feat: Allow building a session when receiving

lib/src/omemo/sessionmanager.dart

@@ -26,14 +26,15 @@ class EncryptionResult {
 
   /// Mapping of the device Id to the key for decrypting ciphertext, encrypted
   /// for the ratchet with said device Id
-  final Map<int, List<int>> encryptedKeys;
+  final List<EncryptedKey> encryptedKeys;
 }
 
 class EncryptedKey {
 
-  const EncryptedKey(this.rid, this.value);
+  const EncryptedKey(this.rid, this.value, this.kex);
   final int rid;
   final String value;
+  final bool kex;
 }
 
 class OmemoSessionManager {
@@ -127,8 +128,8 @@ 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) async {
-    final encryptedKeys = <int, List<int>>{};
+  Future<EncryptionResult> encryptToJid(String jid, String plaintext, { OmemoBundle? newSession }) async {
+    final encryptedKeys = List<EncryptedKey>.empty(growable: true);
 
     // Generate the key and encrypt the plaintext
     final key = generateRandomBytes(32);
@@ -141,11 +142,35 @@ class OmemoSessionManager {
     final hmac = await truncatedHmac(ciphertext, keys.authenticationKey);
     final concatKey = concat([key, hmac]);
 
+    OmemoKeyExchange? kex;
+    if (newSession != null) {
+      kex = await addSessionFromBundle(jid, newSession.id, newSession);
+    }
+    
     await _lock.synchronized(() async {
       // We assume that the user already checked if the session exists
       for (final deviceId in _deviceMap[jid]!) {
         final ratchet = _ratchetMap[deviceId]!;
-        encryptedKeys[deviceId] = (await ratchet.ratchetEncrypt(concatKey)).ciphertext;
+        final ciphertext = (await ratchet.ratchetEncrypt(concatKey)).ciphertext;
+
+        if (kex != null && deviceId == newSession?.id) {
+          kex.message = OmemoAuthenticatedMessage.fromBuffer(ciphertext);
+          encryptedKeys.add(
+            EncryptedKey(
+              deviceId,
+              base64.encode(kex.writeToBuffer()),
+              true,
+            ),
+          );
+        } else {
+          encryptedKeys.add(
+            EncryptedKey(
+              deviceId,
+              base64.encode(ciphertext),
+              false,
+            ),
+          );
+        }
       }
     });
 
@@ -166,6 +191,22 @@ class OmemoSessionManager {
       throw NotEncryptedForDeviceException();
     }
 
+    final decodedRawKey = base64.decode(rawKey.value);
+    OmemoAuthenticatedMessage authMessage;
+    if (rawKey.kex) {
+      // TODO(PapaTutuWawa): Only do this when we should
+      final kex = OmemoKeyExchange.fromBuffer(decodedRawKey);
+      await addSessionFromKeyExchange(
+        senderJid,
+        senderDeviceId,
+        kex,
+      );
+
+      authMessage = kex.message!;
+    } else {
+      authMessage = OmemoAuthenticatedMessage.fromBuffer(decodedRawKey);
+    }
+    
     final devices = _deviceMap[senderJid];
     if (devices == null) {
       throw NoDecryptionKeyException();
@@ -174,12 +215,15 @@ class OmemoSessionManager {
       throw NoDecryptionKeyException();
     }
 
-    final decodedRawKey = base64.decode(rawKey.value);
-    final authMessage = OmemoAuthenticatedMessage.fromBuffer(decodedRawKey);
     final message = OmemoMessage.fromBuffer(authMessage.message!);
     
     final ratchet = _ratchetMap[senderDeviceId]!;
-    final keyAndHmac = await ratchet.ratchetDecrypt(message, decodedRawKey);
+    List<int> keyAndHmac;
+    if (rawKey.kex) {
+      keyAndHmac = await ratchet.ratchetDecrypt(message, authMessage.writeToBuffer());
+    } else {
+      keyAndHmac = await ratchet.ratchetDecrypt(message, decodedRawKey);
+    }
     final key = keyAndHmac.sublist(0, 32);
     final hmac = keyAndHmac.sublist(32, 48);
     final derivedKeys = await deriveEncryptionKeys(key, omemoPayloadInfoString);

test/omemo_test.dart

@@ -1,4 +1,3 @@
-import 'dart:convert';
 import 'package:omemo_dart/omemo_dart.dart';
 import 'package:test/test.dart';
 
@@ -11,21 +10,13 @@ void main() {
     final aliceSession = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
     final bobSession = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
 
-    // Perform the X3DH
-    final kex = await aliceSession.addSessionFromBundle(
-      bobJid,
-      bobSession.device.id,
-      await bobSession.device.toBundle(),
-    );
-    await bobSession.addSessionFromKeyExchange(
-      aliceJid,
-      aliceSession.device.id,
-      kex,
-    );
-
     // Alice encrypts a message for Bob
     const messagePlaintext = 'Hello Bob!';
-    final aliceMessage = await aliceSession.encryptToJid(bobJid, messagePlaintext);
+    final aliceMessage = await aliceSession.encryptToJid(
+      bobJid,
+      messagePlaintext,
+      newSession: await bobSession.device.toBundle(),
+    );
     expect(aliceMessage.encryptedKeys.length, 1);
 
     // Alice sends the message to Bob
@@ -36,12 +27,7 @@ void main() {
       aliceMessage.ciphertext,
       aliceJid,
       aliceSession.device.id,
-      [
-        EncryptedKey(
-          bobSession.device.id,
-          base64.encode(aliceMessage.encryptedKeys[bobSession.device.id]!),
-        ),
-      ],
+      aliceMessage.encryptedKeys,
     );
 
     expect(messagePlaintext, bobMessage);