decrypt function
Implementation
Future<List<int>> decrypt(B64 key, String box) {
var parts = box.split(".");
if (parts.length != 3) {
throw ExInvalidCipherText;
}
var nonce = B64(parts[0]).decode();
var mac = B64(parts[1]).decode();
var text = B64(parts[2]).decode();
return cryptography.Xchacha20.poly1305Aead().decrypt(
cryptography.SecretBox(text.toList(),
nonce: nonce.toList(), mac: cryptography.Mac(mac.toList())),
secretKey: cryptography.SecretKey(key.decode().toList()));
}