add automatic session completion

app/src/main/java/im/conversations/android/axolotl/AxolotlService.java

@@ -96,6 +96,9 @@ public class AxolotlService extends AbstractAccountService {
             throws NoSessionException {
         final var session = getExistingSession(axolotlAddress);
         if (session == null) {
+            // TODO When receiving a message that is not an OMEMOKeyExchange from a device there is
+            // no session with, clients SHOULD create a session with that device and notify it about
+            // the new session by responding with an empty OMEMO message as per Sending a message.
             throw new NoSessionException(
                     String.format("No session for %s", axolotlAddress.toString()));
         }
@@ -166,15 +169,15 @@ public class AxolotlService extends AbstractAccountService {
                             new AxolotlAddress(from.asBareJid(), header.getSourceDevice().get()));
             keyWithAuthTag = session.sessionCipher.decrypt(signalMessage);
         }
-        if (keyWithAuthTag.length < 32) {
-            throw new OutdatedSenderException(
-                    "Key did not contain auth tag. Sender needs to update their OMEMO client");
-        }
         final var inDeviceList = database.axolotlDao().hasDeviceId(account, session.axolotlAddress);
         if (payload == null) {
             return new AxolotlPayload(
                     session.axolotlAddress, session.identityKey, preKeyMessage, inDeviceList, null);
         }
+        if (keyWithAuthTag.length < 32) {
+            throw new OutdatedSenderException(
+                    "Key did not contain auth tag. Sender needs to update their OMEMO client");
+        }
         final byte[] key = new byte[16];
         final byte[] authTag = new byte[16];
         final byte[] iv = header.getIv();

app/src/main/java/im/conversations/android/axolotl/EncryptionBuilder.java

@@ -31,7 +31,7 @@ public class EncryptionBuilder {
 
     private Long sourceDeviceId;
 
-    private ArrayList<AxolotlSession> sessions;
+    private final ArrayList<AxolotlSession> sessions = new ArrayList<>();
 
     private byte[] payload;
 
@@ -76,6 +76,33 @@ public class EncryptionBuilder {
         return encrypted;
     }
 
+    public Encrypted buildKeyTransport() throws AxolotlEncryptionException {
+        try {
+            return buildKeyTransportOrThrow();
+        } catch (final UntrustedIdentityException e) {
+            throw new AxolotlEncryptionException(e);
+        }
+    }
+
+    private Encrypted buildKeyTransportOrThrow() throws UntrustedIdentityException {
+        final long sourceDeviceId =
+                Preconditions.checkNotNull(this.sourceDeviceId, "Specify a source device id");
+        Preconditions.checkState(
+                this.payload == null, "A key transport message should not have a payload");
+        // TODO key transport messages in twomemo (omemo:1) use 32 bytes of zeros instead of a key
+        // TODO if we are not using this using this for actual key transport we can do this in siacs
+        // omemo too (and get rid of the IV)
+        final var sessions = ImmutableList.copyOf(this.sessions);
+        final var key = generateKey();
+        final var iv = generateIv();
+        final var header = buildHeader(sessions, key);
+        header.addIv(iv);
+        header.setSourceDevice(sourceDeviceId);
+        final var encrypted = new Encrypted();
+        encrypted.addExtension(header);
+        return encrypted;
+    }
+
     public EncryptionBuilder payload(final String payload) {
         this.payload = payload.getBytes(StandardCharsets.UTF_8);
         return this;
@@ -97,6 +124,7 @@ public class EncryptionBuilder {
         for (final AxolotlSession session : sessions) {
             final var cipherMessage = session.sessionCipher.encrypt(keyWithAuthTag);
             final var key = header.addExtension(new Key());
+            key.setRemoteDeviceId(session.axolotlAddress.getDeviceId());
             key.setContent(cipherMessage.serialize());
             key.setIsPreKey(cipherMessage.getType() == CiphertextMessage.PREKEY_TYPE);
         }

app/src/main/java/im/conversations/android/xmpp/manager/AxolotlManager.java

@@ -35,6 +35,7 @@ import im.conversations.android.xmpp.model.axolotl.Bundle;
 import im.conversations.android.xmpp.model.axolotl.DeviceList;
 import im.conversations.android.xmpp.model.axolotl.Encrypted;
 import im.conversations.android.xmpp.model.pubsub.Items;
+import im.conversations.android.xmpp.model.stanza.Message;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Locale;
@@ -503,7 +504,40 @@ public class AxolotlManager extends AbstractManager implements AxolotlService.Po
                     "fresh session from {}/{}",
                     axolotlAddress.getJid(),
                     axolotlAddress.getDeviceId());
+            // After receiving an OMEMOKeyExchange and successfully building a new session, the
+            // receiving device SHOULD automatically respond with an empty OMEMO message (as per
+            // Sending a message) to the source of the OMEMOKeyExchange. This is to notify the
+            // device that the session initiation was completed successfully and that the device can
+            // stop sending OMEMOKeyExchanges.
+            sendKeyTransportToCompleteSession(axolotlAddress);
         }
+        // TODO republish device bundle
+    }
+
+    private void sendKeyTransportToCompleteSession(final AxolotlAddress axolotlAddress) {
+        final var existingSession = axolotlService.getExistingSession(axolotlAddress);
+        if (existingSession == null) {
+            return;
+        }
+        final Encrypted encrypted;
+        try {
+            encrypted =
+                    new EncryptionBuilder()
+                            .session(existingSession)
+                            .sourceDeviceId(signalProtocolStore().getLocalRegistrationId())
+                            .buildKeyTransport();
+        } catch (final AxolotlEncryptionException e) {
+            LOGGER.error("Could not create key transport message to complete session", e);
+            return;
+        }
+        final var message = new Message(Message.Type.NORMAL);
+        message.setTo(axolotlAddress.getJid());
+        message.addExtension(encrypted);
+        LOGGER.info(
+                "Sending KeyTransport Message to {}/{}",
+                axolotlAddress.getJid(),
+                axolotlAddress.getDeviceId());
+        connection.sendMessagePacket(message);
     }
 
     @Override