Cracking a 200-Year-Old Cipher to Thomas Jefferson

April 2, 2009

Lawren M. Smithline of the Center for Communications Research (Princeton, N.J.) has deciphered a 19th-century cipher contained in the papers of Thomas Jefferson.

The story goes that on Christmas Day, 1801, President Jefferson received a letter from University of Pennsylvania mathematician Robert Patterson about cryptography. It contained Patterson's vision of a "perfect cipher," one that was adaptable to all languages, easy to memorize, simple to perform, and inscrutable without the key. Patterson then described an encryption technique that he believed met these criteria. The last page of the letter contained a sample cipher, which Patterson characterized as "absolutely inscrutable to all unacquainted with the particular key or secret."  And he included no key with his letter.

Smithline tackled the mystery by using digraph frequencies, guessing, and dynamic programming techniques—akin to those used to assess DNA sequences. It turned out that Patterson had transposed the text so that sentences originally written horizontally were written vertically. The transposition was only one step in creating the conundrum, however, because Patterson had also rearranged rows of transposed text and added letters to the beginning of rows.

"It took more than two centuries for the puzzle to be solved," Smithline wrote in the March-April American Scientist, "using a method that has become common in computational biology. The theory behind the cryptanalysis is of the computer age, but the calculating power necessary is of Patterson's time."

The cipher turned out to be the preamble to the Declaration of Independence. No evidence suggests that Jefferson decoded the text.

Source: American Scientist, March-April, 2009.