int DH_check(const DH *dh, int *ret)
{
    int ok = 0, r;
    BN_CTX *ctx = NULL;
    BIGNUM *t1 = NULL, *t2 = NULL;

    if (!DH_check_params(dh, ret))
        return 0;

    ctx = BN_CTX_new();
    if (ctx == NULL)
        goto err;
    BN_CTX_start(ctx);
    t1 = BN_CTX_get(ctx);
    t2 = BN_CTX_get(ctx);
    if (t2 == NULL)
        goto err;

    if (dh->q) {
        if (BN_cmp(dh->g, BN_value_one()) <= 0)
            *ret |= DH_NOT_SUITABLE_GENERATOR;
        else if (BN_cmp(dh->g, dh->p) >= 0)
            *ret |= DH_NOT_SUITABLE_GENERATOR;
        else {
            /* Check g^q == 1 mod p */
            if (!BN_mod_exp(t1, dh->g, dh->q, dh->p, ctx))
                goto err;
            if (!BN_is_one(t1))
                *ret |= DH_NOT_SUITABLE_GENERATOR;
        }
        r = BN_is_prime_ex(dh->q, DH_NUMBER_ITERATIONS_FOR_PRIME, ctx, NULL);
        if (r < 0)
            goto err;
        if (!r)
            *ret |= DH_CHECK_Q_NOT_PRIME;
        /* Check p == 1 mod q  i.e. q divides p - 1 */
        if (!BN_div(t1, t2, dh->p, dh->q, ctx))
            goto err;
        if (!BN_is_one(t2))
            *ret |= DH_CHECK_INVALID_Q_VALUE;
        if (dh->j && BN_cmp(dh->j, t1))
            *ret |= DH_CHECK_INVALID_J_VALUE;
    }

    r = BN_is_prime_ex(dh->p, DH_NUMBER_ITERATIONS_FOR_PRIME, ctx, NULL);
    if (r < 0)
        goto err;
    if (!r)
        *ret |= DH_CHECK_P_NOT_PRIME;
    else if (!dh->q) {
        if (!BN_rshift1(t1, dh->p))
            goto err;
        r = BN_is_prime_ex(t1, DH_NUMBER_ITERATIONS_FOR_PRIME, ctx, NULL);
        if (r < 0)
            goto err;
        if (!r)
            *ret |= DH_CHECK_P_NOT_SAFE_PRIME;
    }
    ok = 1;
 err:
    BN_CTX_end(ctx);
    BN_CTX_free(ctx);
    return ok;
}