bn_rand.c

/*
 * Copyright 1995-2019 The OpenSSL Project Authors. All Rights Reserved.
 *
 * Licensed under the OpenSSL license (the "License").  You may not use
 * this file except in compliance with the License.  You can obtain a copy
 * in the file LICENSE in the source distribution or at
 * https://www.openssl.org/source/license.html
 */

#include <stdio.h>
#include <time.h>
#include "internal/cryptlib.h"
#include "bn_local.h"
#include <openssl/rand.h>
#include <openssl/sha.h>

typedef enum bnrand_flag_e {
    NORMAL, TESTING, PRIVATE
} BNRAND_FLAG;

static int bnrand(BNRAND_FLAG flag, BIGNUM *rnd, int bits, int top, int bottom)
{
    unsigned char *buf = NULL;
    int b, ret = 0, bit, bytes, mask;

    if (bits == 0) {
        if (top != BN_RAND_TOP_ANY || bottom != BN_RAND_BOTTOM_ANY)
            goto toosmall;
        BN_zero(rnd);
        return 1;
    }
    if (bits < 0 || (bits == 1 && top > 0))
        goto toosmall;

    bytes = (bits + 7) / 8;
    bit = (bits - 1) % 8;
    mask = 0xff << (bit + 1);

    buf = OPENSSL_malloc(bytes);
    if (buf == NULL) {
        BNerr(BN_F_BNRAND, ERR_R_MALLOC_FAILURE);
        goto err;
    }

    /* make a random number and set the top and bottom bits */
    b = flag == NORMAL ? RAND_bytes(buf, bytes) : RAND_priv_bytes(buf, bytes);
    if (b <= 0)
        goto err;

    if (flag == TESTING) {
        /*
         * generate patterns that are more likely to trigger BN library bugs
         */
        int i;
        unsigned char c;

        for (i = 0; i < bytes; i++) {
            if (RAND_bytes(&c, 1) <= 0)
                goto err;
            if (c >= 128 && i > 0)
                buf[i] = buf[i - 1];
            else if (c < 42)
                buf[i] = 0;
            else if (c < 84)
                buf[i] = 255;
        }
    }

    if (top >= 0) {
        if (top) {
            if (bit == 0) {
                buf[0] = 1;
                buf[1] |= 0x80;
            } else {
                buf[0] |= (3 << (bit - 1));
            }
        } else {
            buf[0] |= (1 << bit);
        }
    }
    buf[0] &= ~mask;
    if (bottom)                 /* set bottom bit if requested */
        buf[bytes - 1] |= 1;
    if (!BN_bin2bn(buf, bytes, rnd))
        goto err;
    ret = 1;
 err:
    OPENSSL_clear_free(buf, bytes);
    bn_check_top(rnd);
    return ret;

toosmall:
    BNerr(BN_F_BNRAND, BN_R_BITS_TOO_SMALL);
    return 0;
}

int BN_rand(BIGNUM *rnd, int bits, int top, int bottom)
{
    return bnrand(NORMAL, rnd, bits, top, bottom);
}

int BN_bntest_rand(BIGNUM *rnd, int bits, int top, int bottom)
{
    return bnrand(TESTING, rnd, bits, top, bottom);
}

int BN_priv_rand(BIGNUM *rnd, int bits, int top, int bottom)
{
    return bnrand(PRIVATE, rnd, bits, top, bottom);
}

/* random number r:  0 <= r < range */
static int bnrand_range(BNRAND_FLAG flag, BIGNUM *r, const BIGNUM *range)
{
    int n;
    int count = 100;

    if (range->neg || BN_is_zero(range)) {
        BNerr(BN_F_BNRAND_RANGE, BN_R_INVALID_RANGE);
        return 0;
    }

    n = BN_num_bits(range);     /* n > 0 */

    /* BN_is_bit_set(range, n - 1) always holds */

    if (n == 1)
        BN_zero(r);
    else if (!BN_is_bit_set(range, n - 2) && !BN_is_bit_set(range, n - 3)) {
        /*
         * range = 100..._2, so 3*range (= 11..._2) is exactly one bit longer
         * than range
         */
        do {
            if (!bnrand(flag, r, n + 1, BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))
                return 0;

