Unformatted text preview: and confirms that RB has the same value as it did in step (2). 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 , also uses symmetric cryptography and Trent. (1) Alice sends a message to Trent consisting of her name, Bob’s name, and a random number. A,B,RA (2) Trent generates a random session key. He encrypts a message consisting of a random session key and Alice’s name with the secret key he shares with Bob. Then he encrypts Alice’s random value, Bob’s name, the key, and the encrypted message with the secret key he shares with Alice. Finally, he sends her the encrypted message: EA(RA,B,K,EB(K,A)) (3) Alice decrypts the message and extracts K. She confirms that RA is the same value that she sent Trent in step (1). Then she sends Bob the message that Trent encrypted in his key. EB(K,A) (4) Bob decrypts the message and extracts K. He then generates another random value, RB. He encrypts the message with K and sends it to Alice. EK(RB) (5) Alice decrypts the message with K. She generates RB - 1 and encrypts it with K. Then she sends the message back to Bob. EK(RB - 1) (6) Bob decrypts the message with K and verifies that it is RB - 1. 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 Trent’s 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 Alice’s messages are not replays from an earlier execution of the protocol. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home Use of this site is subjec...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips