SRP_Calc_A
BIGNUM *SRP_Calc_A(const BIGNUM *a, const BIGNUM *N, const BIGNUM *g);
if (!(s->srp_ctx.A = SRP_Calc_A(s->srp_ctx.a, s->srp_ctx.N, s->srp_ctx.g)))
Apub = SRP_Calc_A(a, GN->N, GN->g);
Apub = SRP_Calc_A(a, GN->N, GN->g);