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The [[VIC cipher]] uses a similar [[lagged Fibonacci generator]].
== 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]]
There are several ways to improve the security. The first and most obvious is to use a secret mixed alphabet tableau instead of a ''tabula recta''. This does indeed greatly complicate matters but it is not a complete solution.
Another possibility is to use a key text that has more entropy per character than typical English. For this purpose, the [[KGB]] advised agents to use documents like [[almanac]]s and trade reports, which often contain long lists of random-looking numbers.
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