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openssl/crypto/dh/dh_key.c
Richard Levitte ec061bf8ff Make DH_check_pub_key() and DH_generate_key() safer yet 
We already check for an excessively large P in DH_generate_key(), but not in
DH_check_pub_key(), and none of them check for an excessively large Q.

This change adds all the missing excessive size checks of P and Q.

It's to be noted that behaviours surrounding excessively sized P and Q
differ.  DH_check() raises an error on the excessively sized P, but only
sets a flag for the excessively sized Q.  This behaviour is mimicked in
DH_check_pub_key().

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22453)
2023-11-06 07:55:01 +00:00

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/*
* Copyright 1995-2023 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
*/
/*
* DH low level APIs are deprecated for public use, but still ok for
* internal use.
*/
#include "internal/deprecated.h"
#include <stdio.h>
#include "internal/cryptlib.h"
#include "dh_local.h"
#include "crypto/bn.h"
#include "crypto/dh.h"
#include "crypto/security_bits.h"
#ifdef FIPS_MODULE
# define MIN_STRENGTH 112
#else
# define MIN_STRENGTH 80
#endif
static int generate_key(DH *dh);
static int dh_bn_mod_exp(const DH *dh, BIGNUM *r,
const BIGNUM *a, const BIGNUM *p,
const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx);
static int dh_init(DH *dh);
static int dh_finish(DH *dh);
/*
* See SP800-56Ar3 Section 5.7.1.1
* Finite Field Cryptography Diffie-Hellman (FFC DH) Primitive
*/
int ossl_dh_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
{
BN_CTX *ctx = NULL;
BN_MONT_CTX *mont = NULL;
BIGNUM *z = NULL, *pminus1;
int ret = -1;
if (BN_num_bits(dh->params.p) > OPENSSL_DH_MAX_MODULUS_BITS) {
ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_LARGE);
goto err;
}
if (dh->params.q != NULL
&& BN_num_bits(dh->params.q) > OPENSSL_DH_MAX_MODULUS_BITS) {
ERR_raise(ERR_LIB_DH, DH_R_Q_TOO_LARGE);
goto err;
}
if (BN_num_bits(dh->params.p) < DH_MIN_MODULUS_BITS) {
ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_SMALL);
return 0;
}
ctx = BN_CTX_new_ex(dh->libctx);
if (ctx == NULL)
goto err;
BN_CTX_start(ctx);
pminus1 = BN_CTX_get(ctx);
z = BN_CTX_get(ctx);
if (z == NULL)
goto err;
if (dh->priv_key == NULL) {
ERR_raise(ERR_LIB_DH, DH_R_NO_PRIVATE_VALUE);
goto err;
}
if (dh->flags & DH_FLAG_CACHE_MONT_P) {
mont = BN_MONT_CTX_set_locked(&dh->method_mont_p,
dh->lock, dh->params.p, ctx);
BN_set_flags(dh->priv_key, BN_FLG_CONSTTIME);
if (!mont)
goto err;
}
/* (Step 1) Z = pub_key^priv_key mod p */
if (!dh->meth->bn_mod_exp(dh, z, pub_key, dh->priv_key, dh->params.p, ctx,
mont)) {
ERR_raise(ERR_LIB_DH, ERR_R_BN_LIB);
goto err;
}
/* (Step 2) Error if z <= 1 or z = p - 1 */
if (BN_copy(pminus1, dh->params.p) == NULL
|| !BN_sub_word(pminus1, 1)
|| BN_cmp(z, BN_value_one()) <= 0
|| BN_cmp(z, pminus1) == 0) {
ERR_raise(ERR_LIB_DH, DH_R_INVALID_SECRET);
goto err;
}
/* return the padded key, i.e. same number of bytes as the modulus */
ret = BN_bn2binpad(z, key, BN_num_bytes(dh->params.p));
err:
BN_clear(z); /* (Step 2) destroy intermediate values */
BN_CTX_end(ctx);
BN_CTX_free(ctx);
return ret;
}
/*-
* NB: This function is inherently not constant time due to the
* RFC 5246 (8.1.2) padding style that strips leading zero bytes.
