A 24-wheel reproduction of Jefferson's 26-wheel cipher,
otherwise built according to his instructions.
The most advanced cipher of its time, according to historian David Kahn, was Thomas Jefferson's wheel cipher.1 Jefferson created it in the early 1790s while he was President George Washington's secretary of state. Sometime prior to the end of March, 1802, Jefferson described his device, in part, as follows:
Turn a cylinder of white wood of about 2. Inches diameter & 6. or 8. I. long. bore through it's center a hole sufficient to recieve an iron spindle or axis of 1/8 or 1/4 I. diam. divide the periphery into 26. equal parts (for the 26. letters of the alphabet) and, with a sharp point, draw parallel lines through all the points of division, from one end to the other of the cylinder, & trace those lines with ink to make them plain. then cut the cylinder crosswise into pieces of about 1/6 of an inch thick. they will resemble back-gammon men with plane sides. number each of them, as they are cut off, on one side, that they may be arrangeable in any order you please. on the periphery of each, and between the black lines, put all the letters of the alphabet, not in their established order, but jumbled, & without order, so that no two shall be alike. now string them in their numerical order on an iron axis, one end of which has a head, and the other a nut and screw; the use of which is to hold them firm in any given position when you chuse it. they are now ready for use, your correspondent having a similar cylinder, similarly arranged. . . . numbers [are] represented by letters with dots over them.2
To cipher a phrase such as "your favor of the 22d is received," the letter y is located on the wheel at the left end of the instrument; the next wheel is turned until the letter o is adjacent to the y; the third wheel is turned until u is next to the o; and so on, until all words in the message are spelled out in up to 26 letters, with no spaces between them. Each of the remaining rows of letters on the cylinder represents a different encryption of the message, or plaintext. To transmit the cipher, the sender copies the sequence of letters in one row in a letter and mails it off. The recipient arranges the letters on any row of his cylinder into the sequence received, then rotates the whole cylinder until a meaningful series of words can be recognized. For messages of more than 26 letters, the same procedure is followed as many times as necessary. By merely changing the order of the wheels in the cylinder, Jefferson pointed out, "an immense variety of different cyphers may be produced for different correspondents."
The main drawback to Jefferson's wheel cipher was that replicas had to be distributed in advance to all potential correspondents, which included several in the capitals of Europe, a process that could take months. Jefferson soon turned to written cipher systems that were more convenient. In 1890, however, a French cryptologist devised a similar device. Then the U.S. Army introduced an electrical version prior to World War I, and used it continually until the early 1940s. Since the early 1970s, microprocessors have been used to facilitate encryptions that penetrate the daily lives of most people throughout the world, either directly or indirectly.
1. David Kahn, The Codebreakers: The Story of Secret Writing (New York: Macmillan, 1967), 195.
2. Ralph E. Weber, United States Diplomatic Codes and Ciphers, 1775-1938 (Chicago: Precedent Publishing Inc., 1979), 170-72.
Funded in part by a grant from the National Park Service's Challenge-Cost Share Program.