GATE | GATE-CS-2017 (Set 1) | Question 13

A sender S sends a message m to receiver R, which is digitally signed by S with its private key. In this scenario, one or more of the following security violations can take place.

(I) S can launch a birthday attack to replace m with a fraudulent message.

(II) A third party attacker can launch a birthday attack to replace m with a fraudulent message.

(III) R can launch a birthday attack to replace m with a fraudulent message.
(A) (I) and (II) only
(B) (I) only
(C) (II) only
(D) (II) and (III) only


Answer: (B)

Explanation: First let’s know what is Birthday attack :

Using Birthday Attack, some fraudulent message can be generated which has same hash value and digital signature as the original message.

But this question is more about Private Key. Let’s analyse each option one by one –

I. Instead of intended message, Sender can take some fraudulent message and encrypt it with own private key and then receiver’s public key. This is POSSIBLE.
II. Third party don’t have that Private Key to encrypt the message. So this is NOT possible.
III. And the same way R also don’t have that Private key to encrypt the message.

So, the only possibility is – (I) S can launch a birthday attack to replace m with a fraudulent message.
Option (B) is correct.

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