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- The Jefferson Disk is a simple encryption device using numbered disks with random letter arrangements, requiring both parties to have identical sets and agree on wheel order.
- Encryption involves lining up the message on one row, then sending a different row of garbled text; decryption reverses this by aligning the ciphertext to find the readable plaintext row.
- The security of the system relies on the unknown wheel order and disk arrangement, making it effective even without a computer for permutation attacks.
- Historical similar devices include Gripenstierna's 57-disk machine and Bazeries' 20-disk cylinder, with the US military adopting the M-94 (25 disks) in 1922 and later the M-138A using strips.
- Jefferson described the device in the 1790s but did not build it, and the system was largely forgotten until the 20th century, predating rotor-based machines like Enigma.