Initial implementation of social backup client.

bramble-api/src/main/java/org/briarproject/bramble/api/crypto/AuthenticatedCipher.java

@@ -1,12 +1,11 @@
-package org.briarproject.bramble.crypto;
+package org.briarproject.bramble.api.crypto;
 
-import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.security.GeneralSecurityException;
 
 @NotNullByDefault
-interface AuthenticatedCipher {
+public interface AuthenticatedCipher {
 
 	/**
 	 * Initializes this cipher for encryption or decryption with a key and an

bramble-api/src/main/java/org/briarproject/bramble/api/identity/IdentityManager.java

@@ -26,6 +26,11 @@ public interface IdentityManager {
 	 */
 	void registerIdentity(Identity i);
 
+	/**
+	 * Returns the cached local identity or loads it from the database.
+	 */
+	Identity getIdentity(Transaction txn) throws DbException;
+
 	/**
 	 * Returns the cached local identity or loads it from the database.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/properties/TransportPropertyManager.java

@@ -74,6 +74,13 @@ public interface TransportPropertyManager {
 	TransportProperties getRemoteProperties(ContactId c, TransportId t)
 			throws DbException;
 
+	/**
+	 * Returns the remote transport properties for the given contact and
+	 * transport.
+	 */
+	TransportProperties getRemoteProperties(Transaction txn, ContactId c,
+			TransportId t) throws DbException;
+
 	/**
 	 * Merges the given properties with the existing local properties for the
 	 * given transport.

bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoComponentImpl.java

@@ -6,6 +6,7 @@ import net.i2p.crypto.eddsa.KeyPairGenerator;
 
 import org.briarproject.bramble.api.crypto.AgreementPrivateKey;
 import org.briarproject.bramble.api.crypto.AgreementPublicKey;
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.crypto.DecryptionException;
 import org.briarproject.bramble.api.crypto.KeyPair;

bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoModule.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.crypto.KeyAgreementCrypto;
 import org.briarproject.bramble.api.crypto.PasswordStrengthEstimator;

bramble-core/src/main/java/org/briarproject/bramble/crypto/StreamDecrypterFactoryImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.crypto.StreamDecrypter;
 import org.briarproject.bramble.api.crypto.StreamDecrypterFactory;

bramble-core/src/main/java/org/briarproject/bramble/crypto/StreamDecrypterImpl.java

@@ -1,6 +1,7 @@
 package org.briarproject.bramble.crypto;
 
 import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.crypto.StreamDecrypter;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;

bramble-core/src/main/java/org/briarproject/bramble/crypto/StreamEncrypterFactoryImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.crypto.StreamEncrypter;

bramble-core/src/main/java/org/briarproject/bramble/crypto/StreamEncrypterImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.crypto.StreamEncrypter;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;

bramble-core/src/main/java/org/briarproject/bramble/crypto/XSalsa20Poly1305AuthenticatedCipher.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.spongycastle.crypto.DataLengthException;

bramble-core/src/main/java/org/briarproject/bramble/identity/IdentityManagerImpl.java

@@ -118,6 +118,11 @@ class IdentityManagerImpl implements IdentityManager, OpenDatabaseHook {
 		return cached.getLocalAuthor();
 	}
 
+	@Override
+	public Identity getIdentity(Transaction txn) throws DbException {
+		return getCachedIdentity(txn);
+	}
+
 	@Override
 	public LocalAuthor getLocalAuthor(Transaction txn) throws DbException {
 		return getCachedIdentity(txn).getLocalAuthor();

bramble-core/src/main/java/org/briarproject/bramble/properties/TransportPropertyManagerImpl.java

@@ -294,7 +294,13 @@ class TransportPropertyManagerImpl implements TransportPropertyManager,
 	public TransportProperties getRemoteProperties(ContactId c, TransportId t)
 			throws DbException {
 		return db.transactionWithResult(true, txn ->
-				getRemoteProperties(txn, db.getContact(txn, c), t));
+				getRemoteProperties(txn, c, t));
+	}
+
+	@Override
+	public TransportProperties getRemoteProperties(Transaction txn,
+			ContactId c, TransportId t) throws DbException {
+		return getRemoteProperties(txn, db.getContact(txn, c), t);
 	}
 
 	@Override

bramble-core/src/test/java/org/briarproject/bramble/crypto/StreamDecrypterImplTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.test.BrambleTestCase;
 import org.briarproject.bramble.test.TestUtils;

bramble-core/src/test/java/org/briarproject/bramble/crypto/StreamEncrypterImplTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.test.BrambleTestCase;
 import org.briarproject.bramble.test.TestUtils;

bramble-core/src/test/java/org/briarproject/bramble/crypto/TestAuthenticatedCipher.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 

bramble-core/src/test/java/org/briarproject/bramble/crypto/XSalsa20Poly1305AuthenticatedCipherTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.test.BrambleTestCase;
 import org.briarproject.bramble.util.StringUtils;

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/BackupExistsException.java

@@ -0,0 +1,11 @@
+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.db.DbException;
+
+/**
+ * Thrown when an attempt is made to create a social account backup but a
+ * backup already exists. This exception may occur due to concurrent updates
+ * and does not indicate a database error.
+ */
+public class BackupExistsException extends DbException {
+}

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/BackupMetadata.java

@@ -0,0 +1,42 @@
+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class BackupMetadata {
+
+	private final SecretKey secret;
+	private final List<Author> custodians;
+	private final int threshold, version;
+
+	public BackupMetadata(SecretKey secret, List<Author> custodians,
+			int threshold, int version) {
+		this.secret = secret;
+		this.custodians = custodians;
+		this.threshold = threshold;
+		this.version = version;
+	}
+
+	public SecretKey getSecret() {
+		return secret;
+	}
+
+	public List<Author> getCustodians() {
+		return custodians;
+	}
+
+	public int getThreshold() {
+		return threshold;
+	}
+
+	public int getVersion() {
+		return version;
+	}
+}

