decryptString method

Implementation

String decryptString(String encryptedText) {
  final encryptedData = base64.decode(encryptedText);

  // Validate minimum length (at least IV = 16 bytes)
  if (encryptedData.length < 16) {
    throw ArgumentError('Invalid encrypted data: too short');
  }

  // Extract IV and encrypted bytes
  final iv = encryptedData.sublist(0, 16);
  final encryptedBytes = encryptedData.sublist(16);

  // Handle empty string case (no encrypted data, just IV)
  if (encryptedBytes.isEmpty) {
    return '';
  }

  // Setup AES cipher in CTR mode
  final cipher = CTRStreamCipher(AESEngine());
  final params = ParametersWithIV(KeyParameter(_key), iv);

  cipher.init(false, params); // false = decrypt

  final decryptedBytes = cipher.process(Uint8List.fromList(encryptedBytes));

  try {
    return utf8.decode(decryptedBytes);
  } catch (e) {
    throw ArgumentError('Failed to decrypt: invalid key or corrupted data');
  }
}