntegrationTest
 
 		// send invitation again
 		forumSharingManager0
-				.sendInvitation(forum0.getId(), contactId1From0, "Hi!");
+				.sendInvitation(forum0.getId(), contactId1From0, "Hi!",
+						clock.currentTimeMillis());
 
 		// sync first request message
 		sync0To1(1, true);
@@ -799,8 +705,10 @@ public class ForumSharingIntegrationTest
 				requestReceived = true;
 				Forum f = event.getShareable();
 				try {
-					Contact c = contactManager0.getContact(contactId1From0);
-					forumSharingManager0.respondToInvitation(f, c, true);
+					if (respond) {
+						Contact c = contactManager0.getContact(contactId1From0);
+						forumSharingManager0.respondToInvitation(f, c, true);
+					}
 				} catch (DbException ex) {
 					eventWaiter.rethrow(ex);
 				} finally {
@@ -814,7 +722,6 @@ public class ForumSharingIntegrationTest
 	private class InviteeListener implements EventListener {
 
 		private volatile boolean requestReceived = false;
-		private volatile boolean responseReceived = false;
 
 		private final boolean accept, answer;
 
@@ -836,13 +743,13 @@ public class ForumSharingIntegrationTest
 				if (!answer) return;
 				Forum f = event.getShareable();
 				try {
-					eventWaiter.assertEquals(1,
-							forumSharingManager1.getInvitations().size());
-					SharingInvitationItem invitation =
-							forumSharingManager1.getInvitations().iterator()
-									.next();
-					eventWaiter.assertEquals(f, invitation.getShareable());
 					if (respond) {
+						eventWaiter.assertEquals(1,
+								forumSharingManager1.getInvitations().size());
+						SharingInvitationItem invitation =
+								forumSharingManager1.getInvitations().iterator()
+										.next();
+						eventWaiter.assertEquals(f, invitation.getShareable());
 						Contact c =
 								contactManager1
 										.getContact(event.getContactId());
@@ -859,7 +766,6 @@ public class ForumSharingIntegrationTest
 				ForumInvitationResponseReceivedEvent event =
 						(ForumInvitationResponseReceivedEvent) e;
 				eventWaiter.assertEquals(contactId0From1, event.getContactId());
-				responseReceived = true;
 				eventWaiter.resume();
 			}
 		}

briar-core/src/test/java/org/briarproject/briar/sharing/ForumSharingValidatorTest.java

@@ -4,340 +4,314 @@ import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.UniqueId;
 import org.briarproject.bramble.api.client.BdfMessageContext;
 import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
 import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.bramble.test.TestUtils;
 import org.briarproject.bramble.test.ValidatorTestCase;
-import org.briarproject.briar.api.client.SessionId;
+import org.briarproject.briar.api.forum.Forum;
+import org.briarproject.briar.api.forum.ForumFactory;
+import org.jmock.Expectations;
 import org.junit.Test;
 
+import java.util.Collection;
+
 import javax.annotation.Nullable;
 
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_NAME;
-import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT;
+import static org.briarproject.bramble.test.TestUtils.getRandomId;
 import static org.briarproject.briar.api.forum.ForumConstants.FORUM_SALT_LENGTH;
 import static org.briarproject.briar.api.forum.ForumConstants.MAX_FORUM_NAME_LENGTH;
-import static org.briarproject.briar.api.sharing.SharingConstants.INVITATION_MSG;
-import static org.briarproject.briar.api.sharing.SharingConstants.LOCAL;
 import static org.briarproject.briar.api.sharing.SharingConstants.MAX_INVITATION_MESSAGE_LENGTH;
-import static org.briarproject.briar.api.sharing.SharingConstants.SESSION_ID;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_ABORT;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_ACCEPT;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_DECLINE;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_INVITATION;
-import static org.briarproject.briar.api.sharing.SharingConstants.SHARE_MSG_TYPE_LEAVE;
-import static org.briarproject.briar.api.sharing.SharingConstants.TIME;
-import static org.briarproject.briar.api.sharing.SharingConstants.TYPE;
+import static org.briarproject.briar.sharing.MessageType.ABORT;
+import static org.briarproject.briar.sharing.MessageType.ACCEPT;
+import static org.briarproject.briar.sharing.MessageType.DECLINE;
+import static org.briarproject.briar.sharing.MessageType.INVITE;
+import static org.briarproject.briar.sharing.MessageType.LEAVE;
 import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
 
