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Effect of Partial Key Knowledge
For strong cryptologic algorithms, it is often assumed that exhaustive search (AKA "brute force) will take 2b trials, where b is the number of bits of the secret key. What happens, though, if an adversary gains partial knowledge of the secret key? Perhaps he has intercepted a garbled transmission of the key, where he knows the maximum number of garbles, but not where they occur, or perhaps he knows the probability of each bit being correct. How much does this help him?