Editing Substitution cipher (section)

=== Homophonic ===<!-- This section is linked from [[Homophonic substitution cipher]]. See [[WP:MOS#Section management]] -->
An early attempt to increase the difficulty of frequency analysis attacks on substitution ciphers was to disguise plaintext letter frequencies by '''[[homophony (writing)|homophony]]'''. In these ciphers, plaintext letters map to more than one ciphertext symbol. Usually, the highest-frequency plaintext symbols are given more equivalents than lower frequency letters. In this way, the frequency distribution is flattened, making analysis more difficult.

Since more than 26 characters will be required in the ciphertext alphabet, various solutions are employed to invent larger alphabets. Perhaps the simplest is to use a numeric substitution 'alphabet'. Another method consists of simple variations on the existing alphabet; uppercase, lowercase, upside down, etc. More artistically, though not necessarily more securely, some homophonic ciphers employed wholly invented alphabets of fanciful symbols.

The [[book cipher]] is a type of homophonic cipher, one example being the [[Beale ciphers]]. This is a story of buried treasure that was described in 1819–21 by use of a ciphered text that was keyed to the Declaration of Independence. Here each ciphertext character was represented by a number.  The number was determined by taking the plaintext character and finding a word in the Declaration of Independence that started with that character and using the numerical position of that word in the Declaration of Independence as the encrypted form of that letter.  Since many words in the Declaration of Independence start with the same letter, the encryption of that character could be any of the numbers associated with the words in the Declaration of Independence that start with that letter. Deciphering the encrypted text character ''X'' (which is a number) is as simple as looking up the Xth word of the Declaration of Independence and using the first letter of that word as the decrypted character.

Another homophonic cipher was described by Stahl<ref>Stahl, Fred A., ''On Computational Security'', University of Illinois, 1974</ref><ref>Stahl, Fred A. "[http://www.computer.org/plugins/dl/pdf/proceedings/afips/1973/5081/00/50810565.pdf?template=1&loginState=1&userData=anonymous-IP%3A%3AAddress%3A+67.188.203.8%2C+%5B172.16.161.5%2C+67.188.203.8%2C+127.0.0.1%5D%20 A homophonic cipher for computational cryptography] {{Webarchive|url=https://web.archive.org/web/20160409131649/https://www.computer.org/plugins/dl/pdf/proceedings/afips/1973/5081/00/50810565.pdf?template=1&loginState=1&userData=anonymous-IP::Address:+67.188.203.8,+%5B172.16.161.5,+67.188.203.8,+127.0.0.1%5D%20 |date=2016-04-09 }}", afips, pp. 565, 1973 Proceedings of the National Computer Conference, 1973</ref> and was one of the first{{Citation needed|reason=An early method, sure, but the first...?|date=June 2014}} attempts to provide for computer security of data systems in computers through encryption. Stahl constructed the cipher in such a way that the number of homophones for a given character was in proportion to the frequency of the character, thus making frequency analysis much more difficult.

[[Francesco I Gonzaga]], [[Duke of Mantua]], used the earliest known example of a homophonic substitution cipher in 1401 for correspondence with one Simone de Crema.<ref>David Salomon. [https://books.google.com/books?id=A88kvYwIVu0C&dq=homophonic+cipher+mantua+1400s&pg=PA224 Coding for Data and Computer Communications]. Springer, 2005.</ref><ref>Fred A. Stahl. "[http://www.computer.org/csdl/proceedings/afips/1973/5081/00/50810565.pdf A homophonic cipher for computational cryptography]" Proceedings of the national computer conference and exposition (AFIPS '73), pp. 123–126, New York, USA, 1973.</ref>

[[Mary, Queen of Scots]], while imprisoned by Elizabeth I, during the years from 1578 to 1584 used homophonic ciphers with additional encryption using a nomenclator for frequent prefixes, suffixes, and proper names while communicating with her allies including [[Michel de Castelnau]].<ref>{{cite journal | doi=10.1080/01611194.2022.2160677 | title=Deciphering Mary Stuart's lost letters from 1578-1584 | year=2023 | last1=Lasry | first1=George | last2=Biermann | first2=Norbert | last3=Tomokiyo | first3=Satoshi | journal=Cryptologia | volume=47 | issue=2 | pages=101–202 | s2cid=256720092 | doi-access=free }}</ref>