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/Shard.java

@@ -0,0 +1,37 @@
+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class Shard {
+
+	private final byte[] secretId, shard;
+	private final int numShards, threshold;
+
+	public Shard(byte[] secretId, int numShards, int threshold,
+			byte[] shard) {
+		this.secretId = secretId;
+		this.numShards = numShards;
+		this.threshold = threshold;
+		this.shard = shard;
+	}
+
+	public byte[] getSecretId() {
+		return secretId;
+	}
+
+	public int getNumShards() {
+		return numShards;
+	}
+
+	public int getThreshold() {
+		return threshold;
+	}
+
+	public byte[] getShard() {
+		return shard;
+	}
+}

briar-api/src/main/java/org/briarproject/briar/api/socialbackup/SocialBackupManager.java

@@ -0,0 +1,47 @@
+package org.briarproject.briar.api.socialbackup;
+
+import org.briarproject.bramble.api.contact.ContactId;
+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 java.util.List;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+public interface SocialBackupManager {
+
+	/**
+	 * The unique ID of the social backup client.
+	 */
+	ClientId CLIENT_ID = new ClientId("pw.darkcrystal.backup");
+
+	/**
+	 * The current major version of the social backup client.
+	 */
+	int MAJOR_VERSION = 0;
+
+	/**
+	 * The current minor version of the social backup client.
+	 */
+	int MINOR_VERSION = 0;
+
+	/**
+	 * Returns the metadata for this device's backup, or null if no backup has
+	 * been created.
+	 */
+	@Nullable
+	BackupMetadata getBackupMetadata(Transaction txn) throws DbException;
+
+	/**
+	 * Creates a backup for this device using the given custodians and
+	 * threshold. The encrypted backup and a shard of the backup key will be
+	 * sent to each custodian.
+	 *
+	 * @throws BackupExistsException If a backup already exists
+	 */
+	void createBackup(Transaction txn, List<ContactId> custodianIds,
+			int threshold) throws DbException;
+}

briar-core/src/main/java/org/briarproject/briar/BriarCoreEagerSingletons.java

@@ -10,6 +10,7 @@ import org.briarproject.briar.messaging.MessagingModule;
 import org.briarproject.briar.privategroup.PrivateGroupModule;
 import org.briarproject.briar.privategroup.invitation.GroupInvitationModule;
 import org.briarproject.briar.sharing.SharingModule;
+import org.briarproject.briar.socialbackup.SocialBackupModule;
 
 public interface BriarCoreEagerSingletons {
 
@@ -33,6 +34,8 @@ public interface BriarCoreEagerSingletons {
 
 	void inject(SharingModule.EagerSingletons init);
 
+	void inject(SocialBackupModule.EagerSingletons init);
+
 	class Helper {
 
 		public static void injectEagerSingletons(BriarCoreEagerSingletons c) {
@@ -46,6 +49,7 @@ public interface BriarCoreEagerSingletons {
 			c.inject(new SharingModule.EagerSingletons());
 			c.inject(new IdentityModule.EagerSingletons());
 			c.inject(new IntroductionModule.EagerSingletons());
+			c.inject(new SocialBackupModule.EagerSingletons());
 		}
 	}
 }

briar-core/src/main/java/org/briarproject/briar/BriarCoreModule.java

@@ -13,6 +13,7 @@ import org.briarproject.briar.messaging.MessagingModule;
 import org.briarproject.briar.privategroup.PrivateGroupModule;
 import org.briarproject.briar.privategroup.invitation.GroupInvitationModule;
 import org.briarproject.briar.sharing.SharingModule;
+import org.briarproject.briar.socialbackup.SocialBackupModule;
 import org.briarproject.briar.test.TestModule;
 
 import dagger.Module;
@@ -31,6 +32,7 @@ import dagger.Module;
 		MessagingModule.class,
 		PrivateGroupModule.class,
 		SharingModule.class,
+		SocialBackupModule.class,
 		TestModule.class
 })
 public class BriarCoreModule {

briar-core/src/main/java/org/briarproject/briar/socialbackup/BackupMetadataEncoder.java

@@ -0,0 +1,11 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.BackupMetadata;
+
+@NotNullByDefault
+interface BackupMetadataEncoder {
+
+	BdfDictionary encodeBackupMetadata(BackupMetadata backupMetadata);
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/BackupMetadataEncoderImpl.java

@@ -0,0 +1,42 @@
+package org.briarproject.briar.socialbackup;
+
+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.identity.Author;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.BackupMetadata;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_CUSTODIANS;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_SECRET;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_THRESHOLD;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_VERSION;
+
+@Immutable
+@NotNullByDefault
+class BackupMetadataEncoderImpl implements BackupMetadataEncoder {
+
+	private final ClientHelper clientHelper;
+
+	@Inject
+	BackupMetadataEncoderImpl(ClientHelper clientHelper) {
+		this.clientHelper = clientHelper;
+	}
+
+	@Override
+	public BdfDictionary encodeBackupMetadata(BackupMetadata backupMetadata) {
+		BdfList custodians = new BdfList();
+		for (Author custodian : backupMetadata.getCustodians()) {
+			custodians.add(clientHelper.toList(custodian));
+		}
+		BdfDictionary meta = new BdfDictionary();
+		meta.put(GROUP_KEY_SECRET, backupMetadata.getSecret().getBytes());
+		meta.put(GROUP_KEY_CUSTODIANS, custodians);
+		meta.put(GROUP_KEY_THRESHOLD, backupMetadata.getThreshold());
+		meta.put(GROUP_KEY_VERSION, backupMetadata.getVersion());
+		return meta;
+	}
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/BackupMetadataParser.java

