fuzz/bignum.c

/*
 * Copyright 2016-2025 The OpenSSL Project Authors. All Rights Reserved.
 *
 * Licensed under the Apache License 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * https://www.openssl.org/source/license.html
 * or in the file LICENSE in the source distribution.
 */

/*
 * Confirm that a^b mod c agrees when calculated cleverly vs naively, for
 * random a, b and c.
 */

#include <stdio.h>
#include <openssl/bn.h>
#include <openssl/err.h>
#include "fuzzer.h"

int FuzzerInitialize(int *argc, char ***argv)
{
    OPENSSL_init_crypto(OPENSSL_INIT_LOAD_CRYPTO_STRINGS, NULL);
    ERR_clear_error();

    return 1;
}

int FuzzerTestOneInput(const uint8_t *buf, size_t len)
{
    size_t l1 = 0, l2 = 0, l3 = 0;
    int s1 = 0, s3 = 0;
    BN_CTX *ctx;
    BIGNUM *b1;
    BIGNUM *b2;
    BIGNUM *b3;
    BIGNUM *b4;
    BIGNUM *b5;

    b1 = BN_new();
    b2 = BN_new();
    b3 = BN_new();
    b4 = BN_new();
    b5 = BN_new();
    ctx = BN_CTX_new();

    if (b1 == NULL || b2 == NULL || b3 == NULL || b4 == NULL || b5 == NULL
        || ctx == NULL)
        goto done;

    /* Divide the input into three parts, using the values of the first two
     * bytes to choose lengths, which generate b1, b2 and b3. Use three bits
     * of the third byte to choose signs for the three numbers.
     */
    if (len > 2) {
        len -= 3;
        /* limit l1, l2, and l3 to be no more than 512 bytes */
        l1 = ((buf[0] * len) / 255) % 512;
        ++buf;
        l2 = ((buf[0] * (len - l1)) / 255) % 512;
        ++buf;
        l3 = (len - l1 - l2) % 512;

        s1 = buf[0] & 1;
        s3 = buf[0] & 4;
        ++buf;
    }
    if (BN_bin2bn(buf, (int)l1, b1) != b1)
        goto done;
    BN_set_negative(b1, s1);
    if (BN_bin2bn(buf + l1, (int)l2, b2) != b2)
        goto done;
    if (BN_bin2bn(buf + l1 + l2, (int)l3, b3) != b3)
        goto done;
    BN_set_negative(b3, s3);

    /* mod 0 is undefined */
    if (BN_is_zero(b3))
        goto done;

    if (!BN_mod_exp(b4, b1, b2, b3, ctx))
        goto done;
    if (!BN_mod_exp_simple(b5, b1, b2, b3, ctx))
        goto done;

    if (BN_cmp(b4, b5) != 0) {
        BN_print_fp(stdout, b1);
        putchar('\n');
        BN_print_fp(stdout, b2);
        putchar('\n');
        BN_print_fp(stdout, b3);
        putchar('\n');
        BN_print_fp(stdout, b4);
        putchar('\n');
        BN_print_fp(stdout, b5);
        putchar('\n');
    }

done:
    BN_free(b1);
    BN_free(b2);
    BN_free(b3);
    BN_free(b4);
    BN_free(b5);
    BN_CTX_free(ctx);
    ERR_clear_error();

    return 0;
}

void FuzzerCleanup(void)
{
}