Implement RSA-based OTP (SecurID)

[Streep]

It would be nice if RSA multi-factor authentication OTP keys could be implemented, such as from the widely deployed OTP RSA Authenticator app (formerly "SecurID").

The nice work here can possibly be used for a straightforward implementation: https://github.com/stoken-dev/stoken (written in C, so the important bits need to be ported).

Additional inspiration can possibly be drawn from https://github.com/privacyidea (also has an RSA-implementation, server-side, I believe).