Running key cipher: Difference between revisions

Revision as of 17:57, 13 January 2024

Revision as of 09:47, 19 January 2024

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== Security and cryptanalysis ==

If the running key is truly random, never reused, and kept secret, the result is a [[one-time pad]], a method that provides [[perfect secrecy]] (reveals no information about the plaintext). However, if (as usual) the running key is a block of text in a [[natural language]], security actually becomes fairly poor, since that text will have non-random characteristics which can be used to aid cryptanalysis: for example, [[William F. Friedman]] suggested a [[ciphertext-only attack]] during WWI against most frequent letters encoded by other most frequent letters.<ref>{{Cite web |title=Cryptology: Running-Text Ciphers – Cryptanalysis According to Friedman |url=https://www.staff.uni-mainz.de/pommeren/Cryptology/Classic/7_Aperiodic/AnalFR.html |access-date=2024-01-10 |website=www.staff.uni-mainz.de}}</ref>. As a result, the [[information entropy|entropy]] per character of both plaintext and running key is low, and the combining operation is easily inverted.

To attack the cipher, a [[cryptanalysis|cryptanalyst]] may run [[Known-plaintext attack|guessed probable plaintexts]] along the ciphertext, subtracting them out from each possible position. When the result is a chunk of something intelligible, there is a high probability that the guessed plain text is correct for that position (as either actual plaintext, or part of the running key). The 'chunk of something intelligible' can then often be extended at either end, thus providing even more probable plaintext, which can in turn be extended, and so on (for more detailed explanation refer to [[Autokey cipher#Cryptanalysis|Autokey cipher]]). Eventually it is likely that the source of the running key will be identified, and the jig is up.