Paper 2014/257

Handycipher: a Low-tech, Randomized, Symmetric-key Cryptosystem

Bruce Kallick

Abstract

Handycipher is a low-tech, randomized, symmetric-key, stream cipher, simple enough to permit pen-and-paper encrypting and decrypting of messages, while providing a significantly high level of security. It combines a simple 31-character substitution cipher with a 3,045-token nondeterministic homophonic substitution cipher, and employs the insertion of randomly chosen decoy characters at random locations in the ciphertext. A deniable encryption scheme based on the cipher is described, as well as a way of extending the cipher by using randomly generated session keys.

Note: The current version has been further strengthened by by adding another ten characters to the ciphertext alphabet, using a 51-character key instead of 41, increasing the number of null characters from 15 to 25, and by interweaving random non-null "noise" characters in the Core-cipher encryption before the null characters are added.