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The scheme you consider is the original ElGamal signature. This scheme is known to be existentially forgeable. By definition, a valid original ElGamal signature on a message $m \in \{1, \dots, p-1\}$ is a pair $(r,s)$ satisfying $g^m \equiv y^r \cdot r^s \pmod p$. With $r = g^e \cdot y^v \bmod p$ and $s = -r\cdot v^{-1} \bmod (p-1)$ for random integers $...

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