Unformatted text preview: authentication and key exchange protocols A B EA EB I K L Alice’s name Bob’s name Encryption with a key Trent shares with Alice Encryption with a key Trent shares with Bob Index number A random session key Lifetime TA,TB A timestamp RA,RB A random number, sometimes called a nonce, chosen by Alice and Bob respectively (1) Alice concatenates a timestamp, Bob’s name, and a random session key and encrypts the whole message with the key she shares with Trent. She sends this to Trent, along with her name. A,EA(TA,B,K) (2) Trent decrypts the message from Alice. Then he concatenates a new timestamp, Alice’s name, and the random session key; he encrypts the whole message with the key he shares with Bob. Trent sends to Bob: EB(TB,A,K) The biggest assumption made in this protocol is that Alice is competent enough to generate good session keys. Remember that random numbers aren’t 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]. (1) Alice concatenates her name and a random number, and sends it to Bob. A,RA (2) Bob concatenates Alice’s name, Alice’s random number, his own random number, and encrypts it with the key he shares with Trent. He sends this to Trent, along with his name. B,EB(A,RA,RB) (3) Trent generates two messages. The first consists of Bob’s name, a random session key, Alice’s random number, and Bob’s random number, all encrypted with the key he shares with Alice. The second consists of Alice’s Zname and the random session key, encrypted with the key he shares with Bob. He sends both messages to Alice. EA(B,K,RA,RB),EB(A,K) (4) Alice decrypts the first message, 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),EK(RB) (5) Bob decrypts the message encrypted with his key, extracts K,...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips