From Sixteenth-Century Cryptography to the New Millennium — The Last 500 Years - Codes: The Guide to Secrecy from Ancient to Modern Times - page 66

code since the British were still using the same keys. This allowed the colonial
forces to prevent reinforcements from reaching Cornwallis, who surrendered five
days later on October 19, 1781. This victory at Yorktown ended the fighting and
ensured Washington's victory at the end of the American War for Independence.

One of the founding fathers who sought to
improve the means of secret communications
was certainly the most forward-thinking of
them all, Thomas Jefferson 2.3 (1743-1826),
(see Figure 2.4) the draftsman of the Decla-
ration of Independence of the United States,
the nation's first secretary of state (1789-
1794), its second vice-president (1797-1801),
and its third president (1801-1809). In
1785, he compiled a nomenclator to aid in
his correspondence with Madison and Mon-
roe, a method that he used until 1793.
Perhaps Jefferson's most important crypto-
graphic contribution was what he called his
wheel cypher , (see Figure 2.5) invented in the
1790s. 2.4 This device consisted of 36 concen-
tric wooden disks, each approximately 1 / 6of
an inch thick, and 2 inches in diameter with
a mix of the English alphabet inscribed on the outer edge. Moreover, each
disk had its own number, and the key consisted of an agreed-upon sequence of
these numbers for correspondents to use. The correspondent would assemble
their disks in this (key) sequence on a metal spindle. Here is how it worked.

Figure 2.4: Thomas Jefferson.

To encrypt the first 36
letters of the plaintext, the
sender found the first letter
on the first wheel, second
letter on the second wheel,
lined up with the first, then
the third in the third wheel
lined up with the first two,
and so forth. The ciphertext
consisted of any of the 25 re-
maining parallel rows of let-
ters on the disk. The sender
would select one of them,
write it down as the cipher-
text for the first 36 letters of
plaintext, then repeat the above process for each remaining block of 36 plaintext

Figure 2.5: Wheel cypher.

2.3 The above lithograph of Thomas Jefferson is courtesy of the Library of Congress, repro-
duction no. LC-USZ62-117117, Pictures and Photographs Division, created around 1828.
2.4 The above replica of Jefferson's wheel cypher is courtesy of the National Cryptologic
Museum of the National Security Agency. See http://www.nsa.gov/museum/wheel.html .