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

If alice gives it to him when they are face to face

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Unformatted text preview: t key, are vanishingly small. This is called a nonlinear keyspace, because all the keys are not equally strong. (The opposite is a linear, or flat, keyspace.) An easy way to do this is to create the key as two parts: the key itself and some fixed string encrypted with that key. The module decrypts the string with the key; if it gets the fixed string it uses the key normally, if not it uses a different, weak algorithm. If the algorithm has a 128-bit key and a 64-bit block size, the overall key is 192 bits; this gives the algorithm an effective key of 2128, but makes the odds of randomly choosing a good key one in 264. You can be even subtler. You can design an algorithm such that certain keys are stronger than others. An algorithm can have no weak keys—keys that are obviously very poor—and can still have a nonlinear keyspace. This only works if the algorithm is secret and the enemy can’t reverse-engineer it, or if the difference in key strength is subtle enough that the enemy can’t figure it out. The NSA did this with the secret algorithms in their Overtake modules (see Section 25.1). Did they do the same thing with Skipjack (see Section 13.12)? No one knows. 8.3 Transferring Keys Alice and Bob are going to use a symmetric cryptographic algorithm to communicate securely; they need the same key. Alice generates a key using a random-key generator. Now she has to give it to Bob—securely. If Alice can meet Bob somewhere (a back alley, a windowless room, or one of Jupiter’s moons), she can give him a copy of the key. Otherwise, they have a problem. Public-key cryptography solves the problem nicely and with a minimum of prearrangement, but these techniques are not always available (see Section 3.1). Some systems use alternate channels known to be secure. Alice could send Bob the key with a trusted messenger. She could send it by certified mail or via an overnight delivery service. She could set up another communications channel with Bob and hope no one is eavesdropping on that one. Alice...
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