Brian Nolan
September 13, 2004
Cryptography: History of the Wheel Cypher
During the 15th century Leo Batista Alberti invented an encryption tool called a wheel cypher. The device consisted of several independent discs mounted on a rod with letters placed in random order along the outside of each disk. Although Leo Batista Alberti was the first person to envision the wheel cypher, it was Thomas Jefferson who was credited for creating and who strengthened the device over two centuries later.
While serving as the Secretary of State under George Washington, Thomas Jefferson needed a secure way to send correspondence knowing that third parties would intercept any message sent out. Without prior knowledge of Leo Batista Alberti's work on the wheel cypher, Thomas Jefferson reinvented the cryptographic tool as a means of communicating securely through the mail. Thomas Jefferson's device consisted of 26 to 36 disks each with randomly placed letters of the alphabet on the outside part of each disk. Given that every disk had all the letters randomly placed there were no two disks alike on each wheel cypher, allowing for well over a million different combinations of disk arrangements. This new tool allowed the United States of America to communicate securely between governmental officials during its infancy and also designated Thomas Jefferson as the Father of Cryptography in America.
After 1802 the wheel cypher was abandoned as new devices and methods for securing communications were invented. However, the wheel cypher would be reinvented by the French around 1890 and then again by an American officer in 1917.
When Major Joseph Mauborgne of the U.S. Army reinvented the wheel cypher he used a design that incorporated a total of 26 disks. The device was used from 1922 until 1943 when it was deemed too weak and slow by military standards. Nevertheless, during World War II the U.S. Navy adopted the device, under the name CSP-488, as did the U.S. Army, under the name M-94, as a means of secure communications between commanders. As with Thomas Jefferson's original design, the original message was aligned straight across the wheel, twenty-five letters at a time, using one letter from each disk. Next, the individual writing the message would rotate the entire wheel and copy down one row of letters to be the encrypted message sent. Should the sender be using a unique combination of the disks they would also need to send a key corresponding to the order of the disks. Finally, once the message was received by the appropriate party the person deciphering the message would place their disks in the correct order, set one row of the wheel to match the letters of the enciphered text, and then rotate the entire wheel unit until a readable text was found.
The primary ways of breaking the cipher were to obtain a message of considerable length for a statistical analysis or else by obtaining an exact copy of the wheel cypher being used. Despite this basic flaw of being limited to the number of letters encrypted at a time, allowing for easier analysis of messages, the wheel cypher was considered strong enough for use for nearly two hundred years with only slight modifications made to the original design.
Bibliography
Ann M. Lucas, Monticello Research Department, September 1995
with revisions by Chad Wollerton, December 2003, Monticello The Home of Thomas Jefferson <http://www.monticello.org/reports/interests/wheel_cipher.html>
Wikipedia, the free encyclopedia.6 Sep 2004
<http://en.wikipedia.org/wiki/M-94>