Editing Symmetric-key algorithm (section)

== Reciprocal cipher ==
{{More citations needed section|date=December 2015}}

A reciprocal cipher is a cipher where, just as one enters the [[plaintext]] into the [[cryptography]] system to get the [[ciphertext]], one could enter the ciphertext into the same place in the system to get the plaintext. A reciprocal cipher is also sometimes referred as '''self-reciprocal cipher'''.<ref>
Paul Reuvers and Marc Simons.
Crypto Museum.
[https://www.cryptomuseum.com/crypto/enigma/uhr/ "Enigma Uhr"].
2009.
</ref><ref>
Chris Christensen.
[https://www.nku.edu/~christensen/section%201%20substitution%20ciphers.pdf "Simple Substitution Ciphers"].
2006.
</ref>

Practically all mechanical cipher machines implement a reciprocal cipher, a [[involution (mathematics)|mathematical involution]] on each typed-in letter.
Instead of designing two kinds of machines, one for encrypting and one for decrypting, all the machines can be identical and can be set up (keyed) the same way.<ref>
Greg Goebel.
[http://vc.airvectors.net/ttcode_05.html "The Mechanization of Ciphers"].
2018.
</ref>

Examples of reciprocal ciphers include:
* [[Atbash]]
* [[Beaufort cipher]]<ref>
"... the true Beaufort cipher. Notice that we have ''reciprocal encipherment''; encipherment and decipherment are identically the same thing."
--
Helen F. Gaines.
[https://books.google.com/books?id=Zb2RBQAAQBAJ "Cryptanalysis: A Study of Ciphers and Their Solution"].
2014.
p. 121.
</ref>
* [[Enigma machine]]<ref>
Greg Goebel.
[http://vc.airvectors.net/ttcode_05.html "The Mechanization of Ciphers"].
2018.
</ref>
* Marie Antoinette and [[Axel von Fersen the Younger|Axel von Fersen]] communicated with a self-reciprocal cipher.<ref>
Friedrich L. Bauer.
[https://books.google.com/books?id=hfWTDr_bvMwC "Decrypted Secrets: Methods and Maxims of Cryptology"].
2006.
p. 144
</ref>
* the Porta polyalphabetic cipher is self-reciprocal.<ref>
David Salomon.
[https://books.google.com/books?id=Zr9bjEpXKnIC "Coding for Data and Computer Communications"].
2006.
p. 245
</ref>
* [[Purple cipher]]<ref>
Greg Goebel.
[http://vc.airvectors.net/ttcode_06.html "US Codebreakers In The Shadow Of War"].
2018.
</ref>
* [[RC4]]
* [[ROT13]]
* [[XOR cipher]]
* [[Vatsyayana cipher]]

The majority of all modern ciphers can be classified as either a [[stream cipher]], most of which use a reciprocal [[XOR cipher]] combiner, or a [[block cipher]], most of which use a [[Feistel cipher]] or [[Lai–Massey scheme]] with a reciprocal transformation in each round.{{cn|date=May 2024}}