Cryptography & Network Security

2. (15 points) User Authentication. (a) (7 points) Consider the following proposed authentication protocol between Alice and Bob based on symmetric encryption with a trusted third party (TTP) which is sup- posed to establish a shared session key K between the two users. Alice + TTP: Alice, Bob TTP + Alice : {K}KAT, {K}Ket Alice + Bob : {K}KBT Bob + Alice : {Hello Alice! This is Bob}K Show an attack against this protocol by demonstrating a scenario where Eve can impersonate Bob to Alice (i.e. Alice will think they are talking to Bob when they are in fact talking to Eve). Then provide a fix to the protocol to prevent this attack. 2 (b) (8 points) Consider the following proposed authentication protocol between Alice and Bob based on asymmetric encryption which is supposed to establish a shared session key K between the two users. Alice's public key is KA and Bob's public key is KB. Alice + Bob : {Alice, NAHKB Bob Alice : {K, NAHKA Alice Bob: {Hello Bob! This is Alice, NAHK Show an attack against this protocol by demonstrating a scenario where Eve can impersonate Alice to Bob (i.e. Bob will think they are talking to Alice when they are in fact talking to Eve). Then provide a fix to the protocol to prevent this attack