  In the not so unlikely scenario of total compromise of computers accessible
to a group of users, they might be tempted to resort to human-computable
paper-and-pencil cryptographic methods aided by a classic Tabula Recta, which
helps to perform addition and subtraction directly with letters. But do these
classic algorithms, or some new ones using the same simple tools, have any
chance against computer-aided cryptanalysis? In this paper I discuss how some
human-computable algorithms can indeed afford sufficient security in this
situation, drawing conclusions from computer-based statistical analysis. Three
kinds of algorithms are discussed: those that concentrate entropy from shared
text sources, stream ciphers based on arithmetic of non-binary spaces, and
hash-like algorithms that may be used to generate a password from a challenge
text.