 public class ForumSharingValidatorTest extends ValidatorTestCase {
 
-	private final SessionId sessionId = new SessionId(TestUtils.getRandomId());
+	private final MessageEncoder messageEncoder =
+			context.mock(MessageEncoder.class);
+	private final ForumFactory forumFactory = context.mock(ForumFactory.class);
+	private final ForumSharingValidator v =
+			new ForumSharingValidator(messageEncoder, clientHelper,
+					metadataEncoder, clock, forumFactory);
+
+	private final MessageId previousMsgId = new MessageId(getRandomId());
 	private final String forumName =
 			TestUtils.getRandomString(MAX_FORUM_NAME_LENGTH);
 	private final byte[] salt = TestUtils.getRandomBytes(FORUM_SALT_LENGTH);
+	private final Forum forum = new Forum(group, forumName, salt);
+	private final BdfList descriptor = BdfList.of(forumName, salt);
 	private final String content =
 			TestUtils.getRandomString(MAX_INVITATION_MESSAGE_LENGTH);
+	private final BdfDictionary meta =
+			BdfDictionary.of(new BdfEntry("meta", "data"));
 
 	@Test
 	public void testAcceptsInvitationWithContent() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectCreateForum(forumName);
+		expectEncodeMetadata(INVITE);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						salt, content));
-		assertExpectedContextForInvitation(messageContext, forumName, content);
+				BdfList.of(INVITE.getValue(), previousMsgId, descriptor,
+						content));
+		assertExpectedContext(messageContext, previousMsgId);
 	}
 
 	@Test
-	public void testAcceptsInvitationWithoutContent() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+	public void testAcceptsInvitationWithNullContent() throws Exception {
+		expectCreateForum(forumName);
+		expectEncodeMetadata(INVITE);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						salt));
-		assertExpectedContextForInvitation(messageContext, forumName, null);
+				BdfList.of(INVITE.getValue(), previousMsgId, descriptor, null));
+		assertExpectedContext(messageContext, previousMsgId);
+	}
+
+	@Test
+	public void testAcceptsInvitationWithNullPreviousMsgId() throws Exception {
+		expectCreateForum(forumName);
+		expectEncodeMetadata(INVITE);
+		BdfMessageContext messageContext = v.validateMessage(message, group,
+				BdfList.of(INVITE.getValue(), null, descriptor, null));
+		assertExpectedContext(messageContext, null);
 	}
 
 	@Test
 	public void testAcceptsAccept() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectEncodeMetadata(ACCEPT);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_ACCEPT, sessionId));
-		assertExpectedContext(messageContext, SHARE_MSG_TYPE_ACCEPT);
+				BdfList.of(ACCEPT.getValue(), groupId, previousMsgId));
+		assertExpectedContext(messageContext, previousMsgId);
 	}
 
 	@Test
 	public void testAcceptsDecline() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectEncodeMetadata(DECLINE);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_DECLINE, sessionId));
-		assertExpectedContext(messageContext, SHARE_MSG_TYPE_DECLINE);
+				BdfList.of(DECLINE.getValue(), groupId, previousMsgId));
+		assertExpectedContext(messageContext, previousMsgId);
 	}
 
 	@Test
 	public void testAcceptsLeave() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectEncodeMetadata(LEAVE);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_LEAVE, sessionId));
-		assertExpectedContext(messageContext, SHARE_MSG_TYPE_LEAVE);
+				BdfList.of(LEAVE.getValue(), groupId, previousMsgId));
+		assertExpectedContext(messageContext, previousMsgId);
 	}
 
