bn_rand.c
/*
* Copyright 1995-2024 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 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 "crypto/rand.h"
#include "bn_local.h"
#include <openssl/rand.h>
#include <openssl/sha.h>
#include <openssl/evp.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 int strength, BN_CTX *ctx)
{
unsigned char *buf = NULL;
int b, ret = 0, bit, bytes, mask;
OSSL_LIB_CTX *libctx = ossl_bn_get_libctx(ctx);
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)
goto err;
/* make a random number and set the top and bottom bits */
b = flag == NORMAL ? RAND_bytes_ex(libctx, buf, bytes, strength)
: RAND_priv_bytes_ex(libctx, buf, bytes, strength);
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_ex(libctx, &c, 1, strength) <= 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:
ERR_raise(ERR_LIB_BN, BN_R_BITS_TOO_SMALL);
return 0;
}
int BN_rand_ex(BIGNUM *rnd, int bits, int top, int bottom,
unsigned int strength, BN_CTX *ctx)
{
return bnrand(NORMAL, rnd, bits, top, bottom, strength, ctx);
}
#ifndef FIPS_MODULE
int BN_rand(BIGNUM *rnd, int bits, int top, int bottom)
{
return bnrand(NORMAL, rnd, bits, top, bottom, 0, NULL);
}
int BN_bntest_rand(BIGNUM *rnd, int bits, int top, int bottom)
{
return bnrand(TESTING, rnd, bits, top, bottom, 0, NULL);
}
#endif
int BN_priv_rand_ex(BIGNUM *rnd, int bits, int top, int bottom,
unsigned int strength, BN_CTX *ctx)
{
return bnrand(PRIVATE, rnd, bits, top, bottom, strength, ctx);
}
#ifndef FIPS_MODULE
int BN_priv_rand(BIGNUM *rnd, int bits, int top, int bottom)
{
return bnrand(PRIVATE, rnd, bits, top, bottom, 0, NULL);
}
#endif
/* random number r: 0 <= r < range */
static int bnrand_range(BNRAND_FLAG flag, BIGNUM *r, const BIGNUM *range,
unsigned int strength, BN_CTX *ctx)
{
int n;
int count = 100;
if (r == NULL) {
ERR_raise(ERR_LIB_BN, ERR_R_PASSED_NULL_PARAMETER);
return 0;
}
if (range->neg || BN_is_zero(range)) {
ERR_raise(ERR_LIB_BN, 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,
strength, ctx))
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) {
ERR_raise(ERR_LIB_BN, 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,
strength, ctx))
return 0;
if (!--count) {
ERR_raise(ERR_LIB_BN, BN_R_TOO_MANY_ITERATIONS);
return 0;
}
}
while (BN_cmp(r, range) >= 0);
}
bn_check_top(r);
return 1;
}
int BN_rand_range_ex(BIGNUM *r, const BIGNUM *range, unsigned int strength,
BN_CTX *ctx)
{
return bnrand_range(NORMAL, r, range, strength, ctx);
}
#ifndef FIPS_MODULE
int BN_rand_range(BIGNUM *r, const BIGNUM *range)
{
return bnrand_range(NORMAL, r, range, 0, NULL);
}
#endif
int BN_priv_rand_range_ex(BIGNUM *r, const BIGNUM *range, unsigned int strength,
BN_CTX *ctx)
{
return bnrand_range(PRIVATE, r, range, strength, ctx);
}
#ifndef FIPS_MODULE
int BN_priv_rand_range(BIGNUM *r, const BIGNUM *range)
{
return bnrand_range(PRIVATE, r, range, 0, NULL);
}
# ifndef OPENSSL_NO_DEPRECATED_3_0
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);
}
# endif
#endif
int ossl_bn_priv_rand_range_fixed_top(BIGNUM *r, const BIGNUM *range,
unsigned int strength, BN_CTX *ctx)
{
int n;
int count = 100;
if (r == NULL) {
ERR_raise(ERR_LIB_BN, ERR_R_PASSED_NULL_PARAMETER);
return 0;
}
if (range->neg || BN_is_zero(range)) {
ERR_raise(ERR_LIB_BN, 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 {
