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Alberti cipher disc, artwork. This device was described in 1467 by the Italian cryptographer Leon Battista Alberti (1404-1472). The inner ring of lower-case letters is on a moveable disc that is rotated past the outer ring of upper-case letters and numbers. The position of the discs is determined by key letters included in the encoded message itself. Each ring has 24 characters. The outer ring is used for the uncoded message (the plaintext), and the inner ring is the corresponding letter in the coded message (the ciphertext). This is known as polyalphabetic substitution. The four numbers are used to encode references to phrases in a codebook.
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