 	@Test
 	public void testAcceptsAbort() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectEncodeMetadata(ABORT);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_ABORT, sessionId));
-		assertExpectedContext(messageContext, SHARE_MSG_TYPE_ABORT);
+				BdfList.of(ABORT.getValue(), groupId, previousMsgId));
+		assertExpectedContext(messageContext, previousMsgId);
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNullMessageType() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
-		v.validateMessage(message, group, BdfList.of(null, sessionId));
+		v.validateMessage(message, group,
+				BdfList.of(null, groupId, previousMsgId));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNonLongMessageType() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
-		v.validateMessage(message, group, BdfList.of("", sessionId));
+		v.validateMessage(message, group,
+				BdfList.of("", groupId, previousMsgId));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsInvalidMessageType() throws Exception {
-		int invalidMessageType = SHARE_MSG_TYPE_ABORT + 1;
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		int invalidMessageType = ABORT.getValue() + 1;
 		v.validateMessage(message, group,
-				BdfList.of(invalidMessageType, sessionId));
+				BdfList.of(invalidMessageType, groupId, previousMsgId));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNullSessionId() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_ABORT, null));
+				BdfList.of(ABORT.getValue(), null, previousMsgId));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNonRawSessionId() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
-		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_ABORT, 123));
+		v.validateMessage(message, group, BdfList.of(ABORT.getValue(), 123));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooShortSessionId() throws Exception {
-		byte[] invalidSessionId = TestUtils.getRandomBytes(UniqueId.LENGTH - 1);
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		byte[] invalidGroupId = TestUtils.getRandomBytes(UniqueId.LENGTH - 1);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_ABORT, invalidSessionId));
+				BdfList.of(ABORT.getValue(), invalidGroupId, previousMsgId));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooLongSessionId() throws Exception {
-		byte[] invalidSessionId = TestUtils.getRandomBytes(UniqueId.LENGTH + 1);
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		byte[] invalidGroupId = TestUtils.getRandomBytes(UniqueId.LENGTH + 1);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_ABORT, invalidSessionId));
+				BdfList.of(ABORT.getValue(), invalidGroupId, previousMsgId));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooShortBodyForAbort() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
-		v.validateMessage(message, group, BdfList.of(SHARE_MSG_TYPE_ABORT));
+		v.validateMessage(message, group,
+				BdfList.of(ABORT.getValue(), groupId));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooLongBodyForAbort() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_ABORT, sessionId, 123));
+				BdfList.of(ABORT.getValue(), groupId, previousMsgId, 123));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooShortBodyForInvitation() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName));
+				BdfList.of(INVITE.getValue(), previousMsgId, descriptor));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooLongBodyForInvitation() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						salt, content, 123));
+				BdfList.of(INVITE.getValue(), previousMsgId, descriptor, null,
+						123));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNullForumName() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of(null, salt);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, null,
-						salt, content));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNonStringForumName() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of(123, salt);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, 123,
-						salt, content));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooShortForumName() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of("", salt);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, "",
-						salt, content));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test
 	public void testAcceptsMinLengthForumName() throws Exception {
 		String shortForumName = TestUtils.getRandomString(1);
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList validDescriptor = BdfList.of(shortForumName, salt);
+		expectCreateForum(shortForumName);
+		expectEncodeMetadata(INVITE);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, shortForumName,
-						salt, content));
-		assertExpectedContextForInvitation(messageContext, shortForumName,
-				content);
+				BdfList.of(INVITE.getValue(), previousMsgId, validDescriptor,
+						null));
+		assertExpectedContext(messageContext, previousMsgId);
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooLongForumName() throws Exception {
 		String invalidForumName =
 				TestUtils.getRandomString(MAX_FORUM_NAME_LENGTH + 1);
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of(invalidForumName, salt);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId,
-						invalidForumName, salt, content));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNullSalt() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of(forumName, null);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						null, content));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNonRawSalt() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of(forumName, 123);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						123, content));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooShortSalt() throws Exception {
 		byte[] invalidSalt = TestUtils.getRandomBytes(FORUM_SALT_LENGTH - 1);
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of(forumName, invalidSalt);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						invalidSalt, content));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooLongSalt() throws Exception {
 		byte[] invalidSalt = TestUtils.getRandomBytes(FORUM_SALT_LENGTH + 1);
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		BdfList invalidDescriptor = BdfList.of(forumName, invalidSalt);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						invalidSalt, content));
-	}
-
-	@Test(expected = FormatException.class)
-	public void testRejectsNullContent() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
-		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						salt, null));
+				BdfList.of(INVITE.getValue(), previousMsgId, invalidDescriptor,
+						null));
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsNonStringContent() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectCreateForum(forumName);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						salt, 123));
+				BdfList.of(INVITE.getValue(), previousMsgId, descriptor,
+						123));
 	}
 
