feat: Add a basic OMEMO session manager

lib/src/omemo/sessionmanager.dart

@@ -0,0 +1,84 @@
+import 'dart:convert';
+import 'package:omemo_dart/src/crypto.dart';
+import 'package:omemo_dart/src/double_ratchet/double_ratchet.dart';
+import 'package:omemo_dart/src/helpers.dart';
+import 'package:synchronized/synchronized.dart';
+
+/// The info used for when encrypting the AES key for the actual payload.
+const omemoPayloadInfoString = 'OMEMO Payload';
+
+class EncryptionResult {
+
+  const EncryptionResult(this.ciphertext, this.encryptedKeys);
+  
+  /// The actual message that was encrypted
+  final List<int> ciphertext;
+
+  /// Mapping of the device Id to the key for decrypting ciphertext, encrypted
+  /// for the ratchet with said device Id
+  final Map<String, List<int>> encryptedKeys;
+}
+
+class OmemoSessionManager {
+
+  OmemoSessionManager() : _ratchetMap = {}, _deviceMap = {}, _lock = Lock();
+
+  /// Lock for _ratchetMap and _bundleMap
+  final Lock _lock;
+  
+  /// Mapping of the Device Id to its OMEMO session
+  final Map<String, OmemoDoubleRatchet> _ratchetMap;
+
+  /// Mapping of a bare Jid to its Device Ids
+  final Map<String, List<String>> _deviceMap;
+
+  /// Add a session [ratchet] with the [deviceId] to the internal tracking state.
+  Future<void> addSession(String jid, String deviceId, OmemoDoubleRatchet ratchet) async {
+    await _lock.synchronized(() async {
+      // Add the bundle Id
+      if (!_deviceMap.containsKey(jid)) {
+        _deviceMap[jid] = [deviceId];
+      } else {
+        _deviceMap[jid]!.add(deviceId);
+      }
+
+      // Add the ratchet session
+      if (!_ratchetMap.containsKey(deviceId)) {
+        _ratchetMap[deviceId] = ratchet;
+      } else {
+        // TODO(PapaTutuWawa): What do we do now?
+        throw Exception();
+      }
+    });
+  }
+  
+  /// 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 = <String, List<int>>{};
+
+    // Generate the key and encrypt the plaintext
+    final key = generateRandomBytes(32);
+    final keys = await deriveEncryptionKeys(key, omemoPayloadInfoString);
+    final ciphertext = await aes256CbcEncrypt(
+      utf8.encode(plaintext),
+      keys.encryptionKey,
+      keys.iv,
+    );
+    final hmac = await truncatedHmac(ciphertext, keys.authenticationKey);
+    final concatKey = concat([keys.encryptionKey, hmac]);
+    
+    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;
+      }
+    });
+
+    return EncryptionResult(
+      ciphertext,
+      encryptedKeys,
+    );
+  }
+}