Jefferson disk
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Jefferson's_disk_cipher.jpg
During 1795, Thomas Jefferson invented a cipher system, the Jefferson disk, using 26 wheels, each with the letters of the alphabet arranged randomly around them.
The key to this system lies in how the wheels are ordered around the axis. The user(s) can devise a code word which corresponded to the ordering of the wheels. Once the order of wheels along the axis has been devised, the user can rotate each wheel up and down until a desired message is spelled out in one row. Then the user can copy any row of text on the wheels other than the one that contains the message. The recipient simply has to put the discs in the agreed-upon order, spell out the encrypted message by rotating the wheels, and then look around the rows until he sees the plaintext message, i.e. the row that's not complete gibberish. There is an extremely small chance that there would be two readable messages, but that can be checked quickly by the person coding.
This system is considered quite secure against modern codebreaking if the message is short, and the ordering of letters and wheels is not known to the codebreaker. As messages get larger, it is easy to apply the statistics of English language letter frequency, find patterns, and break the code.
This system was later used by the United States Army from 1923 until 1942 as the M-94. However, the U.S. Army never knew of Jefferson's invention; it reinvented it. Jefferson's disk cipher was 100 years ahead of its time