The Jefferson cipher was created by Thomas Jefferson (yes, that Thomas Jefferson!), and is a brilliant piece of work. He created the cipher in the last decade of the 1800s for possible use in the American Foreign Policy department to protect messages from France and Britain. The invention he came up with was by far the strongest created during its time (and for many years to come), yet it did not win use in the department because Jefferson though a simple column transposition cipher submitted by his friend would be better suited. This means one of two things: Jefferson wanted France and Britain to have a chance to read American communications or he truly didn't know the amazing amount of security offered by his system. (Or perhaps he didn't want other nations to find out about his system and use it, preventing American cryptanalysis). The more probable is the second 'naïve choice' although that option means that Jefferson truly must not have had much education in ciphers and cryptanalysis. Despite its inventor's knowledge in cryptography, the Jefferson 'wheel cyper', as he called it, is amazing.
The device is quite simple. It consisted of one metal cylinder and 36 disks that fit on the cylinder. The circumference of each disk was split into 26 equal sections each of which held one letter. The order of the letters on each disk was random and different. In addition to this basic design, holders could be used to keep the disks from turning when necessary. In all, Jefferson suggested that the device could be about 6 inches long, and used easily with the fingers.
To use the Jefferson cipher, one finds the first letter of the plaintext on the left-most disk. Then, the second disk from the left is used to for the second letter. The first and second disk, then, should show the first two letters of the message on the same line. The encoder continues this process until he reaches the end of the 36 disks. Across one of the rows, the encoder should be able to see the plaintext message. For the equivilent ciphertext of those letters, the coder chooses any of the other 25 rows and copies down the row in full.
To decode the cipertext, the receiver rotates the disks to appropriately mimic the ciphertext line. When this is completed, he scans the rest of the lines around the line cylinder looking for one that should stand out as the plaintext.
The Jefferson cipher is amazing in several ways. For starters, it's simple, practical, and easy to understand. The Navy independently developed a device strikingly similar to the Jefferson cipher in 1922, over a century after it's original creation. In fact, the Navy was still using the Jefferson cipher equivalent up until the 70s.
The Jefferson cipher's security came from the fact that the encoder could read off any of the 25 non plaintext lines and submit it as the ciphertext. This added a bit of randomness to the ciphertext which is hard to quantify in cryptanalysis of the system's messages. Although we know the period is 36 for the system, splitting the message into groups of 36 does not create a series of mono alphabetic substitutions. Within each of those columns, you see, there are actually 25 alphabets being used in a random fashion. Any letter in the column can represent any other letter than itself. The Jefferson successfully uses a randomizing factor because the device allows the human decoder to nullify the randomness. In the process of 'scanning the lines' looking for plaintext, the randomness is removed. But how can one remove the randomness if they don't have the device?
Possibly because the Jefferson cipher system was in use for such a long period of time, there are no formal discussions specifically directed at how to crack the Jeffeson cipher system. However, the following method helps (though it requires time) and can be used.
