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Introduction
Published in Jonathan Katz, Yehuda Lindell, Introduction to Modern Cryptography, 2020
The mono-alphabetic substitution cipher. In the shift cipher, the key defines a map from each letter of the (plaintext) alphabet to some letter of the (ciphertext) alphabet, where the map is a fixed shift determined by the key. In the mono-alphabetic substitution cipher the key also defines a map on the alphabet, but the map is now allowed to be arbitrary subject only to the constraint that it be one-to-one (so that decryption is possible). The key space thus consists of all bijections, or permutations, of the alphabet. So, for example, the key that defines the following permutation (in which a maps to X, etc.) would encrypt the message tellhimaboutme to GDOOKVCXEFLGCD. The name of this cipher comes from the fact that the key defines a (fixed) substitution for individual characters of the plaintext.
HMM Applications
Published in Mark Stamp, Introduction to Machine Learning with Applications in Information Security, 2017
A simple substitution cipher is based on a fixed permutation of the plaintext characters. For example, in the well-known Caesar’s cipher,4 a message is encrypted by substituting the letter that is 3 positions ahead in the alphabet. In general, any permutation of the alphabet can serve as a simple substitution key [137].
Deciphering the Hermeticae Philosophiae Medulla: Textual Cultures of Alchemical Secrecy
Published in Ambix, 2023
Megan Piorko, Sarah Lang, Richard Bean
The cipher contained in Sloane MS 1902 is a polyalphabetic substitution cipher type, which consists of substituting one letter for another following a predetermined scheme.27 Simple substitution ciphers are monoalphabetic, meaning that a fixed set of numbers, symbols, or letters are assigned to the alphabet of the plaintext, in which there is a single corresponding symbol attached to each letter in the alphabet that does not change throughout the ciphertext. Monoalphabetic ciphers were historically the most widely used method of textual encryption, despite being especially vulnerable to frequency analysis because the statistical letter frequencies are distinctive and the substitutions constant. One means of complicating a substitution cipher, for the purpose of added secrecy, is through the use of several alphabetic tables instead of one, adding variation to the substitutions.28