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

Those who claim to have an unbreakable cipher simply

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Unformatted text preview: when the United States broke the Japanese diplomatic code PURPLE during World War II [794]—but they often do. If the algorithm is being used in a commercial security program, it is simply a matter of time and money to disassemble the program and recover the algorithm. If the algorithm is being used in a military communications system, it is simply a matter of time and money to buy (or steal) the equipment and reverse-engineer the algorithm. Those who claim to have an unbreakable cipher simply because they can’t break it are either geniuses or fools. Unfortunately, there are more of the latter in the world. Beware of people who extol the virtues of their algorithms, but refuse to make them public; trusting their algorithms is like trusting snake oil. Good cryptographers rely on peer review to separate the good algorithms from the bad. Security of Algorithms Different algorithms offer different degrees of security; it depends on how hard they are to break. If the cost required to break an algorithm is greater than the value of the encrypted data, then you’re probably safe. If the time required to break an algorithm is longer than the time the encrypted data must remain secret, then you’re probably safe. If the amount of data encrypted with a single key is less than the amount of data necessary to break the algorithm, then you’re probably safe. I say “probably” because there is always a chance of new breakthroughs in cryptanalysis. On the other hand, the value of most data decreases over time. It is important that the value of the data always remain less than the cost to break the security protecting it. Lars Knudsen classified these different categories of breaking an algorithm. In decreasing order of severity [858]: 1. Total break. A cryptanalyst finds the key, K, such that DK(C) = P. 2. Global deduction. A cryptanalyst finds an alternate algorithm, A, equivalent to DK(C), without knowing K. 3. Instance (or local) deduction. A cryptanalyst finds the plaintext of an intercepted ci...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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