New Ideas on a New Old Type of Cipher:The Mixed-Radix One-Time Pad

Abstract

In a short 2012 preprint, an unconventional cipher was introduced, now in this note we take that representational core, formalize it, and use it as the basis for a clean generalization of the one-time pad to non-uniform bases, which we call the Mixed-Radix One-Time Pad (MR-OTP). And we prove that the MR-OTP achieves Shannon perfect secrecy, show that the classical binary OTP is exactly the all-bases-equal-2 special case, and that fixed-base variants recover OTPs over arbitrary alphabets. We then examine whether secret bases can lower the key entropy required for perfect secrecy (they cannot). We close with a usable session protocol based on key rolling that preserves perfect secrecy, and with an honest account of the open problems.