code: func (k SecretKey) Unbox(sender saltpack.BoxPublicKey, nonce saltpack.Nonce, msg []byte) ([]byte, error) {
  	ret, ok := box.Open([]byte{}, msg, (*[24]byte)(&nonce), (*[32]byte)(sender.ToRawBoxKeyPointer()), (*[32]byte)(&k.sec))
  	if !ok {
  		return nil, saltpack.ErrDecryptionFailed
  	}
  	return ret, nil
  }
docs: // Unbox runs the NaCl unbox operation on the given ciphertext and nonce,
  // using the receiver as the secret key.