Sidon sets and statistics of the ElGamal function

Abstract: In the ElGamal signature and encryption schemes, an element $x$ of the underlying group $G = \mathbb{Z}p^\times = {1, \ldots, p-1 }$ for a prime $p$ is also considered as an exponent, for example in $g^x$, where $g$ is a generator of G. This ElGamal map $x \mapsto g^x$ is poorly understood, and one may wonder whether it has some randomness properties. The underlying map from $G$ to $\mathbb{Z}{p-1}$ with $x \mapsto x$ is trivial from a computer science point of view, but does not seem to have any mathematical structure. This work presents two pieces of evidence for randomness. Firstly, experiments with small primes suggest that the map behaves like a uniformly random permutation with respect to two properties that we consider. Secondly, the theory of Sidon sets shows that the graph of this map is equidistributed in a suitable sense. It remains an open question to prove more randomness properties, for example, that the ElGamal map is pseudorandom.