feat: Track the old Signed Prekey after rotation

lib/src/helpers.dart

@@ -1,4 +1,7 @@
+import 'dart:convert';
 import 'dart:math';
+import 'package:cryptography/cryptography.dart';
+import 'package:omemo_dart/src/keys.dart';
 
 /// Flattens [inputs] and concatenates the elements.
 List<int> concat(List<List<int>> inputs) {
@@ -39,3 +42,34 @@ List<int> generateRandomBytes(int length) {
 int generateRandom32BitNumber() {
   return Random.secure().nextInt(4294967295 /*pow(2, 32) - 1*/);
 }
+
+OmemoPublicKey? decodeKeyIfNotNull(Map<String, dynamic> map, String key, KeyPairType type) {
+  if (map[key] == null) return null;
+
+  return OmemoPublicKey.fromBytes(
+    base64.decode(map[key]! as String),
+    type,
+  );
+}
+
+List<int>? base64DecodeIfNotNull(Map<String, dynamic> map, String key) {
+  if (map[key] == null) return null;
+
+  return base64.decode(map[key]! as String);
+}
+
+String? base64EncodeIfNotNull(List<int>? bytes) {
+  if (bytes == null) return null;
+
+  return base64.encode(bytes);
+}
+
+OmemoKeyPair? decodeKeyPairIfNotNull(String? pk, String? sk, KeyPairType type) {
+  if (pk == null || sk == null) return null;
+
+  return OmemoKeyPair.fromBytes(
+    base64.decode(pk),
+    base64.decode(sk),
+    type,
+  );
+}