Operating Systems: Concurrent and Distributed Software Design

8.4. Algorithms

Cryptographic algorithms traditionally substituted characters for one another or transposed characters. Successful algorithms typically did multiple substitutions or transpositions. Computerized algorithms work on bits instead of characters but the algorithms still employ substitution and transposition.

8.4.1. Substitution ciphers

In a simple substitution cipher each character in the plaintext is replaced by another character to form the ciphertext. Decryption is simply the reverse substitution. In the Caesar cipher, each character is replaced by the character three places to its right, mod 26, in the alphabet. In UNIX systems ROT13 replaces each character by that 13 places to the right, mod 26, in the alphabet.

Figure 8.3 shows ...