BN_mod_sub
int BN_mod_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m, BN_CTX *ctx);
LCRYPTO_ALIAS(BN_mod_sub);
LCRYPTO_USED(BN_mod_sub);
if (!BN_mod_sub(diff, want, got, p, ctx))
if (!BN_mod_sub(diff, want, got, p, ctx))