Question

Consider our authentication protocol 4.0, in which Alice authenticates herself to Bob, which we saw works well (i.e., we found no flaws in it.). Now suppose that at the same time that Alice authenticates herself to Bob, Bob must authenticate himself to Alice. Give a scenario by which Trudy, pretending to be Alice, can now authenticate herself to Bob as Alice.

Answer 1

According to protocol 4.0, the scenario can be defined in terms of Bob, Alice and Trudy as follows:

The communication is done between Bob and Alice only in whom they share a secret key KAB with each other. The Trudy is an intruder who wants to get the information that is being communicated between Alice and Bob. To do so, Trudy wants Bob to initiate the communication by authenticating herself her Alice. Thus, Trudy waits for the Bob to start in-order to make the Bob authenticate himself.

Here are the steps how the communication is done between the Bob and Trudy:

Step 1: Bob starts the communication by himself to other side and waits for the reply.

Step 2: Trudy starts to disguise as Alice herself and authenticate herself to Bob.

Step 3: After looking at the reply, Bob sends the nonce key to Trudy. At this step, still Trudy does not know the KAB(R) in-order to reply to Bob.

Step 4: At this point Trudy response to the step-1 while Bob still continuing to authentication. Trudy picks the nonce for the Bob to encrypt the message with the Bob sent nonce in the Step-3.

Step 5: Now Bob completes his own authentication to himself on the other side of encrypting the nonce he sent at step 4. Where, Trudy gets the nonce key KAB( R).

Step 6: Now, Trudy completes her authentication in responding to R that is sent by Bob in step 3. At this point, Trudy has responded properly, so Bob thinks that he is communicating with Alice(Trudy).

The actual scenario with respect to above steps, the communication is done as follows:

Bob: “I am Bob”

Trudy: “I am Alice”

Bob: “R”

Trudy: “R”

Bob: “KAB (R )” – Bob completes his authentication.

Trudy: Get the KAB (R ) – Trudy complete her authentication.

Thus, the communication is started between the Bob and Trudy instead of Bob and Alice according to protocol 4.0.