Authentication/Key Distribution(2+4+2+2) Consider the following protocol for Aut

Question

Authentication/Key Distribution(2+4+2+2) Consider the following protocol for Authentication/Key Distribution: (X and Y are two nodes, A is a CA/KDC, N_1 and N_2 are random numbers, and E_x means encrypted with the secret key of X.) 1. X rightarrow A: X, Y, N_1 2. A rightarrow X: E_x (N_1, X, Y, K, E_y (K, X)) 3. X rightarrow Y: E_y(K, X) 4. Y rightarrow X: E_k(N_2) 5. X rightarrow Y: E_K(N_2 - 1) What is purpose of N_1 in message 2? Show that if an attacker were to break an old K, he/she can impersonate user X to user Y. How to solve this problem? What is the purpose of Steps 4 and 5?

Explanation / Answer

1
N1 determines the freshness of the response. I reduces the chance of replay attack.

2
Let C be the Intruder with knowledge of K, which was also the secret in earlier transaction. C is also aware of Ey(K, X)
C-->Y Ey(K, X) //replay attack
Y-->C Ek(N2)
C-->Y Ek(N2 - 1)
Successfully starts communication with Y

3
Inclusion of freshness token in the while sending the access token to Y will prohibit the use of replay attack. e.g.

X-->A X, Y, N1
A-->X Ex(N1, N2, X, Y, K, Ey(K,X,N2))
X-->Y Ey(K, X, N2)
Y-->X Ek(N2)
X-->Y Ek(N2 - 1)

Stopping replay attack is enough to secure this communication channel

4.
Last two steps are used for hand shake where both user try to perform handshake using the secret token. Once handshake process is executed sucessfully, X and Y agrees upon K and N2 and continue communication based oh K and N2

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