 	@Test
 	public void testAcceptsMinLengthContent() throws Exception {
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectCreateForum(forumName);
+		expectEncodeMetadata(INVITE);
 		BdfMessageContext messageContext = v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						salt, ""));
-		assertExpectedContextForInvitation(messageContext, forumName, "");
+				BdfList.of(INVITE.getValue(), previousMsgId, descriptor, "1"));
+		assertExpectedContext(messageContext, previousMsgId);
 	}
 
 	@Test(expected = FormatException.class)
 	public void testRejectsTooLongContent() throws Exception {
 		String invalidContent =
 				TestUtils.getRandomString(MAX_INVITATION_MESSAGE_LENGTH + 1);
-		ForumSharingValidator v = new ForumSharingValidator(clientHelper,
-				metadataEncoder, clock);
+		expectCreateForum(forumName);
 		v.validateMessage(message, group,
-				BdfList.of(SHARE_MSG_TYPE_INVITATION, sessionId, forumName,
-						salt, invalidContent));
+				BdfList.of(INVITE.getValue(), previousMsgId, descriptor,
+						invalidContent));
 	}
 
-	private void assertExpectedContextForInvitation(
-			BdfMessageContext messageContext, String forumName,
-			@Nullable String content) throws FormatException {
-		BdfDictionary meta = messageContext.getDictionary();
-		if (content == null) {
-			assertEquals(6, meta.size());
-		} else {
-			assertEquals(7, meta.size());
-			assertEquals(content, meta.getString(INVITATION_MSG));
-		}
-		assertEquals(forumName, meta.getString(FORUM_NAME));
-		assertEquals(salt, meta.getRaw(FORUM_SALT));
-		assertEquals(SHARE_MSG_TYPE_INVITATION, meta.getLong(TYPE).intValue());
-		assertEquals(sessionId.getBytes(), meta.getRaw(SESSION_ID));
-		assertFalse(meta.getBoolean(LOCAL));
-		assertEquals(timestamp, meta.getLong(TIME).longValue());
-		assertEquals(0, messageContext.getDependencies().size());
+	private void expectCreateForum(final String name) {
+		context.checking(new Expectations() {{
+			oneOf(forumFactory).createForum(name, salt);
+			will(returnValue(forum));
+		}});
+	}
+
+	private void expectEncodeMetadata(final MessageType type) {
+		context.checking(new Expectations() {{
+			oneOf(messageEncoder)
+					.encodeMetadata(type, groupId, timestamp, false, false,
+							false, false);
+			will(returnValue(meta));
+		}});
 	}
 
 	private void assertExpectedContext(BdfMessageContext messageContext,
-			int type) throws FormatException {
-		BdfDictionary meta = messageContext.getDictionary();
-		assertEquals(4, meta.size());
-		assertEquals(type, meta.getLong(TYPE).intValue());
-		assertEquals(sessionId.getBytes(), meta.getRaw(SESSION_ID));
-		assertFalse(meta.getBoolean(LOCAL));
-		assertEquals(timestamp, meta.getLong(TIME).longValue());
-		assertEquals(0, messageContext.getDependencies().size());
+			@Nullable MessageId previousMsgId) throws FormatException {
+		Collection<MessageId> dependencies = messageContext.getDependencies();
+		if (previousMsgId == null) {
+			assertTrue(dependencies.isEmpty());
+		} else {
+			assertEquals(1, dependencies.size());
+			assertTrue(dependencies.contains(previousMsgId));
+		}
+		assertEquals(meta, messageContext.getDictionary());
 	}
+
 }