*/
int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
{
int ret = 0, i;
volatile size_t npad = 0, mask = 1;
/* compute the key; ret is constant unless compute_key is external */
#ifdef FIPS_MODULE
ret = ossl_dh_compute_key(key, pub_key, dh);
#else
ret = dh->meth->compute_key(key, pub_key, dh);
#endif
if (ret <= 0)
return ret;
/* count leading zero bytes, yet still touch all bytes */
for (i = 0; i < ret; i++) {
mask &= !key[i];
npad += mask;
}
/* unpad key */
ret -= npad;
/* key-dependent memory access, potentially leaking npad / ret */
memmove(key, key + npad, ret);
/* key-dependent memory access, potentially leaking npad / ret */
memset(key + ret, 0, npad);
return ret;
}
int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh)
{
int rv, pad;
/* rv is constant unless compute_key is external */
#ifdef FIPS_MODULE
rv = ossl_dh_compute_key(key, pub_key, dh);
#else
rv = dh->meth->compute_key(key, pub_key, dh);
#endif
if (rv <= 0)
return rv;
pad = BN_num_bytes(dh->params.p) - rv;
/* pad is constant (zero) unless compute_key is external */
if (pad > 0) {
memmove(key + pad, key, rv);
memset(key, 0, pad);
}
return rv + pad;
}
static DH_METHOD dh_ossl = {
"OpenSSL DH Method",
generate_key,
ossl_dh_compute_key,
dh_bn_mod_exp,
dh_init,
dh_finish,
DH_FLAG_FIPS_METHOD,
NULL,
NULL
};
static const DH_METHOD *default_DH_method = &dh_ossl;
const DH_METHOD *DH_OpenSSL(void)
{
return &dh_ossl;
}
const DH_METHOD *DH_get_default_method(void)
{
return default_DH_method;
}
static int dh_bn_mod_exp(const DH *dh, BIGNUM *r,
const BIGNUM *a, const BIGNUM *p,
const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx)
{
#ifdef S390X_MOD_EXP
return s390x_mod_exp(r, a, p, m, ctx, m_ctx);
#else
return BN_mod_exp_mont(r, a, p, m, ctx, m_ctx);
#endif
}
static int dh_init(DH *dh)
{
dh->flags |= DH_FLAG_CACHE_MONT_P;
dh->dirty_cnt++;
return 1;
}
static int dh_finish(DH *dh)
{
BN_MONT_CTX_free(dh->method_mont_p);
return 1;
}
#ifndef FIPS_MODULE
void DH_set_default_method(const DH_METHOD *meth)
{
default_DH_method = meth;
}
#endif /* FIPS_MODULE */
int DH_generate_key(DH *dh)
{
#ifdef FIPS_MODULE
return generate_key(dh);
#else
return dh->meth->generate_key(dh);
#endif
}
int ossl_dh_generate_public_key(BN_CTX *ctx, const DH *dh,
const BIGNUM *priv_key, BIGNUM *pub_key)
{
int ret = 0;
BIGNUM *prk = BN_new();
BN_MONT_CTX *mont = NULL;
if (prk == NULL)
return 0;
if (dh->flags & DH_FLAG_CACHE_MONT_P) {
/*
* We take the input DH as const, but we lie, because in some cases we
* want to get a hold of its Montgomery context.
*
* We cast to remove the const qualifier in this case, it should be
* fine...
*/
BN_MONT_CTX **pmont = (BN_MONT_CTX **)&dh->method_mont_p;
mont = BN_MONT_CTX_set_locked(pmont, dh->lock, dh->params.p, ctx);
if (mont == NULL)
goto err;
}
BN_with_flags(prk, priv_key, BN_FLG_CONSTTIME);
/* pub_key = g^priv_key mod p */
if (!dh->meth->bn_mod_exp(dh, pub_key, dh->params.g, prk, dh->params.p,
ctx, mont))
goto err;
ret = 1;
err:
BN_clear_free(prk);
return ret;
}
static int generate_key(DH *dh)
{
int ok = 0;
int generate_new_key = 0;
#ifndef FIPS_MODULE
unsigned l;
#endif
BN_CTX *ctx = NULL;
BIGNUM *pub_key = NULL, *priv_key = NULL;
if (BN_num_bits(dh->params.p) > OPENSSL_DH_MAX_MODULUS_BITS) {
ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_LARGE);
return 0;
}
if (dh->params.q != NULL
&& BN_num_bits(dh->params.q) > OPENSSL_DH_MAX_MODULUS_BITS) {
ERR_raise(ERR_LIB_DH, DH_R_Q_TOO_LARGE);
return 0;
}
if (BN_num_bits(dh->params.p) < DH_MIN_MODULUS_BITS) {
ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_SMALL);
return 0;
}
ctx = BN_CTX_new_ex(dh->libctx);
if (ctx == NULL)
goto err;
