HOW WE SOLVED THE $100,000 DECIPHER PUZZLE (16 HOURS TOO LATE)
Robert W. Baldwin, Alan T. Sherman · Cryptologia · 1990
On Saturday March 30, 1985, we solved a cryptography puzzle that had remained unsolved for over two years. Unfortunately for us, we missed by one day the deadline for sharing what had become a $117,000 prize. This paper describes the puzzle and how we solved it. The $100,000 Decipher Puzzle consists of a simple two-sided jigsaw puzzle that contains a sequence of 376 codenumbers, ranging from 1 to 1252, which were encrypted by a multiple-substitution cipher similar to the Beale cipher. Clues state that the key was derived from some keytext in the public domain. We solved the puzzle by building a Zetalisp program that tested candidate keytexts by trying a variety of ways to extract candidate keys from each keytext and by checking the resulting candidate plaintexts for English. Our program exploited a novel “windowing” technique that detected when part of a candidate key was correct and an effective test for English based on digraph frequencies.