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In the ElGamal signature scheme we calculate a signature for a (hashed) message $m$ as

$r \equiv_p g^k$,

$s \equiv_{p-1} (m - xr) k^{-1}$

and verify it by checking

$g^m \equiv_p y^r r^s$.

Here $r$ is a group element which is in some cases used as an exponent. This is not a problem, as long as the group elements are just natural numbers, but using a different group, this causes complications (like in EC-DSA, where we have to use just one coordinate of $r$ when it ought to be an exponent).

Even though this is admittedly not a big problem, my question is: Why is it even necessary to include $r$ as an exponent in $s$? Would changing the calculation of $s$ to just

$s \equiv_{p-1} (m - x) k^{-1}$

and the verification condition to

$g^m \equiv_p y r^s$

be insecure in some way? So far I couldn't find a way to forge a signature or extract $x$ or $k$ from this modified scheme.

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  • $\begingroup$ Choose arbitrary $s$ in $Z_{p-1}^*$ (i.e. invertible) and set $r=(y^{-1}g^m)^{s^{-1}}$. $\endgroup$ – DrLecter Aug 20 '14 at 20:00
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With your proposed modification of the ElGamal signature scheme you can produce forgeries for arbitrary (hashed) messages $m$.

By looking at the verification equation $$g^m = yr^s$$ you just have to set $r$ to $r=(g^my^{-1})^{s^{-1}}$ (just by rearranging the verification equation) which you can do for any $s$ from $\mathbb{Z}_{p-1}^*$, i.e., every $s$ that has a multiplicative inverse in $\mathbb{Z}_{p-1}$.

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