We use the proof as given in protocol 6.1 in https://eprint.iacr.org/2017/552.pdf
Statement: (c, pk, Q, G)
witness (x, r, sk) such that Q = xG, c = Enc(pk, x, r) and Dec(sk, c) = x.
note that because of the range proof, the proof is sound only for x < q/3
We use the proof as given in proof PIi in https://eprint.iacr.org/2016/013.pdf.
This proof ws taken from the proof 6.3 (left side ) in https://www.cs.unc.edu/~reiter/papers/2004/IJIS.pdf