stretch method
Implementation
Key stretch(int desiredKeyLength,
{int iterationCount = 100, Uint8List? salt}) {
salt ??= _SecureRandom(desiredKeyLength).bytes;
final params = Pbkdf2Parameters(salt, iterationCount, desiredKeyLength);
final pbkdf2 = PBKDF2KeyDerivator(Mac('SHA-1/HMAC'))..init(params);
return Key(pbkdf2.process(_bytes));
}