@@ -0,0 +1,16 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.BackupMetadata;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+interface BackupMetadataParser {
+
+	@Nullable
+	BackupMetadata parseBackupMetadata(BdfDictionary meta)
+			throws FormatException;
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/BackupMetadataParserImpl.java

@@ -0,0 +1,51 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.BackupMetadata;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_CUSTODIANS;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_SECRET;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_THRESHOLD;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.GROUP_KEY_VERSION;
+
+@Immutable
+@NotNullByDefault
+class BackupMetadataParserImpl implements BackupMetadataParser {
+
+	private final ClientHelper clientHelper;
+
+	@Inject
+	BackupMetadataParserImpl(ClientHelper clientHelper) {
+		this.clientHelper = clientHelper;
+	}
+
+	@Nullable
+	@Override
+	public BackupMetadata parseBackupMetadata(BdfDictionary meta)
+			throws FormatException {
+		if (meta.isEmpty()) return null;
+		SecretKey secret = new SecretKey(meta.getRaw(GROUP_KEY_SECRET));
+		BdfList bdfCustodians = meta.getList(GROUP_KEY_CUSTODIANS);
+		List<Author> custodians = new ArrayList<>(bdfCustodians.size());
+		for (int i = 0; i < bdfCustodians.size(); i++) {
+			BdfList author = bdfCustodians.getList(i);
+			custodians.add(clientHelper.parseAndValidateAuthor(author));
+		}
+		int threshold = meta.getLong(GROUP_KEY_THRESHOLD).intValue();
+		int version = meta.getLong(GROUP_KEY_VERSION).intValue();
+		return new BackupMetadata(secret, custodians, threshold, version);
+	}
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/BackupPayload.java

@@ -0,0 +1,15 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.Bytes;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class BackupPayload extends Bytes {
+
+	BackupPayload(byte[] payload) {
+		super(payload);
+	}
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/BackupPayloadEncoder.java

@@ -0,0 +1,20 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.identity.Identity;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.util.List;
+import java.util.Map;
+
+@NotNullByDefault
+interface BackupPayloadEncoder {
+
+	BackupPayload encodeBackupPayload(SecretKey secret, Identity identity,
+			List<Contact> contacts,
+			List<Map<TransportId, TransportProperties>> properties,
+			int version);
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/BackupPayloadEncoderImpl.java

@@ -0,0 +1,94 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.crypto.AuthenticatedCipher;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.identity.Identity;
+import org.briarproject.bramble.api.identity.LocalAuthor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.security.GeneralSecurityException;
+import java.security.SecureRandom;
+import java.util.List;
+import java.util.Map;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+import javax.inject.Provider;
+
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.AUTH_TAG_BYTES;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.NONCE_BYTES;
+
+@Immutable
+@NotNullByDefault
+class BackupPayloadEncoderImpl implements BackupPayloadEncoder {
+
+	private final ClientHelper clientHelper;
+	private final Provider<AuthenticatedCipher> cipherProvider;
+	private final SecureRandom secureRandom;
+
+	@Inject
+	BackupPayloadEncoderImpl(ClientHelper clientHelper,
+			Provider<AuthenticatedCipher> cipherProvider,
+			SecureRandom secureRandom) {
+		this.clientHelper = clientHelper;
+		this.cipherProvider = cipherProvider;
+		this.secureRandom = secureRandom;
+	}
+
+	@Override
+	public BackupPayload encodeBackupPayload(SecretKey secret,
+			Identity identity, List<Contact> contacts,
+			List<Map<TransportId, TransportProperties>> properties,
+			int version) {
+		if (contacts.size() != properties.size()) {
+			throw new IllegalArgumentException();
+		}
+		// Encode the local identity
+		BdfList identityData = new BdfList();
+		LocalAuthor localAuthor = identity.getLocalAuthor();
+		identityData.add(clientHelper.toList(localAuthor));
+		identityData.add(localAuthor.getPrivateKey().getEncoded());
+		identityData.add(identity.getHandshakePublicKey().getEncoded());
+		identityData.add(identity.getHandshakePrivateKey().getEncoded());
+		// Encode the contacts
+		BdfList contactData = new BdfList();
+		for (int i = 0; i < contacts.size(); i++) {
+			Contact contact = contacts.get(i);
+			Map<TransportId, TransportProperties> props = properties.get(i);
+			BdfList data = new BdfList();
+			data.add(clientHelper.toList(contact.getAuthor()));
+			data.add(contact.getAlias());
+			PublicKey pub = requireNonNull(contact.getHandshakePublicKey());
+			data.add(pub.getEncoded());
+			data.add(clientHelper.toDictionary(props));
+			contactData.add(data);
+		}
+		// Encode and encrypt the payload
+		BdfList backup = new BdfList();
+		backup.add(version);
+		backup.add(identityData);
+		backup.add(contactData);
+		try {
+			byte[] plaintext = clientHelper.toByteArray(backup);
+			byte[] ciphertext = new byte[plaintext.length + AUTH_TAG_BYTES];
+			byte[] nonce = new byte[NONCE_BYTES];
+			secureRandom.nextBytes(nonce);
+			AuthenticatedCipher cipher = cipherProvider.get();
+			cipher.init(true, secret, nonce);
+			int encrypted = cipher.process(plaintext, 0, plaintext.length,
+					ciphertext, 0);
+			if (encrypted != ciphertext.length) throw new AssertionError();
+			return new BackupPayload(ciphertext);
+		} catch (FormatException | GeneralSecurityException e) {
+			throw new AssertionError(e);
+		}
+	}
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystal.java

