U4.8 protocols using trusted 3rd parties

U4.8 protocols using trusted 3rd parties

1. In large systems where n users with shared keys are using an encryption based authentication protocol, the numbers of keys required would be huge (n-1)/2

   • Distribution & storage of many keys is difficult
     • Keys may accidentally be shared
       • the alternative to using large numbers of shared secret keys, is using public-key techniques with trust in keys based on certificates

2. Using a trusted 3rd party Key Distribution Centre

   • Users would only need to look after their own private key and have access to authentic public keys for the other users

3. Could be a corporate network, with the KDC being a server managed by the company IT department,

4. Could be a campus network, where the authentication protocol is used to control access to different services located on the network eg: file and print

5. The Needham–Schroeder protocol

   • Allows users to make use of the KDC to allow them to authenticate one another and to agree session keys
     • More complex because there are 3 entities involved in the protocol
       • 1. Alice asks Trent for a session Key she can use to communicate with Bob
         • 2. Trent replies to Alice. The entire message is encrypted under the long-term key KA,T that Alice and Trent share
           • This msg contains Alice's nonce NA. The inclusion of this nonce authenticates Trent to Alice: only Trent knows the long-term key KA,T, so only Trent can respond with an encrypted message containing Alice's nonce.
             • Secondly it contains B's identity
               • Thirdly it contains the session key K that Alice and Bob will use. Notice that this session key is chosen by Trent
                 • Fourthly, it contains a string {K || A}KB,T, which is the plaintext K || A encrypted under the long-term key shared between Bob and Trent
                   • 3. Alice forwards to Bob the ciphertext
                     • It contains Bob's copy of the session Key
                       • Bob receives this message, he decrypts it, extracts K and checks that the message included A's identity
                         • 4. and 5. A & B engage in unilateral authentication protocol
                           • A is authenticated to B
                             • Bob issues a challenge
                               • Alice rsponds