feat: Allow initiating multiple sessions for a message

lib/src/omemo/sessionmanager.dart

@@ -128,7 +128,7 @@ class OmemoSessionManager {
   
   /// Encrypt the key [plaintext] for all known bundles of [jid]. Returns a map that
   /// maps the Bundle Id to the ciphertext of [plaintext].
-  Future<EncryptionResult> encryptToJid(String jid, String plaintext, { OmemoBundle? newSession }) async {
+  Future<EncryptionResult> encryptToJid(String jid, String plaintext, { List<OmemoBundle>? newSessions }) async {
     final encryptedKeys = List<EncryptedKey>.empty(growable: true);
 
     // Generate the key and encrypt the plaintext
@@ -142,9 +142,11 @@ class OmemoSessionManager {
     final hmac = await truncatedHmac(ciphertext, keys.authenticationKey);
     final concatKey = concat([key, hmac]);
 
-    OmemoKeyExchange? kex;
+    final kex = <int, OmemoKeyExchange>{};
-    if (newSession != null) {
+    if (newSessions != null) {
-      kex = await addSessionFromBundle(jid, newSession.id, newSession);
+      for (final newSession in newSessions) {
+        kex[newSession.id] = await addSessionFromBundle(jid, newSession.id, newSession);
+      }
     }
     
     await _lock.synchronized(() async {
@@ -153,12 +155,13 @@ class OmemoSessionManager {
         final ratchet = _ratchetMap[deviceId]!;
         final ciphertext = (await ratchet.ratchetEncrypt(concatKey)).ciphertext;
 
-        if (kex != null && deviceId == newSession?.id) {
+        if (kex.isNotEmpty && kex.containsKey(deviceId)) {
-          kex.message = OmemoAuthenticatedMessage.fromBuffer(ciphertext);
+          final k = kex[deviceId]!
+            ..message = OmemoAuthenticatedMessage.fromBuffer(ciphertext);
           encryptedKeys.add(
             EncryptedKey(
               deviceId,
-              base64.encode(kex.writeToBuffer()),
+              base64.encode(k.writeToBuffer()),
               true,
             ),
           );

test/omemo_test.dart

@@ -15,7 +15,9 @@ void main() {
     final aliceMessage = await aliceSession.encryptToJid(
       bobJid,
       messagePlaintext,
-      newSession: await bobSession.device.toBundle(),
+      newSessions: [
+        await bobSession.device.toBundle(),
+      ],
     );
     expect(aliceMessage.encryptedKeys.length, 1);
 
@@ -32,7 +34,63 @@ void main() {
     expect(messagePlaintext, bobMessage);
 
     // Bob responds to Alice
-    final bobResponseText = 'Oh, hello Alice!';
+    const bobResponseText = 'Oh, hello Alice!';
+    final bobResponseMessage = await bobSession.encryptToJid(
+      aliceJid,
+      bobResponseText,
+    );
+
+    // Bob sends the message to Alice
+    // ...
+
+    // Alice decrypts it
+    final aliceReceivedMessage = await aliceSession.decryptMessage(
+      bobResponseMessage.ciphertext,
+      bobJid,
+      bobSession.device.id,
+      bobResponseMessage.encryptedKeys,
+    );
+    expect(bobResponseText, aliceReceivedMessage);
+  });
+
+  test('Test using OMEMO sessions with only two devices for the receiver', () async {
+    const aliceJid = 'alice@server.example';
+    const bobJid = 'bob@other.server.example';
+      
+    // Alice and Bob generate their sessions
+    final aliceSession = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
+    final bobSession = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
+    // Bob's other device
+    final bobSession2 = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
+
+    // Alice encrypts a message for Bob
+    const messagePlaintext = 'Hello Bob!';
+    final aliceMessage = await aliceSession.encryptToJid(
+      bobJid,
+      messagePlaintext,
+      newSessions: [
+        await bobSession.device.toBundle(),
+        await bobSession2.device.toBundle(),
+      ],
+    );
+    expect(aliceMessage.encryptedKeys.length, 2);
+    expect(aliceMessage.encryptedKeys[0].kex, true);
+    expect(aliceMessage.encryptedKeys[1].kex, true);
+
+    // Alice sends the message to Bob
+    // ...
+
+    // Bob decrypts it
+    final bobMessage = await bobSession.decryptMessage(
+      aliceMessage.ciphertext,
+      aliceJid,
+      aliceSession.device.id,
+      aliceMessage.encryptedKeys,
+    );
+    expect(messagePlaintext, bobMessage);
+
+    // Bob responds to Alice
+    const bobResponseText = 'Oh, hello Alice!';
     final bobResponseMessage = await bobSession.encryptToJid(
       aliceJid,
       bobResponseText,