@@ -0,0 +1,13 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.Shard;
+
+import java.util.List;
+
+@NotNullByDefault
+interface DarkCrystal {
+
+	List<Shard> createShards(SecretKey secret, int shards, int threshold);
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/DarkCrystalStub.java

@@ -0,0 +1,36 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.Shard;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+import javax.inject.Inject;
+
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.SECRET_ID_BYTES;
+
+@NotNullByDefault
+class DarkCrystalStub implements DarkCrystal {
+
+	@Inject
+	DarkCrystalStub() {
+	}
+
+	@Override
+	public List<Shard> createShards(SecretKey secret, int numShards,
+			int threshold) {
+		Random random = new Random();
+		byte[] secretId = new byte[SECRET_ID_BYTES];
+		random.nextBytes(secretId);
+		List<Shard> shards = new ArrayList<>(numShards);
+		for (int i = 0; i < numShards; i++) {
+			byte[] shard = new byte[123];
+			random.nextBytes(shard);
+			shards.add(new Shard(secretId, numShards, threshold, shard));
+		}
+		return shards;
+	}
+}

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

@@ -0,0 +1,12 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.Shard;
+
+@NotNullByDefault
+interface MessageEncoder {
+
+	byte[] encodeShardMessage(Shard shard);
+
+	byte[] encodeBackupMessage(int version, BackupPayload payload);
+}

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

@@ -0,0 +1,55 @@
+package org.briarproject.briar.socialbackup;
+
+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.socialbackup.Shard;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.briar.socialbackup.MessageType.BACKUP;
+import static org.briarproject.briar.socialbackup.MessageType.SHARD;
+
+@Immutable
+@NotNullByDefault
+class MessageEncoderImpl implements MessageEncoder {
+
+	private final ClientHelper clientHelper;
+
+	@Inject
+	MessageEncoderImpl(ClientHelper clientHelper) {
+		this.clientHelper = clientHelper;
+	}
+
+	@Override
+	public byte[] encodeShardMessage(Shard shard) {
+		BdfList body = BdfList.of(
+				SHARD.getValue(),
+				shard.getSecretId(),
+				shard.getNumShards(),
+				shard.getThreshold(),
+				shard.getShard()
+		);
+		return encodeBody(body);
+	}
+
+	@Override
+	public byte[] encodeBackupMessage(int version, BackupPayload payload) {
+		BdfList body = BdfList.of(
+				BACKUP.getValue(),
+				version,
+				payload.getBytes()
+		);
+		return encodeBody(body);
+	}
+
+	private byte[] encodeBody(BdfList body) {
+		try {
+			return clientHelper.toByteArray(body);
+		} catch (FormatException e) {
+			throw new AssertionError(e);
+		}
+	}
+}

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

@@ -0,0 +1,13 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.briar.api.socialbackup.Shard;
+
+@NotNullByDefault
+interface MessageParser {
+
+	Shard parseShardMessage(byte[] body) throws FormatException;
+
+	BackupPayload parseBackupMessage(byte[] body) throws FormatException;
+}

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

@@ -0,0 +1,41 @@
+package org.briarproject.briar.socialbackup;
+
+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.socialbackup.Shard;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+@Immutable
+@NotNullByDefault
+class MessageParserImpl implements MessageParser {
+
+	private final ClientHelper clientHelper;
+
+	@Inject
+	MessageParserImpl(ClientHelper clientHelper) {
+		this.clientHelper = clientHelper;
+	}
+
+	@Override
+	public Shard parseShardMessage(byte[] body) throws FormatException {
+		BdfList list = clientHelper.toList(body);
+		// Message type, secret ID, num shards, threshold, shard
+		byte[] secretId = list.getRaw(1);
+		int numShards = list.getLong(2).intValue();
+		int threshold = list.getLong(3).intValue();
+		byte[] shard = list.getRaw(4);
+		return new Shard(secretId, numShards, threshold, shard);
+	}
+
+	@Override
+	public BackupPayload parseBackupMessage(byte[] body)
+			throws FormatException {
+		BdfList list = clientHelper.toList(body);
+		// Message type, backup payload
+		return new BackupPayload(list.getRaw(1));
+	}
+}

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

@@ -0,0 +1,28 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+enum MessageType {
+
+	SHARD(0), BACKUP(1);
+
+	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/socialbackup/SocialBackupConstants.java

@@ -0,0 +1,35 @@
+package org.briarproject.briar.socialbackup;
+
+interface SocialBackupConstants {
+
+	// Group metadata keys
+	String GROUP_KEY_SECRET = "secret";
+	String GROUP_KEY_CUSTODIANS = "custodians";
+	String GROUP_KEY_THRESHOLD = "threshold";
+	String GROUP_KEY_VERSION = "version";
+
+	// Message metadata keys
+	String MSG_KEY_MESSAGE_TYPE = "messageType";
+	String MSG_KEY_LOCAL = "local";
+	String MSG_KEY_VERSION = "version";
+
+	/**
+	 * The length of the authenticated cipher's nonce in bytes.
+	 */
+	int NONCE_BYTES = 24;
+
+	/**
+	 * The length of the authenticated cipher's authentication tag in bytes.
+	 */
+	int AUTH_TAG_BYTES = 16;
+
+	/**
+	 * The length of the secret ID in bytes.
+	 */
+	int SECRET_ID_BYTES = 32;
+
+	/**
+	 * The maximum length of a shard in bytes.
+	 */
+	int MAX_SHARD_BYTES = 1024;
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/SocialBackupManagerImpl.java

