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