static int test_exp_mod_zero()
{
    BIGNUM *a = NULL, *p = NULL, *m = NULL;
    BIGNUM *r = NULL;
    BN_ULONG one_word = 1;
    BN_CTX *ctx = BN_CTX_new();
    int ret = 1, failed = 0;

    m = BN_new();
    if (!m)
        goto err;
    BN_one(m);

    a = BN_new();
    if (!a)
        goto err;
    BN_one(a);

    p = BN_new();
    if (!p)
        goto err;
    BN_zero(p);

    r = BN_new();
    if (!r)
        goto err;

    if (!BN_rand(a, 1024, 0, 0))
        goto err;

    if (!BN_mod_exp(r, a, p, m, ctx))
        goto err;

    if (!a_is_zero_mod_one("BN_mod_exp", r, a))
        failed = 1;

    if (!BN_mod_exp_recp(r, a, p, m, ctx))
        goto err;

    if (!a_is_zero_mod_one("BN_mod_exp_recp", r, a))
        failed = 1;

    if (!BN_mod_exp_simple(r, a, p, m, ctx))
        goto err;

    if (!a_is_zero_mod_one("BN_mod_exp_simple", r, a))
        failed = 1;

    if (!BN_mod_exp_mont(r, a, p, m, ctx, NULL))
        goto err;

    if (!a_is_zero_mod_one("BN_mod_exp_mont", r, a))
        failed = 1;

    if (!BN_mod_exp_mont_consttime(r, a, p, m, ctx, NULL)) {
        goto err;
    }

    if (!a_is_zero_mod_one("BN_mod_exp_mont_consttime", r, a))
        failed = 1;

    /*
     * A different codepath exists for single word multiplication
     * in non-constant-time only.
     */
    if (!BN_mod_exp_mont_word(r, one_word, p, m, ctx, NULL))
        goto err;

    if (!BN_is_zero(r)) {
        fprintf(stderr, "BN_mod_exp_mont_word failed:\n");
        fprintf(stderr, "1 ** 0 mod 1 = r (should be 0)\n");
        fprintf(stderr, "r = ");
        BN_print_fp(stderr, r);
        fprintf(stderr, "\n");
        return 0;
    }

    ret = failed;

 err:
    BN_free(r);
    BN_free(a);
    BN_free(p);
    BN_free(m);
    BN_CTX_free(ctx);

    return ret;
}