@@ -0,0 +1,287 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.BdfIncomingMessageHook;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.client.ContactGroupFactory;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.ContactManager;
+import org.briarproject.bramble.api.contact.ContactManager.ContactHook;
+import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.crypto.SecretKey;
+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.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.identity.Author;
+import org.briarproject.bramble.api.identity.Identity;
+import org.briarproject.bramble.api.identity.IdentityManager;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager.OpenDatabaseHook;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TorConstants;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.properties.TransportProperties;
+import org.briarproject.bramble.api.properties.TransportPropertyManager;
+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.bramble.api.versioning.ClientVersioningManager;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager.ClientVersioningHook;
+import org.briarproject.briar.api.socialbackup.BackupExistsException;
+import org.briarproject.briar.api.socialbackup.BackupMetadata;
+import org.briarproject.briar.api.socialbackup.Shard;
+import org.briarproject.briar.api.socialbackup.SocialBackupManager;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+
+import javax.annotation.Nullable;
+import javax.inject.Inject;
+
+import static java.util.Collections.singletonMap;
+import static org.briarproject.briar.socialbackup.MessageType.BACKUP;
+import static org.briarproject.briar.socialbackup.MessageType.SHARD;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.MSG_KEY_LOCAL;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.MSG_KEY_VERSION;
+
+@NotNullByDefault
+class SocialBackupManagerImpl extends BdfIncomingMessageHook
+		implements SocialBackupManager, OpenDatabaseHook, ContactHook,
+		ClientVersioningHook {
+
+	private final ClientVersioningManager clientVersioningManager;
+	private final TransportPropertyManager transportPropertyManager;
+	private final ContactGroupFactory contactGroupFactory;
+	private final BackupMetadataParser backupMetadataParser;
+	private final BackupMetadataEncoder backupMetadataEncoder;
+	private final BackupPayloadEncoder backupPayloadEncoder;
+	private final MessageEncoder messageEncoder;
+	private final IdentityManager identityManager;
+	private final ContactManager contactManager;
+	private final CryptoComponent crypto;
+	private final DarkCrystal darkCrystal;
+	private final Clock clock;
+	private final Group localGroup;
+
+	@Inject
+	SocialBackupManagerImpl(
+			DatabaseComponent db,
+			ClientHelper clientHelper,
+			MetadataParser metadataParser,
+			ClientVersioningManager clientVersioningManager,
+			TransportPropertyManager transportPropertyManager,
+			ContactGroupFactory contactGroupFactory,
+			BackupMetadataParser backupMetadataParser,
+			BackupMetadataEncoder backupMetadataEncoder,
+			BackupPayloadEncoder backupPayloadEncoder,
+			MessageEncoder messageEncoder,
+			IdentityManager identityManager,
+			ContactManager contactManager,
+			CryptoComponent crypto,
+			DarkCrystal darkCrystal,
+			Clock clock) {
+		super(db, clientHelper, metadataParser);
+		this.clientVersioningManager = clientVersioningManager;
+		this.transportPropertyManager = transportPropertyManager;
+		this.contactGroupFactory = contactGroupFactory;
+		this.backupMetadataParser = backupMetadataParser;
+		this.backupMetadataEncoder = backupMetadataEncoder;
+		this.backupPayloadEncoder = backupPayloadEncoder;
+		this.messageEncoder = messageEncoder;
+		this.identityManager = identityManager;
+		this.contactManager = contactManager;
+		this.crypto = crypto;
+		this.darkCrystal = darkCrystal;
+		localGroup =
+				contactGroupFactory.createLocalGroup(CLIENT_ID, MAJOR_VERSION);
+		this.clock = clock;
+	}
+
+	@Override
+	public void onDatabaseOpened(Transaction txn) throws DbException {
+		if (db.containsGroup(txn, localGroup.getId())) return;
+		db.addGroup(txn, localGroup);
+		// Set things up for any pre-existing contacts
+		for (Contact c : db.getContacts(txn)) addingContact(txn, c);
+	}
+
+	@Override
+	public void addingContact(Transaction txn, Contact c) throws DbException {
+		// Create a group to share with the contact
+		Group g = getContactGroup(c);
+		db.addGroup(txn, g);
+		// Apply the client's visibility to the contact group
+		Visibility client = clientVersioningManager.getClientVisibility(txn,
+				c.getId(), CLIENT_ID, MAJOR_VERSION);
+		db.setGroupVisibility(txn, c.getId(), g.getId(), client);
+		// TODO: Add the contact to our backup, if any
+	}
+
+	@Override
+	public void removingContact(Transaction txn, Contact c) throws DbException {
+		db.removeGroup(txn, getContactGroup(c));
+		// TODO: Remove the contact from our backup, if any
+	}
+
+	@Override
+	public void onClientVisibilityChanging(Transaction txn, Contact c,
+			Visibility v) throws DbException {
+		// Apply the client's visibility to the contact group
+		Group g = getContactGroup(c);
+		db.setGroupVisibility(txn, c.getId(), g.getId(), v);
+	}
+
+	@Override
+	protected boolean incomingMessage(Transaction txn, Message m, BdfList body,
+			BdfDictionary meta) throws DbException, FormatException {
+		MessageType type = MessageType.fromValue(body.getLong(0).intValue());
+		if (type == SHARD) {
+			// TODO: Add the shard to our backup, if any
+		} else if (type == BACKUP) {
+			// Keep the newest version of the backup, delete any older versions
+			int version = meta.getLong(MSG_KEY_VERSION).intValue();
+			BdfDictionary query = BdfDictionary.of(
+					new BdfEntry(MSG_KEY_MESSAGE_TYPE, BACKUP.getValue()),
+					new BdfEntry(MSG_KEY_LOCAL, false));
+			Map<MessageId, BdfDictionary> results =
+					clientHelper.getMessageMetadataAsDictionary(txn,
+							m.getGroupId(), query);
+			if (results.size() > 1) throw new DbException();
+			for (Entry<MessageId, BdfDictionary> e : results.entrySet()) {
+				MessageId prevId = e.getKey();
+				BdfDictionary prevMeta = e.getValue();
+				int prevVersion = prevMeta.getLong(MSG_KEY_VERSION).intValue();
+				if (version > prevVersion) {
+					// This backup is newer - delete the previous backup
+					db.deleteMessage(txn, prevId);
+					db.deleteMessageMetadata(txn, prevId);
+				} else {
+					// We've already received a newer backup - delete this one
+					db.deleteMessage(txn, m.getId());
+					db.deleteMessageMetadata(txn, m.getId());
+				}
+			}
+		}
+		return false;
+	}
+
+	@Nullable
+	@Override
+	public BackupMetadata getBackupMetadata(Transaction txn)
+			throws DbException {
+		try {
+			BdfDictionary meta = clientHelper.getGroupMetadataAsDictionary(txn,
+					localGroup.getId());
+			return backupMetadataParser.parseBackupMetadata(meta);
+		} catch (FormatException e) {
+			throw new DbException(e);
+		}
+	}
+
+	@Override
+	public void createBackup(Transaction txn, List<ContactId> custodianIds,
+			int threshold) throws DbException {
+		if (getBackupMetadata(txn) != null) throw new BackupExistsException();
+		// Load the contacts
+		List<Contact> custodians = new ArrayList<>(custodianIds.size());
+		for (ContactId custodianId : custodianIds) {
+			custodians.add(contactManager.getContact(txn, custodianId));
+		}
+		// Create the encrypted backup payload
+		SecretKey secret = crypto.generateSecretKey();
+		BackupPayload payload = createBackupPayload(txn, secret, 0);
+		// Create the shards
+		List<Shard> shards = darkCrystal.createShards(secret,
+				custodians.size(), threshold);
+		try {
+			// Send the shard and backup messages to the custodians
+			for (int i = 0; i < custodians.size(); i++) {
+				Contact custodian = custodians.get(i);
+				Shard shard = shards.get(i);
+				sendShardMessage(txn, custodian, shard);
+				sendBackupMessage(txn, custodian, 0, payload);
+			}
+			// Store the backup metadata
+			List<Author> authors = new ArrayList<>(custodians.size());
+			for (Contact custodian : custodians) {
+				authors.add(custodian.getAuthor());
+			}
+			BackupMetadata backupMetadata =
+					new BackupMetadata(secret, authors, threshold, 0);
+			BdfDictionary meta =
+					backupMetadataEncoder.encodeBackupMetadata(backupMetadata);
+			clientHelper.mergeGroupMetadata(txn, localGroup.getId(), meta);
+		} catch (FormatException e) {
+			throw new AssertionError(e);
+		}
+	}
+
+	private Group getContactGroup(Contact c) {
+		return contactGroupFactory.createContactGroup(CLIENT_ID,
+				MAJOR_VERSION, c);
+	}
+
+	private BackupPayload createBackupPayload(Transaction txn,
+			SecretKey secret, int version) throws DbException {
+		Identity identity = identityManager.getIdentity(txn);
+		Collection<Contact> contacts = contactManager.getContacts(txn);
+		List<Contact> eligible = new ArrayList<>();
+		List<Map<TransportId, TransportProperties>> properties =
+				new ArrayList<>();
+		// Include all contacts whose handshake public keys we know
+		for (Contact c : contacts) {
+			if (c.getHandshakePublicKey() != null) {
+				eligible.add(c);
+				properties.add(getTransportProperties(txn, c.getId()));
+				// TODO: Include shard received from contact, if any
+			}
+		}
+		return backupPayloadEncoder.encodeBackupPayload(secret, identity,
+				eligible, properties, version);
+	}
+
+	private Map<TransportId, TransportProperties> getTransportProperties(
+			Transaction txn, ContactId c) throws DbException {
+		// TODO: Include filtered properties for other transports
+		TransportProperties p = transportPropertyManager
+				.getRemoteProperties(txn, c, TorConstants.ID);
+		return singletonMap(TorConstants.ID, p);
+	}
+
+	private void sendShardMessage(Transaction txn, Contact custodian,
+			Shard shard) throws DbException, FormatException {
+		GroupId g = getContactGroup(custodian).getId();
+		long timestamp = clock.currentTimeMillis();
+		byte[] body = messageEncoder.encodeShardMessage(shard);
+		Message m = clientHelper.createMessage(g, timestamp, body);
+		BdfDictionary meta = BdfDictionary.of(
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, SHARD.getValue()),
+				new BdfEntry(MSG_KEY_LOCAL, true));
+		clientHelper.addLocalMessage(txn, m, meta, true, false);
+	}
+
+	private void sendBackupMessage(Transaction txn, Contact custodian,
+			int version, BackupPayload payload)
+			throws DbException, FormatException {
+		GroupId g = getContactGroup(custodian).getId();
+		long timestamp = clock.currentTimeMillis();
+		byte[] body = messageEncoder.encodeBackupMessage(version, payload);
+		Message m = clientHelper.createMessage(g, timestamp, body);
+		BdfDictionary meta = BdfDictionary.of(
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, BACKUP.getValue()),
+				new BdfEntry(MSG_KEY_LOCAL, true),
+				new BdfEntry(MSG_KEY_VERSION, version));
+		clientHelper.addLocalMessage(txn, m, meta, true, false);
+	}
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/SocialBackupModule.java