if (dh->priv_key == NULL) {
priv_key = BN_secure_new();
if (priv_key == NULL)
goto err;
generate_new_key = 1;
} else {
priv_key = dh->priv_key;
}
if (dh->pub_key == NULL) {
pub_key = BN_new();
if (pub_key == NULL)
goto err;
} else {
pub_key = dh->pub_key;
}
if (generate_new_key) {
/* Is it an approved safe prime ?*/
if (DH_get_nid(dh) != NID_undef) {
int max_strength =
ossl_ifc_ffc_compute_security_bits(BN_num_bits(dh->params.p));
if (dh->params.q == NULL
|| dh->length > BN_num_bits(dh->params.q))
goto err;
/* dh->length = maximum bit length of generated private key */
if (!ossl_ffc_generate_private_key(ctx, &dh->params, dh->length,
max_strength, priv_key))
goto err;
} else {
#ifdef FIPS_MODULE
if (dh->params.q == NULL)
goto err;
#else
if (dh->params.q == NULL) {
/* secret exponent length, must satisfy 2^(l-1) <= p */
if (dh->length != 0
&& dh->length >= BN_num_bits(dh->params.p))
goto err;
l = dh->length ? dh->length : BN_num_bits(dh->params.p) - 1;
if (!BN_priv_rand_ex(priv_key, l, BN_RAND_TOP_ONE,
BN_RAND_BOTTOM_ANY, 0, ctx))
goto err;
/*
* We handle just one known case where g is a quadratic non-residue:
* for g = 2: p % 8 == 3
*/
if (BN_is_word(dh->params.g, DH_GENERATOR_2)
&& !BN_is_bit_set(dh->params.p, 2)) {
/* clear bit 0, since it won't be a secret anyway */
if (!BN_clear_bit(priv_key, 0))
goto err;
}
} else
#endif
{
/* Do a partial check for invalid p, q, g */
if (!ossl_ffc_params_simple_validate(dh->libctx, &dh->params,
FFC_PARAM_TYPE_DH, NULL))
goto err;
/*
* For FFC FIPS 186-4 keygen
* security strength s = 112,
* Max Private key size N = len(q)
*/
if (!ossl_ffc_generate_private_key(ctx, &dh->params,
BN_num_bits(dh->params.q),
MIN_STRENGTH,
priv_key))
goto err;
}
}
}
if (!ossl_dh_generate_public_key(ctx, dh, priv_key, pub_key))
goto err;
dh->pub_key = pub_key;
dh->priv_key = priv_key;
dh->dirty_cnt++;
ok = 1;
err:
if (ok != 1)
ERR_raise(ERR_LIB_DH, ERR_R_BN_LIB);
if (pub_key != dh->pub_key)
BN_free(pub_key);
if (priv_key != dh->priv_key)
BN_free(priv_key);
BN_CTX_free(ctx);
return ok;
}
int ossl_dh_buf2key(DH *dh, const unsigned char *buf, size_t len)
{
int err_reason = DH_R_BN_ERROR;
BIGNUM *pubkey = NULL;
const BIGNUM *p;
int ret;
if ((pubkey = BN_bin2bn(buf, len, NULL)) == NULL)
goto err;
DH_get0_pqg(dh, &p, NULL, NULL);
if (p == NULL || BN_num_bytes(p) == 0) {
err_reason = DH_R_NO_PARAMETERS_SET;
goto err;
}
/* Prevent small subgroup attacks per RFC 8446 Section 4.2.8.1 */
if (!ossl_dh_check_pub_key_partial(dh, pubkey, &ret)) {
err_reason = DH_R_INVALID_PUBKEY;
goto err;
}
if (DH_set0_key(dh, pubkey, NULL) != 1)
goto err;
return 1;
err:
ERR_raise(ERR_LIB_DH, err_reason);
BN_free(pubkey);
return 0;
}
size_t ossl_dh_key2buf(const DH *dh, unsigned char **pbuf_out, size_t size,
int alloc)
{
const BIGNUM *pubkey;
unsigned char *pbuf = NULL;
const BIGNUM *p;
int p_size;
DH_get0_pqg(dh, &p, NULL, NULL);
DH_get0_key(dh, &pubkey, NULL);
if (p == NULL || pubkey == NULL
|| (p_size = BN_num_bytes(p)) == 0
|| BN_num_bytes(pubkey) == 0) {
ERR_raise(ERR_LIB_DH, DH_R_INVALID_PUBKEY);
return 0;
}
if (pbuf_out != NULL && (alloc || *pbuf_out != NULL)) {
if (!alloc) {
if (size >= (size_t)p_size)
pbuf = *pbuf_out;
if (pbuf == NULL)
ERR_raise(ERR_LIB_DH, DH_R_INVALID_SIZE);
} else {
pbuf = OPENSSL_malloc(p_size);
}
/* Errors raised above */
if (pbuf == NULL)
return 0;
/*
* As per Section 4.2.8.1 of RFC 8446 left pad public
* key with zeros to the size of p
*/
if (BN_bn2binpad(pubkey, pbuf, p_size) < 0) {
if (alloc)
OPENSSL_free(pbuf);
ERR_raise(ERR_LIB_DH, DH_R_BN_ERROR);
return 0;
}
*pbuf_out = pbuf;
}
return p_size;
}
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