Take the 2-minute tour ×  I am trying to implement the Efficient
Robust Private Set Intersection using Additive ElGamal.  For
this, from the client side, my inputs are $C = \{1, 2\}$. So,
the polynomial is $(x-1)(x-2)$ which is $x^2-3x+2$.  The next
step is to encrypt the variable coefficients which are $1$,
$-3$, $2$.  Can someone tell me how to use ElGamal for
encrypting negative numbers. If there is an example which one
can run from Client and Server side, it will be really helpful.
share|improve this question Dr. Lecter, My question is more on
the front if the coefficients of the resulting polynomial can be
negative when (x-1)(x-2) is computed over a finite field? If
not, I would like to know how to compute the product of the
above polynomials? –  user11706 Feb 17 at 14:41 add comment  2
Answers  up vote 2 down vote accepted  I use the notation
introduced in the paper: The parameters of ElGamal are $(G, q,
g)$ where $q$ is prime, $G$ is a cyclic group of order $q$ and
$g$ is a generator. In the paper, th