sign_len method
Implementation
Uint8List sign_len(Uint8List message, final int moff, final int mlen) {
// check message
if (!(message != null && message.length >= (moff + mlen))) return null;
// signed message
Uint8List sm = Uint8List(mlen + signatureLength);
TweetNaclFast.crypto_sign(sm, -1, message, moff, mlen, _mySecretKey);
return sm;
}