Add support for comments and reblogging to Blog Client

Comments and reblogs need to depend on the post they refer to.
Since message dependencies are limited to one group,
the post and also the comments need to be wrapped
when commented on or reblogged to another blog.

For this reason, in addition to comments, two new wrapping message types
are introduced. They retain all data of the original messages and allow
for reconstruction and signature verification.

This commit breaks backwards compatibility with old blog posts.
It removes the content type, title and parent ID from the post
message structure.

briar-core/src/org/briarproject/clients/ClientHelperImpl.java

@@ -2,6 +2,10 @@ package org.briarproject.clients;
 
 import org.briarproject.api.FormatException;
 import org.briarproject.api.clients.ClientHelper;
+import org.briarproject.api.crypto.CryptoComponent;
+import org.briarproject.api.crypto.KeyParser;
+import org.briarproject.api.crypto.PrivateKey;
+import org.briarproject.api.crypto.Signature;
 import org.briarproject.api.data.BdfDictionary;
 import org.briarproject.api.data.BdfList;
 import org.briarproject.api.data.BdfReader;
@@ -23,6 +27,7 @@ import org.briarproject.api.sync.MessageId;
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.security.GeneralSecurityException;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
@@ -40,18 +45,20 @@ class ClientHelperImpl implements ClientHelper {
 	private final BdfWriterFactory bdfWriterFactory;
 	private final MetadataParser metadataParser;
 	private final MetadataEncoder metadataEncoder;
+	private final CryptoComponent cryptoComponent;
 
 	@Inject
 	ClientHelperImpl(DatabaseComponent db, MessageFactory messageFactory,
 			BdfReaderFactory bdfReaderFactory,
 			BdfWriterFactory bdfWriterFactory, MetadataParser metadataParser,
-			MetadataEncoder metadataEncoder) {
+			MetadataEncoder metadataEncoder, CryptoComponent cryptoComponent) {
 		this.db = db;
 		this.messageFactory = messageFactory;
 		this.bdfReaderFactory = bdfReaderFactory;
 		this.bdfWriterFactory = bdfWriterFactory;
 		this.metadataParser = metadataParser;
 		this.metadataEncoder = metadataEncoder;
+		this.cryptoComponent = cryptoComponent;
 	}
 
 	@Override
@@ -303,4 +310,21 @@ class ClientHelperImpl implements ClientHelper {
 			throw new RuntimeException(e);
 		}
 	}
+
+	@Override
+	public BdfList toList(byte[] b) throws FormatException {
+		return toList(b, 0, b.length);
+	}
+
+	@Override
+	public byte[] sign(BdfList toSign, byte[] privateKey)
+			throws FormatException, GeneralSecurityException {
+		Signature signature = cryptoComponent.getSignature();
+		KeyParser keyParser = cryptoComponent.getSignatureKeyParser();
+		PrivateKey key =
+				keyParser.parsePrivateKey(privateKey);
+		signature.initSign(key);
+		signature.update(toByteArray(toSign));
+		return signature.sign();
+	}
 }

briar-core/src/org/briarproject/clients/ClientsModule.java

@@ -26,9 +26,10 @@ public class ClientsModule {
 	ClientHelper provideClientHelper(DatabaseComponent db,
 			MessageFactory messageFactory, BdfReaderFactory bdfReaderFactory,
 			BdfWriterFactory bdfWriterFactory, MetadataParser metadataParser,
-			MetadataEncoder metadataEncoder) {
+			MetadataEncoder metadataEncoder, CryptoComponent cryptoComponent) {
 		return new ClientHelperImpl(db, messageFactory, bdfReaderFactory,
-				bdfWriterFactory, metadataParser, metadataEncoder);
+				bdfWriterFactory, metadataParser, metadataEncoder,
+				cryptoComponent);
 	}
 
 	@Provides