1
vote
0answers
126 views

Why do we need to hash both the message and the $h$ value in ElGamal signature?

The professor left us a question on ElGamal signatures: Given the hash function $H$ and message $M$, choose a random $r$ and compute $h=g^r$ and $H(M||h)$. Show that, if $H(M)$ is used instead of ...
2
votes
1answer
176 views

ElGamal signature: Forging a signature of a specific form

I have a question I can't solve from one of the courses I'm currently taking: Show that given a legitimate ElGamal signature $(S,R)$ on a given message $m$, an attacker can compute a signature ...
2
votes
0answers
970 views

An existential forgery attack on ElGamal [closed]

From an old exam question: Consider this existential forgery attack on ElGamal. Choose $u$, and $v$ such that $\operatorname{gcd}(v, p - 1) = 1$. Compute $r := y^v g^u \mod p$ and $s := ...