Unformatted text preview: name, her random number, and a timestamp, and encrypts with the key he shares with Trent. He sends it to Trent along with his name and a new random number. B,RB,EB(A,RA,TB) (3) Trent generates a random session key. Then he creates two messages. The first is Bob’s name, Alice’s random number, a random session key, and the timestamp, all encrypted with the key he shares with Alice. The second is Alice’s name, the session key, and the timestamp, all encrypted with the key he shares with Bob. He sends these both to Alice, along with Bob’s random number. EA(B,RA,K,TB),EA(A,K,TB),RB (4) Alice decrypts the message encrypted with her key, extracts K, and confirms that RA has the same value as it did in step (1). Alice sends Bob two messages. The first is the message received from Trent, encrypted with Bob’s key. The second is RB, encrypted with the session key. EB(A,K,TB),EK(RB) (5) Bob decrypts the message encrypted with his key, extracts K, and confirms that TB and RB have the same value they did in step (2). Assuming both random numbers and the timestamp match, Alice and Bob are convinced of one another’s identity and share a secret key. Synchronized clocks are not required because the timestamp is only relative to Bob’s clock; Bob only checks the timestamp he generated himself. One nice thing about this protocol is that Alice can use the message she received from Trent for subsequent authentication with Bob, within some predetermined time limit. Assume that Alice and Bob completed the above protocol, communicated, and then terminated the connection. Alice and Bob can reauthenticate in three steps, without having to rely on Trent. (1) Alice sends Bob the message Trent sent her in step (3) and a new random number. EB(A,K,TB),R’A (2) Bob sends Alice another new random number, and Alice’s new random number encrypted in their session key. R’B,EK(R’A) (3) Alice sends Bob his new random number, encrypted in their session key. EK(R’B) The new random numbers prevent replay attacks. DASS
The Distributed Authen...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips