U4.4 Encryption Based Unilateral authentication protocol

Alice messages Bob >Hi, its Alice
1. A-->B "Hi Bob its Alice
Bob responds with a random bit string as a challenge
1. B --> A : R
  1. As its a random string it cant easily be predicted by an attacker
Alice responds by sending an encryption of the challenge concatenated with Bobs ID using their shared key K
1. A--> B: {R II B}K
2. Bob decrypts the message and checks the plaintext contains his challenge
  1. 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
  2. This also means the message cant be a replay because it contains the challenge himself
Random Challenges
1. An attacker (M) even if he sees the challenge cant prepare a response cos he doesnt know the Key K
  1. If M could predict the challenge he could impersonate Bob issue the predicted challenge and get A's response (encrypted with key)
    1. Later M impersonates Alice, when Bob issues the challenge M sends the one he got from A to Bob
      1. Replaying messages later = replay attack
As soon as this protocol is complete it could be hijacked
1. Therefore only providing a guarantee at time of authentication

Resource summary

	Created by Craig Parker over 10 years ago