Hot answers tagged elgamal-signature
8
In this context, "nondeterministic" means that the algorithm to generate the ciphertext (or the signature) takes a random value as one of its inputs, and it can generate many possible ciphertexts (or signatures) based on the random value. ElGamal is nondetermanistic because the encryptor selects a random exponent as a part of encryption method.
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4
That looks about right. Assume we have two messages $m_1$ and $m_2$ and the corresponding signatures $(r,s_1)$ and $(r,s_2)$ generated using the same $k$ (where $r=g^k$ is thus the same for both signatures).
If we could assume that $s_1 - s_2$ and $r$ were invertible modulo $p-1$, we could simply compute
$$ k \equiv (m_1 - m_2)(s_1 - s_2)^{-1} \mod p-1 $$
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1
There seem to be no standardized ElGamal test vectors available in the public domain. However, there are some ElGamal test vectors generated with libgcrypt 1.5.0 available in this fork of the pycrypto project.
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