@@ -0,0 +1,96 @@
+package org.briarproject.briar.socialbackup;
+
+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.validation.ValidationManager;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager;
+import org.briarproject.briar.api.socialbackup.SocialBackupManager;
+
+import javax.inject.Inject;
+import javax.inject.Singleton;
+
+import dagger.Module;
+import dagger.Provides;
+
+import static org.briarproject.briar.api.socialbackup.SocialBackupManager.CLIENT_ID;
+import static org.briarproject.briar.api.socialbackup.SocialBackupManager.MAJOR_VERSION;
+import static org.briarproject.briar.api.socialbackup.SocialBackupManager.MINOR_VERSION;
+
+@Module
+public class SocialBackupModule {
+
+	public static class EagerSingletons {
+		@Inject
+		SocialBackupManager socialBackupManager;
+		@Inject
+		SocialBackupValidator socialBackupValidator;
+	}
+
+	@Provides
+	@Singleton
+	SocialBackupManager socialBackupManager(
+			LifecycleManager lifecycleManager,
+			ContactManager contactManager,
+			ValidationManager validationManager,
+			ClientVersioningManager clientVersioningManager,
+			SocialBackupManagerImpl socialBackupManager) {
+		lifecycleManager.registerOpenDatabaseHook(socialBackupManager);
+		contactManager.registerContactHook(socialBackupManager);
+		validationManager.registerIncomingMessageHook(CLIENT_ID,
+				MAJOR_VERSION, socialBackupManager);
+		clientVersioningManager.registerClient(CLIENT_ID, MAJOR_VERSION,
+				MINOR_VERSION, socialBackupManager);
+		return socialBackupManager;
+	}
+
+	@Provides
+	@Singleton
+	SocialBackupValidator socialBackupValidator(
+			ValidationManager validationManager,
+			ClientHelper clientHelper,
+			MetadataEncoder metadataEncoder,
+			Clock clock) {
+		SocialBackupValidator validator =
+				new SocialBackupValidator(clientHelper, metadataEncoder, clock);
+		validationManager.registerMessageValidator(CLIENT_ID, MAJOR_VERSION,
+				validator);
+		return validator;
+	}
+
+	@Provides
+	BackupMetadataParser backupMetadataParser(
+			BackupMetadataParserImpl backupMetadataParser) {
+		return backupMetadataParser;
+	}
+
+	@Provides
+	BackupMetadataEncoder backupMetadataEncoder(
+			BackupMetadataEncoderImpl backupMetadataEncoder) {
+		return backupMetadataEncoder;
+	}
+
+	@Provides
+	BackupPayloadEncoder backupPayloadEncoder(
+			BackupPayloadEncoderImpl backupPayloadEncoder) {
+		return backupPayloadEncoder;
+	}
+
+	@Provides
+	MessageEncoder messageEncoder(MessageEncoderImpl messageEncoder) {
+		return messageEncoder;
+	}
+
+	@Provides
+	MessageParser messageParser(MessageParserImpl messageParser) {
+		return messageParser;
+	}
+
+	@Provides
+	DarkCrystal darkCrystal(DarkCrystalStub darkCrystal) {
+		// TODO: Replace this with a real implementation
+		return darkCrystal;
+	}
+}

