applied cryptography - protocols, algorithms, and source code in c

A homophonic substitution cipher is like a simple

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Unformatted text preview: osition. Substitution Ciphers A substitution cipher is one in which each character in the plaintext is substituted for another character in the ciphertext. The receiver inverts the substitution on the ciphertext to recover the plaintext. In classical cryptography, there are four types of substitution ciphers: — A simple substitution cipher, or monoalphabetic cipher, is one in which each character of the plaintext is replaced with a corresponding character of ciphertext. The cryptograms in newspapers are simple substitution ciphers. — A homophonic substitution cipher is like a simple substitution cryptosystem, except a single character of plaintext can map to one of several characters of ciphertext. For example, “A” could correspond to either 5, 13, 25, or 56, “B” could correspond to either 7, 19, 31, or 42, and so on. — A polygram substitution cipher is one in which blocks of characters are encrypted in groups. For example, “ABA” could correspond to “RTQ,” “ABB” could correspond to “SLL,” and so on. — A polyalphabetic substitution cipher is made up of multiple simple substitution ciphers. For example, there might be five different simple substitution ciphers used; the particular one used changes with the position of each character of the plaintext. The famous Caesar Cipher, in which each plaintext character is replaced by the character three to the right modulo 26 (“A” is replaced by “D,” “B” is replaced by “E,”..., “W” is replaced by “Z,” “X” is replaced by “A,” “Y” is replaced by “B,” and “Z” is replaced by “C”) is a simple substitution cipher. It’s actually even simpler, because the ciphertext alphabet is a rotation of the plaintext alphabet and not an arbitrary permutation. ROT13 is a simple encryption program commonly found on UNIX systems; it is also a simple substitution cipher. In this cipher, “A” is replaced by “N,” “B” is replaced by “O,” and so on. Every letter is rotated 13 places. Encrypting a file twice with ROT13 restores the ori...
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