# Which answer is true regarding birthday attack on digital signatures?

The actual question is:

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 mm 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.

Which of the following are possible security violations?

A. (I) and (II) only

B. (I) only

C. (II) only

D. (II) and (III) only

so if a sender sends a message m to reciever.. I know that an attacker can perform a birthday attack and send fraudulent message to receiver

But these statements confuse me!

1. Sender can perform birthday attack and send fraudulent message to Receiver

2. Receiver can perform birthday attack and create fraudulent message.

I searched upon this but this is too confusing can someone explain? They both seem to be true but the answer i saw states only one is? Can someone explain birthday attack! Thanks

• you are massively confused. usually the attacker is not the sender or the receiver but a malicious third party. also birthday attack has to do with collisions in messages. the sender already has the message, what on earth does it mean for him to perform a birthday attack? he doesn't need to create a collision, he knows the message. – kodlu Jan 26 '18 at 17:18
• this question needs rewriting, since we can't read your mind. – kodlu Jan 26 '18 at 17:18
• OK but please, how on earth is that the same question you wrote? Why didn't you copy the whole question? There are signatures involved as well which change the context completely. – kodlu Jan 26 '18 at 19:23
• Sorry @kodlu I was in hurry – poda_badu Feb 3 '18 at 4:46

A birthday attacks generates a pair of colliding messages $m, m'$. Because of the way the collision is generated, you can't predict either $m$ or $m'$. If $m$ is fixed, then you need to perform a second-preimage attack instead.
The only person in this scenario who can change $m$ is $S$. $R$ and the third party only receive a fixed message $m$, and have to generate a second preimage $m'$. This is not possible using a birthday attack.