BN_set_flags(r, BN_FLG_CONSTTIME);
do {
if (!bnrand(PRIVATE, r, n + 1, BN_RAND_TOP_ONE, BN_RAND_BOTTOM_ANY,
strength, ctx))
return 0;
if (!--count) {
ERR_raise(ERR_LIB_BN, BN_R_TOO_MANY_ITERATIONS);
return 0;
}
ossl_bn_mask_bits_fixed_top(r, n);
}
while (BN_ucmp(r, range) >= 0);
#ifdef BN_DEBUG
/* With BN_DEBUG on a fixed top number cannot be returned */
bn_correct_top(r);
#endif
}
return 1;
}
/*
* ossl_bn_gen_dsa_nonce_fixed_top 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 ossl_bn_gen_dsa_nonce_fixed_top(BIGNUM *out, const BIGNUM *range,
const BIGNUM *priv,
const unsigned char *message,
size_t message_len, BN_CTX *ctx)
{
EVP_MD_CTX *mdctx = EVP_MD_CTX_new();
/*
* 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|+1 bytes of random output. */
const unsigned num_k_bytes = BN_num_bytes(range) + 1;
unsigned char private_bytes[96];
unsigned char *k_bytes = NULL;
const int max_n = 64; /* Pr(failure to generate) < 2^max_n */
int n;
int ret = 0;
EVP_MD *md = NULL;
OSSL_LIB_CTX *libctx = ossl_bn_get_libctx(ctx);
if (mdctx == NULL)
goto end;
k_bytes = OPENSSL_malloc(num_k_bytes);
if (k_bytes == NULL)
goto end;
/* Ensure top byte is set to avoid non-constant time in bin2bn */
k_bytes[0] = 0xff;
/* 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.
*/
ERR_raise(ERR_LIB_BN, BN_R_PRIVATE_KEY_TOO_LARGE);
goto end;
}
md = EVP_MD_fetch(libctx, "SHA512", NULL);
if (md == NULL) {
ERR_raise(ERR_LIB_BN, BN_R_NO_SUITABLE_DIGEST);
goto end;
}
for (n = 0; n < max_n; n++) {
unsigned char i = 0;
for (done = 1; done < num_k_bytes;) {
if (RAND_priv_bytes_ex(libctx, random_bytes, sizeof(random_bytes),
0) <= 0)
goto end;
if (!EVP_DigestInit_ex(mdctx, md, NULL)
|| !EVP_DigestUpdate(mdctx, &i, sizeof(i))
|| !EVP_DigestUpdate(mdctx, private_bytes,
sizeof(private_bytes))
|| !EVP_DigestUpdate(mdctx, message, message_len)
|| !EVP_DigestUpdate(mdctx, random_bytes,
sizeof(random_bytes))
|| !EVP_DigestFinal_ex(mdctx, digest, NULL))
goto end;
todo = num_k_bytes - done;
if (todo > SHA512_DIGEST_LENGTH)
todo = SHA512_DIGEST_LENGTH;
memcpy(k_bytes + done, digest, todo);
done += todo;
++i;
}
if (!BN_bin2bn(k_bytes, num_k_bytes, out))
goto end;
/* Clear out the top bits and rejection filter into range */
BN_set_flags(out, BN_FLG_CONSTTIME);
ossl_bn_mask_bits_fixed_top(out, BN_num_bits(range));
if (BN_ucmp(out, range) < 0) {
ret = 1;
#ifdef BN_DEBUG
/* With BN_DEBUG on a fixed top number cannot be returned */
bn_correct_top(out);
#endif
goto end;
}
}
/* Failed to generate anything */
ERR_raise(ERR_LIB_BN, ERR_R_INTERNAL_ERROR);
end:
EVP_MD_CTX_free(mdctx);
EVP_MD_free(md);
OPENSSL_clear_free(k_bytes, num_k_bytes);
OPENSSL_cleanse(digest, sizeof(digest));
OPENSSL_cleanse(random_bytes, sizeof(random_bytes));
OPENSSL_cleanse(private_bytes, sizeof(private_bytes));
return ret;
}
int BN_generate_dsa_nonce(BIGNUM *out, const BIGNUM *range,
const BIGNUM *priv, const unsigned char *message,
size_t message_len, BN_CTX *ctx)
{
int ret;
ret = ossl_bn_gen_dsa_nonce_fixed_top(out, range, priv, message,
message_len, ctx);
/*
* This call makes the BN_generate_dsa_nonce non-const-time, thus we
* do not use it internally. But fixed_top BNs currently cannot be returned
* from public API calls.
*/
bn_correct_top(out);
return ret;
}
