feat: Track the old Signed Prekey after rotation

lib/src/omemo/device.dart

@@ -10,7 +10,18 @@ import 'package:omemo_dart/src/x3dh/x3dh.dart';
 @immutable
 class Device {
 
-  const Device(this.jid, this.id, this.ik, this.spk, this.spkId, this.spkSignature, this.opks);
+  const Device(
+    this.jid,
+    this.id,
+    this.ik,
+    this.spk,
+    this.spkId,
+    this.spkSignature,
+    this.oldSpk,
+    this.oldSpkId,
+    this.oldSpkSignature,
+    this.opks,
+  );
 
   /// Deserialize the Device
   factory Device.fromJson(Map<String, dynamic> data) {
@@ -27,6 +38,10 @@ class Device {
       'spk_pub': 'base/64/encoded',
       'spk_id': 123,
       'spk_sig': 'base/64/encoded',
+      'old_spk': 'base/64/encoded',
+      'old_spk_pub': 'base/64/encoded',
+      'old_spk_id': 122,
+      'old_ spk_sig': 'base/64/encoded',
       'opks': [
         {
           'id': 0,
@@ -60,6 +75,13 @@ class Device {
       ),
       data['spk_id']! as int,
       base64.decode(data['spk_sig']! as String),
+      decodeKeyPairIfNotNull(
+        data['old_spk_pub'] as String?,
+        data['old_spk'] as String?,
+        KeyPairType.x25519,
+      ),
+      data['old_spk_id'] as int?,
+      base64DecodeIfNotNull(data, 'old_spk_sig'),
       opks,
     );
   }
@@ -77,7 +99,7 @@ class Device {
       opks[i] = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
     }
 
-    return Device(jid, id, ik, spk, spkId, signature, opks);
+    return Device(jid, id, ik, spk, spkId, signature, null, null, null, opks);
   }
 
   /// Our bare Jid
@@ -96,6 +118,13 @@ class Device {
   /// ...and its signature
   final List<int> spkSignature;
 
+  /// The old Signed Prekey...
+  final OmemoKeyPair? oldSpk;
+  /// its Id, ...
+  final int? oldSpkId;
+  /// ...and its signature
+  final List<int>? oldSpkSignature;
+
   /// Map of an id to the associated Onetime-Prekey
   final Map<int, OmemoKeyPair> opks;
 
@@ -112,6 +141,9 @@ class Device {
       spk,
       spkId,
       spkSignature,
+      oldSpk,
+      oldSpkId,
+      oldSpkSignature,
       opks,
     );
   }
@@ -131,6 +163,9 @@ class Device {
       newSpk,
       newSpkId,
       newSignature,
+      spk,
+      spkId,
+      spkSignature,
       opks,
     );
   }
@@ -175,6 +210,10 @@ class Device {
       'spk_pub': base64.encode(await spk.pk.getBytes()),
       'spk_id': spkId,
       'spk_sig': base64.encode(spkSignature),
+      'old_spk': base64EncodeIfNotNull(await oldSpk?.sk.getBytes()),
+      'old_spk_pub': base64EncodeIfNotNull(await oldSpk?.pk.getBytes()),
+      'old_spk_id': oldSpkId,
+      'old_spk_sig': base64EncodeIfNotNull(oldSpkSignature),
       'opks': serialisedOpks,
     };
   }
@@ -198,12 +237,18 @@ class Device {
     final ikMatch = await ik.equals(other.ik);
     // ignore: invalid_use_of_visible_for_testing_member
     final spkMatch = await spk.equals(other.spk);
+    // ignore: invalid_use_of_visible_for_testing_member
+    final oldSpkMatch = oldSpk != null ? await oldSpk!.equals(other.oldSpk!) : other.oldSpk == null;
+    final oldSpkSigsMatch = oldSpkSignature != null ? listsEqual(oldSpkSignature!, other.oldSpkSignature!) : other.oldSpkSignature == null;
     return id == other.id &&
       ikMatch &&
       spkMatch &&
+      oldSpkMatch &&
       jid == other.jid &&
       listsEqual(spkSignature, other.spkSignature) &&
       spkId == other.spkId &&
+      oldSpkId == other.oldSpkId &&
+      oldSpkSigsMatch &&
       opksMatch;
   }
 }