decryptEnvelope method

String decryptEnvelope(Map<String, dynamic> envelope, String receiverId, { String? timestamp, })

Decrypt and verify envelope

Implementation

String decryptEnvelope(
  Map<String, dynamic> envelope,
  String receiverId, {
  String? timestamp,
}) {
  final time = timestamp ?? envelope['timestamp'];
  final key = keyResonance(envelope['senderId'], timestamp: time);
  final now = DateTime.now().toUtc();
  final msgTime = DateTime.parse(envelope['timestamp']);
  final window = envelope['window'] ?? 120;
  final seq = envelope['seq'];
  // Time window check
  if (now.difference(msgTime).inSeconds.abs() > window) {
    throw Exception('Message expired');
  }
  // Replay protection
  final ledgerKey = '${envelope['senderId']}:$receiverId';
  _replayLedger.putIfAbsent(ledgerKey, () => {});
  if (_replayLedger[ledgerKey]!.contains(seq)) {
    throw Exception('Replay detected');
  }
  _replayLedger[ledgerKey]!.add(seq);
  // Seal check
  final expectedSeal = _seal(envelope, key, time);
  if (envelope['seal'] != expectedSeal) {
    throw Exception('Seal verification failed');
  }
  // Decrypt
  final ct = base64.decode(envelope['ciphertext']);
  final pt = _drum(ct, time);
  return utf8.decode(pt);
}