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Concept
Classical cipher
Résumé
In cryptography, a classical cipher is a type of cipher that was used historically but for the most part, has fallen into disuse. In contrast to modern cryptographic algorithms, most classical ciphers can be practically computed and solved by hand. However, they are also usually very simple to break with modern technology. The term includes the simple systems used since Greek and Roman times, the elaborate Renaissance ciphers, World War II cryptography such as the Enigma machine and beyond.
In contrast, modern strong cryptography relies on new algorithms and computers developed since the 1970s.
Classical ciphers are often divided into transposition ciphers and substitution ciphers, but there are also concealment ciphers.
Substitution cipher
In a substitution cipher, letters (or groups of letters) are systematically replaced throughout the message for other letters (or groups of letters).
A well-known example of a substitution cipher is the Caesar cipher. To encrypt a message with the Caesar cipher, each letter of message is replaced by the letter three positions later in the alphabet. Hence, A is replaced by D, B by E, C by F, etc. Finally, X, Y and Z are replaced by A, B and C respectively. So, for example, "WIKIPEDIA" encrypts as "ZLNLSHGLD". Caesar rotated the alphabet by three letters, but any number works.
Another method of substitution cipher is based on a keyword. All spaces and repeated letters are removed from a word or phrase, which the encoder then uses as the start of the cipher alphabet. The end of the cipher alphabet is the rest of the alphabet in order without repeating the letters in the keyword. For example, if the keyword is CIPHER, the cipher alphabet would look like this:
The previous examples were all examples of monoalphabetic substitution ciphers, where just one cipher alphabet is used. It is also possible to have a polyalphabetic substitution cipher, where multiple cipher alphabets are used. The encoder would make up two or more cipher alphabets using whatever techniques they choose, and then encode their message, alternating what cipher alphabet is used with every letter or word.
Source officielle
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