Pontifex is the name used for the Solitaire algorithm devised by Bruce Schneier of Counterpane Systems in Neal Stephenson's novel Cryptonomicon. There is a more elaborate description available.

There were some errors with the original script presented. Here is the cleaned up version in CGI form. It should be trivial to revert to a command line version.

There are a few limitations: A relatively short message length, and only letters are encrypted.