In cryptography, a classical cipher is a type of cipher used historically but which now have fallen, for the most part, into disuse. In general, classical ciphers operate on an alphabet of letters (such as "A-Z"), and are implemented by hand or with simple mechanical devices. By contrast, modern schemes use computers or other digital technology, and operate on bits and bytes. Classical schemes are often susceptible to ciphertext-only attacks, sometimes even without knowledge of the system itself, using tools such as frequency analysis. Sometimes classed with classical ciphers are more advanced mechanical or electro-mechanical cipher machines, such as the Enigma machine.
Classical ciphers are often divided into transposition ciphers and substitution ciphers. In a substitution cipher, letters (or groups of letters) are systematically replaced throughout the message for other letters (or groups of letters). In a transposition cipher, the letters themselves are kept unchanged, but rather their order within the message is scrambled according to some well-defined scheme. More complex algorithms can be formed by mixing substitution and transposition in a product cipher; modern block ciphers such as DES iterate through several stages of substitution and transposition.
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