Suppose the following challenge-response protocol is used by two parties which s

Question

Suppose the following challenge-response protocol is used by two parties which share a secret K to mutually authenticate each other:

ALICE BOB

--------------------------->>> I'm Alice

R <<<------------------------

------------------------->>> {R}K

The figure shows how Alice authenticates herself to Bob, but Bob can authenticate himself to Alice in a similar way (i.e., exchanging the roles of Alice and Bob in the figure). After authentication, the two parties want to set up a session key. Which of the following are secure to be used as a session key? For those not secure, explain why.

1) K XOR R;

2) EK(R+K);

3) EK(K+1);

4) EK+R(R)

Explanation / Answer

1. K XOR R : Insecure session key as it is possible to arrive at a combination of X and Y such that K!=X and R!=Y and X XOR Y = K XOR R and this would result in an insecure session, as the session key could be cracked.

2. EK(R+K): Secure session key, extremely difficult to factorize

3. EK(K+1): Doesnot contain a component that is a multiple of 'R', that represents authentication of Alice. Insecure session key, would not be unique to a session also. Hence, each session would not be safe from data gathered in other sessions.

4. EK + R(R): Secure session key, extremely difficult to factorize

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