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To access the contents, click the chapter and section titles.
Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth)
3.3 Authentication and Key ExchangeThese protocols combine authentication with key exchange to solve a general computer problem: Alice and Bob are on opposite ends of a network and want to talk securely. How can Alice and Bob exchange a secret key and at the same time each be sure that he or she is talking to the other and not to Mallory? Most of the protocols assume that Trent shares a different secret key with each participant, and that all of these keys are in place before the protocol begins. The symbols used in these protocols are summarized in Table 3.1. Wide-Mouth Frog The Wide-Mouth Frog protocol [283,284] is probably the simplest symmetric key-management protocol that uses a trusted server. Both Alice and Bob share a secret key with Trent. The keys are just used for key distribution and not to encrypt any actual messages between users. Just by using two messages, Alice transfers a session key to Bob:
The biggest assumption made in this protocol is that Alice is competent enough to generate good session keys. Remember that random numbers arent easy to generate; it might be more than Alice can be trusted to do properly. Yahalom In this protocol, both Alice and Bob share a secret key with Trent [283,284].
At the end, Alice and Bob are each convinced that they are talking to the other and not to a third party. The novelty here is that Bob is the first one to contact Trent, who only sends one message to Alice. Needham-Schroeder This protocol, invented by Roger Needham and Michael Schroeder [1159], also uses symmetric cryptography and Trent.
All of this fussing around with RA and RB and RB - 1 is to prevent replay attacks. In this attack, Mallory can record old messages and then use them later in an attempt to subvert the protocol. The presence of RA in step (2) assures Alice that Trents message is legitimate and not a replay of a response from a previous execution of the protocol. When Alice successfully decrypts RB and sends Bob RB - 1 in step (5), Bob is ensured that Alices messages are not replays from an earlier execution of the protocol.
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