U4.4 Encryption Based Unilateral authentication protocol

U4.4 Encryption Based Unilateral authentication protocol

1. Alice messages Bob >Hi, its Alice

   • A-->B "Hi Bob its Alice

2. Bob responds with a random bit string as a challenge

   • B --> A : R
     • As its a random string it cant easily be predicted by an attacker

3. Alice responds by sending an encryption of the challenge concatenated with Bobs ID using their shared key K

   • A--> B: {R II B}K
   • Bob decrypts the message and checks the plaintext contains his challenge
     • Bob can be sure this message came from Alice because they share a secret key, because it includes Bob's identity he knows he didnt generate it humself
     • This also means the message cant be a replay because it contains the challenge himself

4. Random Challenges

   • An attacker (M) even if he sees the challenge cant prepare a response cos he doesnt know the Key K
     • If M could predict the challenge he could impersonate Bob issue the predicted challenge and get A's response (encrypted with key)
       • Later M impersonates Alice, when Bob issues the challenge M sends the one he got from A to Bob
         • Replaying messages later = replay attack

5. As soon as this protocol is complete it could be hijacked

   • Therefore only providing a guarantee at time of authentication