            /*
             * If r < 3*range, use r := r MOD range (which is either r, r -
             * range, or r - 2*range). Otherwise, iterate once more. Since
             * 3*range = 11..._2, each iteration succeeds with probability >=
             * .75.
             */
            if (BN_cmp(r, range) >= 0) {
                if (!BN_sub(r, r, range))
                    return 0;
                if (BN_cmp(r, range) >= 0)
                    if (!BN_sub(r, r, range))
                        return 0;
            }

            if (!--count) {
                BNerr(BN_F_BNRAND_RANGE, BN_R_TOO_MANY_ITERATIONS);
                return 0;
            }

        }
        while (BN_cmp(r, range) >= 0);
    } else {
        do {
            /* range = 11..._2  or  range = 101..._2 */
            if (!bnrand(flag, r, n, BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))
                return 0;

            if (!--count) {
                BNerr(BN_F_BNRAND_RANGE, BN_R_TOO_MANY_ITERATIONS);
                return 0;
            }
        }
        while (BN_cmp(r, range) >= 0);
    }

    bn_check_top(r);
    return 1;
}

int BN_rand_range(BIGNUM *r, const BIGNUM *range)
{
    return bnrand_range(NORMAL, r, range);
}

int BN_priv_rand_range(BIGNUM *r, const BIGNUM *range)
{
    return bnrand_range(PRIVATE, r, range);
}

int BN_pseudo_rand(BIGNUM *rnd, int bits, int top, int bottom)
{
    return BN_rand(rnd, bits, top, bottom);
}

int BN_pseudo_rand_range(BIGNUM *r, const BIGNUM *range)
{
    return BN_rand_range(r, range);
}

/*
 * BN_generate_dsa_nonce generates a random number 0 <= out < range. Unlike
 * BN_rand_range, it also includes the contents of |priv| and |message| in
 * the generation so that an RNG failure isn't fatal as long as |priv|
 * remains secret. This is intended for use in DSA and ECDSA where an RNG
 * weakness leads directly to private key exposure unless this function is
 * used.
 */
int BN_generate_dsa_nonce(BIGNUM *out, const BIGNUM *range,
                          const BIGNUM *priv, const unsigned char *message,
                          size_t message_len, BN_CTX *ctx)
{
    SHA512_CTX sha;
    /*
     * We use 512 bits of random data per iteration to ensure that we have at
     * least |range| bits of randomness.
     */
    unsigned char random_bytes[64];
    unsigned char digest[SHA512_DIGEST_LENGTH];
    unsigned done, todo;
    /* We generate |range|+8 bytes of random output. */
    const unsigned num_k_bytes = BN_num_bytes(range) + 8;
    unsigned char private_bytes[96];
    unsigned char *k_bytes;
    int ret = 0;

    k_bytes = OPENSSL_malloc(num_k_bytes);
    if (k_bytes == NULL)
        goto err;

    /* We copy |priv| into a local buffer to avoid exposing its length. */
    if (BN_bn2binpad(priv, private_bytes, sizeof(private_bytes)) < 0) {
        /*
         * No reasonable DSA or ECDSA key should have a private key this
         * large and we don't handle this case in order to avoid leaking the
         * length of the private key.
         */
        BNerr(BN_F_BN_GENERATE_DSA_NONCE, BN_R_PRIVATE_KEY_TOO_LARGE);
        goto err;
    }

    for (done = 0; done < num_k_bytes;) {
        if (RAND_priv_bytes(random_bytes, sizeof(random_bytes)) != 1)
            goto err;
        SHA512_Init(&sha);
        SHA512_Update(&sha, &done, sizeof(done));
        SHA512_Update(&sha, private_bytes, sizeof(private_bytes));
        SHA512_Update(&sha, message, message_len);
        SHA512_Update(&sha, random_bytes, sizeof(random_bytes));
        SHA512_Final(digest, &sha);

        todo = num_k_bytes - done;
        if (todo > SHA512_DIGEST_LENGTH)
            todo = SHA512_DIGEST_LENGTH;
        memcpy(k_bytes + done, digest, todo);
        done += todo;
    }

    if (!BN_bin2bn(k_bytes, num_k_bytes, out))
        goto err;
    if (BN_mod(out, out, range, ctx) != 1)
        goto err;
    ret = 1;

 err:
    OPENSSL_free(k_bytes);
    OPENSSL_cleanse(private_bytes, sizeof(private_bytes));
    return ret;
}