Securing and authenticating messages is easy thanks to computers. Lately I've been thinking about something like Vigenère cipher with a OTP, how secure it is, and how easy it is to implement without the aid of computers. However, one thing it doesn't provide is any authentication. Decrypting a Vigenère cipher with pen and paper is trivial, but hand calculating HMAC-SHA256 is not something I'd like to attempt. Are there any authentication schemes which are simple; easy to compute without the aid of a computer? Any chance any of those schemes actually provide decent security?