briar-core/src/main/java/org/briarproject/briar/socialbackup/SocialBackupValidator.java

@@ -0,0 +1,80 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.FormatException;
+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.BdfEntry;
+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.system.Clock;
+
+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.socialbackup.MessageType.BACKUP;
+import static org.briarproject.briar.socialbackup.MessageType.SHARD;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.MAX_SHARD_BYTES;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.MSG_KEY_LOCAL;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.MSG_KEY_VERSION;
+import static org.briarproject.briar.socialbackup.SocialBackupConstants.SECRET_ID_BYTES;
+
+@Immutable
+@NotNullByDefault
+class SocialBackupValidator extends BdfMessageValidator {
+
+	@Inject
+	SocialBackupValidator(ClientHelper clientHelper,
+			MetadataEncoder metadataEncoder, Clock clock) {
+		super(clientHelper, metadataEncoder, clock);
+	}
+
+	@Override
+	protected BdfMessageContext validateMessage(Message m, Group g,
+			BdfList body) throws FormatException {
+		MessageType type = MessageType.fromValue(body.getLong(0).intValue());
+		if (type == SHARD) return validateShardMessage(body);
+		else if (type == BACKUP) return validateBackupMessage(body);
+		else throw new AssertionError();
+	}
+
+	private BdfMessageContext validateShardMessage(BdfList body)
+			throws FormatException {
+		// Message type, secret ID, num shards, threshold, shard
+		checkSize(body, 5);
+		byte[] secretId = body.getRaw(1);
+		checkLength(secretId, SECRET_ID_BYTES);
+		int numShards = body.getLong(2).intValue();
+		if (numShards < 2) throw new FormatException();
+		int threshold = body.getLong(3).intValue();
+		if (threshold < 2) throw new FormatException();
+		if (threshold > numShards) throw new FormatException();
+		byte[] shard = body.getRaw(4);
+		checkLength(shard, 1, MAX_SHARD_BYTES);
+		BdfDictionary meta = BdfDictionary.of(
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, SHARD.getValue()),
+				new BdfEntry(MSG_KEY_LOCAL, false));
+		return new BdfMessageContext(meta);
+	}
+
+	private BdfMessageContext validateBackupMessage(BdfList body)
+			throws FormatException {
+		// Message type, version, backup payload
+		checkSize(body, 3);
+		int version = body.getLong(1).intValue();
+		if (version < 0) throw new FormatException();
+		byte[] payload = body.getRaw(2);
+		BdfDictionary meta = BdfDictionary.of(
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, BACKUP.getValue()),
+				new BdfEntry(MSG_KEY_LOCAL, false),
+				new BdfEntry(MSG_KEY_VERSION, version));
+		return new BdfMessageContext(meta);
+	}
+}

briar-core/src/test/java/org/briarproject/briar/socialbackup/SocialBackupIntegrationTest.java

