applied cryptography - protocols, algorithms, and source code in c

Many messages have standard beginnings and endings

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Unformatted text preview: n-plaintext attacks and chosen-plaintext attacks are more common than you might think. It is not unheard-of for a cryptanalyst to get a plaintext message that has been encrypted or to bribe someone to encrypt a chosen message. You may not even have to bribe someone; if you give a message to an ambassador, you will probably find that it gets encrypted and sent back to his country for consideration. Many messages have standard beginnings and endings that might be known to the cryptanalyst. Encrypted source code is especially vulnerable because of the regular appearance of keywords: #define, struct, else, return. Encrypted executable code has the same kinds of problems: functions, loop structures, and so on. Known-plaintext attacks (and even chosen-plaintext attacks) were successfully used against both the Germans and the Japanese during World War II. David Kahn’s books [794,795,796] have historical examples of these kinds of attacks. And don’t forget Kerckhoffs’s assumption: If the strength of your new cryptosystem relies on the fact that the attacker does not know the algorithm’s inner workings, you’re sunk. If you believe that keeping the algorithm’s insides secret improves the security of your cryptosystem more than letting the academic community analyze it, you’re wrong. And if you think that someone won’t disassemble your code and reverse-engineer your algorithm, you’re naïve. (In 1994 this happened with the RC4 algorithm—see Section 17.1.) The best algorithms we have are the ones that have been made public, have been attacked by the world’s best cryptographers for years, and are still unbreakable. (The National Security Agency keeps their algorithms secret from outsiders, but they have the best cryptographers in the world working within their walls—you don’t. Additionally, they discuss their algorithms with one another, relying on peer review to uncover any weaknesses in their work.) Cryptanalysts don’t always have access to the algorithms, as...
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