feat: Allow serialising Device to Json

lib/src/omemo/device.dart

@@ -12,6 +12,56 @@ class Device {
 
   const Device(this.id, this.ik, this.spk, this.spkId, this.spkSignature, this.opks);
 
+  /// Deserialize the Device
+  factory Device.fromJson(Map<String, dynamic> data) {
+    // NOTE: We use the way OpenSSH names their keys, meaning that ik is the Identity
+    //       Keypair's private key, while ik_pub refers to the Identity Keypair's public
+    //       key.
+    /*
+    {
+      'id': 123,
+      'ik': 'base/64/encoded',
+      'ik_pub': 'base/64/encoded',
+      'spk': 'base/64/encoded',
+      'spk_pub': 'base/64/encoded',
+      'spk_id': 123,
+      'spk_sig': 'base/64/encoded',
+      'opks': [
+        {
+          'id': 0,
+          'public': 'base/64/encoded',
+          'private': 'base/64/encoded'
+        }, ...
+      ]
+    }
+    */
+    final opks = <int, OmemoKeyPair>{};
+    for (final opk in data['opks']! as List<Map<String, dynamic>>) {
+      opks[opk['id']! as int] = OmemoKeyPair.fromBytes(
+        base64.decode(opk['public']! as String),
+        base64.decode(opk['private']! as String),
+        KeyPairType.x25519,
+      );
+    }
+
+    return Device(
+      data['id']! as int,
+      OmemoKeyPair.fromBytes(
+        base64.decode(data['ik_pub']! as String),
+        base64.decode(data['ik']! as String),
+        KeyPairType.ed25519,
+      ),
+      OmemoKeyPair.fromBytes(
+        base64.decode(data['spk_pub']! as String),
+        base64.decode(data['spk']! as String),
+        KeyPairType.x25519,
+      ),
+      data['spk_id']! as int,
+      base64.decode(data['spk_sig']! as String),
+      opks,
+    );
+  }
+  
   /// Generate a completely new device, i.e. cryptographic identity.
   static Future<Device> generateNewDevice({ int opkAmount = 100 }) async {
     final id = generateRandom32BitNumber();
@@ -92,4 +142,28 @@ class Device {
       encodedOpks,
     );
   }
+
+  /// Serialise the device information.
+  Future<Map<String, dynamic>> toJson() async {
+    /// Serialise the OPKs
+    final serialisedOpks = List<Map<String, dynamic>>.empty(growable: true);
+    for (final entry in opks.entries) {
+      serialisedOpks.add({
+        'id': entry.key,
+        'public': base64.encode(await entry.value.pk.getBytes()),
+        'private': base64.encode(await entry.value.sk.getBytes()),
+      });
+    }
+    
+    return {
+      'id': id,
+      'ik': base64.encode(await ik.sk.getBytes()),
+      'ik_pub': base64.encode(await ik.pk.getBytes()),
+      'spk': base64.encode(await spk.sk.getBytes()),
+      'spk_pub': base64.encode(await spk.pk.getBytes()),
+      'spk_id': spkId,
+      'spk_sig': base64.encode(spkSignature),
+      'opks': serialisedOpks,
+    };
+  }
 }