@@ -0,0 +1,75 @@
+package org.briarproject.briar.socialbackup;
+
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.test.TestDatabaseConfigModule;
+import org.briarproject.briar.api.socialbackup.BackupMetadata;
+import org.briarproject.briar.api.socialbackup.SocialBackupManager;
+import org.briarproject.briar.test.BriarIntegrationTest;
+import org.briarproject.briar.test.BriarIntegrationTestComponent;
+import org.briarproject.briar.test.DaggerBriarIntegrationTestComponent;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.List;
+
+import static java.util.Arrays.asList;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+
+public class SocialBackupIntegrationTest
+		extends BriarIntegrationTest<BriarIntegrationTestComponent> {
+
+	private SocialBackupManager socialBackupManager0;
+
+	@Before
+	@Override
+	public void setUp() throws Exception {
+		super.setUp();
+		socialBackupManager0 = c0.getSocialBackupManager();
+	}
+
+	@Override
+	protected void createComponents() {
+		BriarIntegrationTestComponent component =
+				DaggerBriarIntegrationTestComponent.builder().build();
+		BriarIntegrationTestComponent.Helper.injectEagerSingletons(component);
+		component.inject(this);
+
+		c0 = DaggerBriarIntegrationTestComponent.builder()
+				.testDatabaseConfigModule(new TestDatabaseConfigModule(t0Dir))
+				.build();
+		BriarIntegrationTestComponent.Helper.injectEagerSingletons(c0);
+
+		c1 = DaggerBriarIntegrationTestComponent.builder()
+				.testDatabaseConfigModule(new TestDatabaseConfigModule(t1Dir))
+				.build();
+		BriarIntegrationTestComponent.Helper.injectEagerSingletons(c1);
+
+		c2 = DaggerBriarIntegrationTestComponent.builder()
+				.testDatabaseConfigModule(new TestDatabaseConfigModule(t2Dir))
+				.build();
+		BriarIntegrationTestComponent.Helper.injectEagerSingletons(c2);
+	}
+
+	@Test
+	public void testCreateBackup() throws Exception {
+		// Create the backup
+		db0.transaction(false, txn -> {
+			assertNull(socialBackupManager0.getBackupMetadata(txn));
+			socialBackupManager0.createBackup(txn,
+					asList(contactId1From0, contactId2From0), 2);
+			BackupMetadata backupMetadata =
+					socialBackupManager0.getBackupMetadata(txn);
+			assertNotNull(backupMetadata);
+			List<Author> expected = asList(contact1From0.getAuthor(),
+					contact2From0.getAuthor());
+			assertEquals(expected, backupMetadata.getCustodians());
+			assertEquals(2, backupMetadata.getThreshold());
+			assertEquals(0, backupMetadata.getVersion());
+		});
+		// Sync the shard and backup messages to the contacts
+		sync0To1(2, true);
+		sync0To2(2, true);
+	}
+}

briar-core/src/test/java/org/briarproject/briar/test/BriarIntegrationTestComponent.java

@@ -27,6 +27,7 @@ import org.briarproject.briar.api.messaging.MessagingManager;
 import org.briarproject.briar.api.messaging.PrivateMessageFactory;
 import org.briarproject.briar.api.privategroup.PrivateGroupManager;
 import org.briarproject.briar.api.privategroup.invitation.GroupInvitationManager;
+import org.briarproject.briar.api.socialbackup.SocialBackupManager;
 import org.briarproject.briar.avatar.AvatarModule;
 import org.briarproject.briar.blog.BlogModule;
 import org.briarproject.briar.forum.ForumModule;
@@ -36,6 +37,7 @@ import org.briarproject.briar.messaging.MessagingModule;
 import org.briarproject.briar.privategroup.PrivateGroupModule;
 import org.briarproject.briar.privategroup.invitation.GroupInvitationModule;
 import org.briarproject.briar.sharing.SharingModule;
+import org.briarproject.briar.socialbackup.SocialBackupModule;
 
 import javax.inject.Singleton;
 
@@ -70,6 +72,8 @@ public interface BriarIntegrationTestComponent
 
 	void inject(SharingModule.EagerSingletons init);
 
+	void inject(SocialBackupModule.EagerSingletons init);
+
 	LifecycleManager getLifecycleManager();
 
 	EventBus getEventBus();
@@ -116,6 +120,8 @@ public interface BriarIntegrationTestComponent
 
 	ConnectionManager getConnectionManager();
 
+	SocialBackupManager getSocialBackupManager();
+
 	class Helper {
 
 		public static void injectEagerSingletons(
@@ -131,6 +137,7 @@ public interface BriarIntegrationTestComponent
 			c.inject(new MessagingModule.EagerSingletons());
 			c.inject(new PrivateGroupModule.EagerSingletons());
 			c.inject(new SharingModule.EagerSingletons());
+			c.inject(new SocialBackupModule.EagerSingletons());
 		}
 	}
 }