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

Broadcasting a message imagine that you have 100

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Unformatted text preview: es for honest participants, describing operations available to all—honest and dishonest—participants, describing the basic building blocks of the protocol, and describing the reduction rules. The point of all this is to show that a given protocol meets its requirements. Tools like the NRL Protocol Analyzer could eventually lead to a protocol that can be proven secure. While much of the work in formal methods involves applying the methods to existing protocols, there is some push towards using formal methods to design the protocols in the first place. Some preliminary steps in this direction are [711]. The NRL Protocol Analyzer also attempts to do this [1512,222,1513]. The application of formal methods to cryptographic protocols is still a fairly new idea and it’s really hard to figure out where it is headed. At this point, the weakest link seems to be the formalization process. 3.5 Multiple-Key Public-Key Cryptography Public-key cryptography uses two keys. A message encrypted with one key can be decrypted with the other. Usually one key is private and the other is public. However, let’s assume that Alice has one key and Bob has the other. Now Alice can encrypt a message so that only Bob can decrypt it, and Bob can encrypt a message so that only Alice can read it. This concept was generalized by Colin Boyd [217]. Imagine a variant of public-key cryptography with three keys: KA, KB, and KC, distributed as shown in Table 3.2. Alice can encrypt a message with KA so that Ellen, with KB and KC, can decrypt it. So can Bob and Carol in collusion. Bob can encrypt a message so that Frank can read it, and Carol can encrypt a message so that Dave can read it. Dave can encrypt a message with KA so that Ellen can read it, with KB so that Frank can read it, or with both KA and KB so that Carol can read it. Similarly, Ellen can encrypt a message so that either Alice, Dave, or Frank can read it. All the possible combinations are summarized in Table 3.3; there are no other ones. TABLE 3.2 Three-Key Key Distribution A...
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