bignum.c

/*
 * Copyright 2016 The OpenSSL Project Authors. All Rights Reserved.
 *
 * Licensed under the OpenSSL licenses, (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 "fuzzer.h"

int FuzzerInitialize(int *argc, char ***argv) {
    return 1;
}

int FuzzerTestOneInput(const uint8_t *buf, size_t len) {
    static BN_CTX *ctx;
    static BIGNUM *b1;
    static BIGNUM *b2;
    static BIGNUM *b3;
    static BIGNUM *b4;
    static BIGNUM *b5;
    int success = 0;
    size_t l1 = 0, l2 = 0, l3 = 0;
    int s1 = 0, s2 = 0, s3 = 0;

    if (ctx == NULL) {
        b1 = BN_new();
        b2 = BN_new();
        b3 = BN_new();
        b4 = BN_new();
        b5 = BN_new();
        ctx = BN_CTX_new();
    }
    /* 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;
        l1 = (buf[0] * len) / 255;
        ++buf;
        l2 = (buf[0] * (len - l1)) / 255;
        ++buf;
        l3 = len - l1 - l2;

        s1 = buf[0] & 1;
        s2 = buf[0] & 2;
        s3 = buf[0] & 4;
        ++buf;
    }
    OPENSSL_assert(BN_bin2bn(buf, l1, b1) == b1);
    BN_set_negative(b1, s1);
    OPENSSL_assert(BN_bin2bn(buf + l1, l2, b2) == b2);
    BN_set_negative(b2, s2);
    OPENSSL_assert(BN_bin2bn(buf + l1 + l2, l3, b3) == b3);
    BN_set_negative(b3, s3);

    /* mod 0 is undefined */
    if (BN_is_zero(b3)) {
        success = 1;
        goto done;
    }

    OPENSSL_assert(BN_mod_exp(b4, b1, b2, b3, ctx));
    OPENSSL_assert(BN_mod_exp_simple(b5, b1, b2, b3, ctx));

    success = BN_cmp(b4, b5) == 0;
    if (!success) {
        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:
    OPENSSL_assert(success);

    return 0;
}