Minor edit ...1111111112. The quest for a human-computable cipher goes back hundreds of years. In fact, it predates the computer era. The Playfair cipher was used by the British during Empire days as a military cipher in the field. It used a key->letterbox method similar to APGs, with a 5x5 letterbox. The radix-rearrangement idea also has a long history. One cute idea mapped the alphabet into a 3x3x3 cube, and mixed the coordinates of the letters with neighboring letters of the message. Of course, adding the requirement that the cipher resist computer attack, makes the problem harder. There's an important practical application: Making a password system where the server challenges the human client, who must compute a response based on his password and the challenge. If the response is human-computable, then the password is never entrusted to the (possibly compromised) user machine. The method must resist an eavesdropper who learns several challenge-response pairs. Rich ----- Quoting "Adam P. Goucher" <apgoucher@gmx.com>:

That's rather well-known. In the same vein:

"Half of all people understand 3-adic numbers, and the other ...1111111111 do not."

Sincerely,

Adam P. Goucher _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun

q> Mr Baker: A little behind the times, are we? This joke was popular q> maybe 10 years ago! Base 23. :) -JimC -- James Cloos <cloos@jhcloos.com> OpenPGP: 1024D/ED7DAEA6