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| jdege | Apr 10 2008, 03:33 PM |
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A good cipher is one that makes it difficult to break any message. But even a weak cipher can be difficult to break, when the plaintext is intentionally chosen to mislead. That is - people who are using ciphers to protect their sensitive communications have to encrypt messages that are part of that communication. People who are using ciphers as a game, to challenge people who break ciphers for fun, are under no such restriction. So you often see, in crypto puzzles, plaintexts chosen to make difficult the life of the crypto buff who is trying to break it. It is possible, if you are familiar with the techniques used to break a particular cipher, to choose a plaintext intended to make those techniques inapplicable. What do people use to break simple substitutions? Letter frequency, digram frequency, short words, pattern words, etc. The ACA's "Cryptogram" has 25 Aristocrats in each issue. The last five are usually of this form - texts chosen with the intent of making the normal techniques of breaking substitution ciphers difficult. A message consisting only of words of six letters with no duplicates, so that pattern word matching is useless. Or a message such as the one I quoted, with an absurd number of repetitions of a normally very low frequency digram. What I quoted wasn't ciphertext, it was plaintext of a type that would produce ciphertext that would be resistant to the ordinary attacks. |
| When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl. | |
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| Can Anyone Break This???? · Challenges | |
2:59 PM Nov 26
