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Substitution over a single letter—simple substitution—can be demonstrated by writing out the alphabet in some order to represent the substitution. The cipher alphabet may be shifted or reversed (creating the Caesar and Atbash ciphers, respectively) or scrambled in a more complex fashion, in which case it is called a mixed alphabet or deranged alphabet.Traditionally, mixed alphabets are created by first writing out a keyword, removing repeated letters in it, then writing all the remaining letters in the alphabet.
Usually, the highest-frequency plaintext symbols are given more equivalents than lower frequency letters.By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered.There are a number of different types of substitution cipher.More artistically, though not necessarily more securely, some homophonic ciphers employed wholly invented alphabets of fanciful symbols.(See Poe's "The Gold-Bug" for a literary example; cf.In lists and catalogues for sales people sometimes a very simple encryption is used to replace numeric digits by letters. A disadvantage of this method of derangement is that the last letters of the alphabet (which are mostly low frequency) tend to stay at the end.
A stronger way of constructing a mixed alphabet is to perform a columnar transposition on the ordinary alphabet using the keyword, but this is not often done.
Originally the code was restricted to the names of important people, hence the name of the cipher; in later years it covered many common words and place names as well.
The symbols for whole words (codewords in modern parlance) and letters (cipher in modern parlance) were not distinguished in the ciphertext.
In other cases, the plaintext can be contrived to have a nearly flat frequency distribution, and much longer plaintexts will then be required by the user.
An early attempt to increase the difficulty of frequency analysis attacks on substitution ciphers was to disguise plaintext letter frequencies by homophony.
the Voynich manuscript.) An interesting variant is the nomenclator.