Consider the following protocol for Authentication/Key Distribution: (X and Y are two principals, A is a Certificate Authority or a Key Distribution Center, RX is a rand on number and EX means encrypted with the secret key of X.)
1. X→A: X, Y, RX
2. A→X: EX (RX, Y, K, EY (K, X))
3. X→Y: EY (K, X)
4. Y→X: EK (RY)
5. X→Y: EK (RY - 1)
(a) What does the presence of RX in message 2 assure?
(b) What problem will be created if an attacker were to break an old K (and the attacker has also copied messages for that session)? Explain your answer.
(c) Suggest a method to solve this problem?