feat: Implement getting fingerprints

lib/src/double_ratchet/double_ratchet.dart

@@ -1,4 +1,3 @@
-import 'dart:convert';
 import 'package:cryptography/cryptography.dart';
 import 'package:hex/hex.dart';
 import 'package:meta/meta.dart';
@@ -356,6 +355,7 @@ class OmemoDoubleRatchet {
       ..message = headerBytes;
   }
 
+  /// Returns a copy of the ratchet.
   OmemoDoubleRatchet clone() {
     return OmemoDoubleRatchet(
       dhs,
@@ -375,6 +375,15 @@ class OmemoDoubleRatchet {
     );
   }
 
+  /// Computes the fingerprint of the double ratchet, according to
+  /// XEP-0384.
+  Future<String> get fingerprint async {
+    final curveKey = await ik.toCurve25519();
+    return HEX.encode(
+      await curveKey.getBytes(),
+    );
+  }
+
   @visibleForTesting
   Future<bool> equals(OmemoDoubleRatchet other) async {
     final dhrMatch = dhr == null

lib/src/omemo/omemo.dart

@@ -755,8 +755,43 @@ class OmemoManager {
     await _leaveRatchetCriticalSection(jid);
   }
 
-  // TODO
-  Future<List<DeviceFingerprint>?> getFingerprintsForJid(String jid) async => null;
+  /// If ratchets with [jid] exists, returns a list of fingerprints for each
+  /// ratchet.
+  /// 
+  /// If not ratchets exists, returns null.
+  Future<List<DeviceFingerprint>?> getFingerprintsForJid(String jid) async {
+    await _getFingerprintsForJidImpl(jid);
+    final result = await _getFingerprintsForJidImpl(jid);
+    await _leaveRatchetCriticalSection(jid);
+
+    return result;
+  }
+
+  Future<List<DeviceFingerprint>?> _getFingerprintsForJidImpl(String jid) async {
+    // Check if we know of the JID.
+    if (!_deviceList.containsKey(jid)) {
+      return null;
+    }
+
+    final devices = _deviceList[jid]!;
+    final fingerprints = List<DeviceFingerprint>.empty(growable: true);
+    for (final device in devices) {
+      final ratchet = _ratchetMap[RatchetMapKey(jid, device)];
+      if (ratchet == null) {
+        _log.warning('getFingerprintsForJid: Ratchet $jid:$device not found.');
+        continue;
+      }
+
+      fingerprints.add(
+        DeviceFingerprint(
+          device,
+          await ratchet.fingerprint,
+        ),
+      );
+    }
+
+    return fingerprints;
+  }
 
   /// Returns the device used for encryption and decryption.
   Future<OmemoDevice> getDevice() => _deviceLock.synchronized(() => _device);