For as long as humans have had writing, there’s been a need to send secret messages. It is easy to think that Enigma machines and their immediate predecessors are old tech, but they are much more recent than ancient systems used by the Greeks and Romans. Even Thomas Jefferson, one of the founding fathers of the United States, was interested in encryption and is often said to have invented the Jefferson Disk machine for encryption. The truth is, the device is probably older than Jefferson, but he certainly thought about using it for secret communications.
Simple but Effective
The idea is simple. We make a series of disks. Each disk has a number on it and, around the edge, all the letters of the alphabet. The placement of each wheel with the same number is the same, but, overall, the arrangement is random. That is, all disks marked #5 might start with XCBYG, but all disks marked with #10 could start with FAYQL. You take one set of disks, and I keep the other set.
When we want to send secret messages, we agree to arrange our disks on an axle in the same order. Jefferson used a 36-disk system, so we might agree to go left to right with the odd numbers first and then the even numbers, or any other setup that we could agree on.
Encryption
Once the wheels are in place, encryption is simple. There’s a bar across the device, and you line up your message using a wheel for each letter: ENEMYCOMESBYSEA, for example. Then you look at any different row, which will now read something crazy like: FSRSSXQCGAEEFOR (plus the random letters on the rest of the disks). That’s the message you send.
Decryption
Upon receipt, you spell out the same message on the disks with the wheels in the agreed order. Once you have FSRSSXQCGAEEFOR lined up, you look at the other rows. It is a good chance that all of them will be gibberish except one. That’s the decoded message.
Of course, you could agree to shift a certain number of rows if you wanted to be sure. The order of the wheels amounts to a key, and while Jefferson wasn’t sure how secure this is, modern analysis says it is actually pretty good. If you don’t have the wheels and you don’t know the order of the wheels, it is actually excellent. If you have the wheels but don’t know the order, it is still very difficult to try all the permutations, especially without a computer.
History
The Swede Fredrik Gripenstierna used a similar machine with 57 disks, but it had a slightly different purpose and operating principle. Jefferson described his device in the 1790s, and while he never claimed to invent it, the story has caught on that he did. Sometimes these are called Bazeries cylinders, as Etienne Bazeries had a 20-disk device in 1891, apparently independently invented.
It doesn’t appear that Jefferson or anyone else actually built the machine he described. There is an old device in the NSA museum that could be from Jefferson, but it isn’t clear that it was actually his or related to his writings.
Jefferson abandoned the scheme after learning about columnar transposition ciphers in 1803.
Use in the 20th Century
While Jefferson’s wheels never saw use, the US Military adapted Bazeries’ cylinder in 1922 and used the M-94 through 1942. The M-94 had 25 aluminum disks on a spindle.
The original prototype of the M-94 used a sliding strip arrangement instead of disks, and the Army returned to that style with the M-138A cipher machine in the 1930s. Each machine had 100 strips, and you had to select 30, further improving key security. It was used for some time, and you can see an example of it in the picture from the National Cryptologic Museum.
Everything Old is New
It is tempting to say the military used Jefferson’s wheel, but, in fact, the papers explaining the device were forgotten until 1922, and the prototype M-94s were developed between 1914 and 1917.
The system probably predates Jefferson anyway. Charles Babbage made references to disks with letters in 1854, although the exact arrangement he had in mind isn’t clear.
Of course, the Enigma and similar rotor-based machines took over during World War II. While making a disk-based code tool yourself would be easier (try styrofoam cups, like in the video below, if you don’t want to 3D print it), you can put in a bit more effort and make your own Enigma.
