Don't use ClientId.toString() for signature labels.

bramble-api/src/main/java/org/briarproject/bramble/api/sync/ClientId.java

@@ -36,4 +36,8 @@ public class ClientId implements Comparable<ClientId> {
 		return id.hashCode();
 	}
 
+	@Override
+	public String toString() {
+		return id;
+	}
 }

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

@@ -17,8 +17,8 @@ import static org.briarproject.briar.api.blog.BlogManager.CLIENT_ID;
 @NotNullByDefault
 public interface BlogPostFactory {
 
-	String SIGNING_LABEL_POST = CLIENT_ID + "/POST";
-	String SIGNING_LABEL_COMMENT = CLIENT_ID + "/COMMENT";
+	String SIGNING_LABEL_POST = CLIENT_ID.getString() + "/POST";
+	String SIGNING_LABEL_COMMENT = CLIENT_ID.getString() + "/COMMENT";
 
 	BlogPost createBlogPost(GroupId groupId, long timestamp,
 			@Nullable MessageId parent, LocalAuthor author, String body)

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

@@ -16,7 +16,7 @@ import static org.briarproject.briar.api.forum.ForumManager.CLIENT_ID;
 @NotNullByDefault
 public interface ForumPostFactory {
 
-	String SIGNING_LABEL_POST = CLIENT_ID + "/POST";
+	String SIGNING_LABEL_POST = CLIENT_ID.getString() + "/POST";
 
 	@CryptoExecutor
 	ForumPost createPost(GroupId groupId, long timestamp,

briar-api/src/main/java/org/briarproject/briar/api/privategroup/GroupMessageFactory.java

@@ -13,8 +13,8 @@ import static org.briarproject.briar.api.privategroup.PrivateGroupManager.CLIENT
 @NotNullByDefault
 public interface GroupMessageFactory {
 
-	String SIGNING_LABEL_JOIN = CLIENT_ID + "/JOIN";
-	String SIGNING_LABEL_POST = CLIENT_ID + "/POST";
+	String SIGNING_LABEL_JOIN = CLIENT_ID.getString() + "/JOIN";
+	String SIGNING_LABEL_POST = CLIENT_ID.getString() + "/POST";
 
 	/**
 	 * Creates a join announcement message for the creator of a group.

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

@@ -12,7 +12,7 @@ import static org.briarproject.briar.api.privategroup.invitation.GroupInvitation
 @NotNullByDefault
 public interface GroupInvitationFactory {
 
-	String SIGNING_LABEL_INVITE = CLIENT_ID + "/INVITE";
+	String SIGNING_LABEL_INVITE = CLIENT_ID.getString() + "/INVITE";
 
 	/**
 	 * Returns a signature to include when inviting a member to join a private

briar-core/src/main/java/org/briarproject/briar/introduction/IntroduceeManager.java

@@ -98,7 +98,8 @@ class IntroduceeManager {
 	private static final Logger LOG =
 			Logger.getLogger(IntroduceeManager.class.getName());
 
-	static final String SIGNING_LABEL_RESPONSE = CLIENT_ID + "/RESPONSE";
+	static final String SIGNING_LABEL_RESPONSE =
+			CLIENT_ID.getString() + "/RESPONSE";
 
 	private final MessageSender messageSender;
 	private final DatabaseComponent db;