Editing Running key cipher (section)

== Variants ==

Modern variants of the running key cipher often replace the traditional ''tabula recta'' with bitwise [[exclusive or]], operate on whole [[byte]]s rather than alphabetic letters, and derive their running keys from large files. Apart from possibly greater entropy density of the files, and the ease of automation, there is little practical difference between such variants and traditional methods.

=== Permutation generated running keys ===

A more compact running key can be used if one combinatorially generates text using several start pointers (or combination rules).  For example, rather than start at one place (a single pointer), one could use several start pointers and xor together the streams to form a new running key, similarly skip rules can be used.  What is exchanged then is a series of pointers to the running key book and/or a series of rules for generating the new permuted running key from the initial key text.  (These may be exchanged via [[public key]] encryption or in person.  They may also be changed frequently without changing the running key book.)

=== Ciphertext appearing to be plaintext ===

Traditional ciphertext appears to be quite different from plaintext. To address this problem, one variant outputs "plaintext" words instead of "plaintext" letters as the ciphertext output.  This is done by creating an "alphabet" of words (in practice multiple words can correspond to each ciphertext output character).  The result is a ciphertext output which looks like a long sequence of plaintext words (the process can be nested).  Theoretically, this is no different from using standard ciphertext characters as output.  However, plaintext-looking ciphertext may result in a "human in the loop" to try to mistakenly interpret it as decoded plaintext.

An example would be BDA (Berkhoff deflater algorithm){{Citation needed|reason=Can't find any explanation about this out there|date=October 2019}}, each ciphertext output character has at least one noun, verb, adjective and adverb associated with it. (E.g. (at least) one of each for every [[ASCII]] character).  Grammatically plausible sentences are generated as ciphertext output.  Decryption requires mapping the words back to ASCII, and then decrypting the characters to the real plaintext using the running key. Nested-BDA will run the output through the reencryption process several times, producing several layers of "plaintext-looking" ciphertext - each one potentially requiring "human-in-the-loop" to try to interpret its non-existent [[semantic]] meaning.

=== Gromark cipher ===

The "Gromark cipher" ("[[Gronsfeld cipher]] with mixed alphabet and running key") uses a running numerical key formed by adding successive pairs of digits.<ref>American Cryptogram Association.
[http://cryptogram.org/docs/acayou16.pdf "The ACA and You"] {{Webarchive|url=https://web.archive.org/web/20160403131720/http://cryptogram.org/docs/acayou16.pdf |date=2016-04-03 }}.
2016.</ref>
The [[VIC cipher]] uses a similar [[lagged Fibonacci generator]].