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openssl/crypto/dsa/dsa_ossl.c

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/*
* 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 "internal/cryptlib.h"
#include "crypto/bn.h"
#include <openssl/bn.h>
#include <openssl/sha.h>
#include "dsa_local.h"
#include <openssl/asn1.h>
static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa);
static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
BIGNUM **rp);
static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
BIGNUM **rp, const unsigned char *dgst, int dlen);
static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
DSA_SIG *sig, DSA *dsa);
static int dsa_init(DSA *dsa);
static int dsa_finish(DSA *dsa);
static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
BN_CTX *ctx);
static DSA_METHOD openssl_dsa_meth = {
"OpenSSL DSA method",
dsa_do_sign,
dsa_sign_setup_no_digest,
dsa_do_verify,
NULL, /* dsa_mod_exp, */
NULL, /* dsa_bn_mod_exp, */
dsa_init,
dsa_finish,
DSA_FLAG_FIPS_METHOD,
NULL,
NULL,
NULL
};
static const DSA_METHOD *default_DSA_method = &openssl_dsa_meth;
void DSA_set_default_method(const DSA_METHOD *meth)
{
default_DSA_method = meth;
}
const DSA_METHOD *DSA_get_default_method(void)
{
return default_DSA_method;
}
const DSA_METHOD *DSA_OpenSSL(void)
{
return &openssl_dsa_meth;
}
static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
{
BIGNUM *kinv = NULL;
BIGNUM *m, *blind, *blindm, *tmp;
BN_CTX *ctx = NULL;
int reason = ERR_R_BN_LIB;
DSA_SIG *ret = NULL;
int rv = 0;
if (dsa->p == NULL || dsa->q == NULL || dsa->g == NULL) {
reason = DSA_R_MISSING_PARAMETERS;
goto err;
}
if (dsa->priv_key == NULL) {
reason = DSA_R_MISSING_PRIVATE_KEY;
goto err;
}
ret = DSA_SIG_new();
if (ret == NULL)
goto err;
ret->r = BN_new();
ret->s = BN_new();
if (ret->r == NULL || ret->s == NULL)
goto err;
ctx = BN_CTX_new();
if (ctx == NULL)
goto err;
m = BN_CTX_get(ctx);
blind = BN_CTX_get(ctx);
blindm = BN_CTX_get(ctx);
tmp = BN_CTX_get(ctx);
if (tmp == NULL)
goto err;
redo:
if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen))
goto err;
if (dlen > BN_num_bytes(dsa->q))
/*
* if the digest length is greater than the size of q use the
* BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
* 4.2
*/
dlen = BN_num_bytes(dsa->q);
if (BN_bin2bn(dgst, dlen, m) == NULL)
goto err;
/*
* The normal signature calculation is:
*
* s := k^-1 * (m + r * priv_key) mod q
*
* We will blind this to protect against side channel attacks
*
* s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q
*/
/* Generate a blinding value */
do {
if (!BN_priv_rand(blind, BN_num_bits(dsa->q) - 1,
BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))
goto err;
} while (BN_is_zero(blind));
BN_set_flags(blind, BN_FLG_CONSTTIME);
BN_set_flags(blindm, BN_FLG_CONSTTIME);
BN_set_flags(tmp, BN_FLG_CONSTTIME);
/* tmp := blind * priv_key * r mod q */
if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->q, ctx))
goto err;
if (!BN_mod_mul(tmp, tmp, ret->r, dsa->q, ctx))
goto err;
/* blindm := blind * m mod q */
if (!BN_mod_mul(blindm, blind, m, dsa->q, ctx))
goto err;
/* s : = (blind * priv_key * r) + (blind * m) mod q */
if (!BN_mod_add_quick(ret->s, tmp, blindm, dsa->q))
goto err;
/* s := s * k^-1 mod q */
if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->q, ctx))
goto err;
/* s:= s * blind^-1 mod q */
if (BN_mod_inverse(blind, blind, dsa->q, ctx) == NULL)
goto err;
if (!BN_mod_mul(ret->s, ret->s, blind, dsa->q, ctx))
goto err;
/*
* Redo if r or s is zero as required by FIPS 186-3: this is very
* unlikely.
*/
if (BN_is_zero(ret->r) || BN_is_zero(ret->s))
goto redo;
rv = 1;
err:
if (rv == 0) {
DSAerr(DSA_F_DSA_DO_SIGN, reason);
DSA_SIG_free(ret);
ret = NULL;
}
BN_CTX_free(ctx);
BN_clear_free(kinv);
return ret;
}
static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in,
BIGNUM **kinvp, BIGNUM **rp)
{
return dsa_sign_setup(dsa, ctx_in, kinvp, rp, NULL, 0);
}
static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
BIGNUM **kinvp, BIGNUM **rp,
const unsigned char *dgst, int dlen)
{
BN_CTX *ctx = NULL;
BIGNUM *k, *kinv = NULL, *r = *rp;
BIGNUM *l;
int ret = 0;
int q_bits, q_words;
if (!dsa->p || !dsa->q || !dsa->g) {
DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
return 0;
}
/* Reject obviously invalid parameters */
if (BN_is_zero(dsa->p) || BN_is_zero(dsa->q) || BN_is_zero(dsa->g)) {
DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_INVALID_PARAMETERS);
return 0;
}
if (dsa->priv_key == NULL) {
DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PRIVATE_KEY);
return 0;
}
k = BN_new();
l = BN_new();
if (k == NULL || l == NULL)
goto err;
if (ctx_in == NULL) {
if ((ctx = BN_CTX_new()) == NULL)
goto err;
} else
ctx = ctx_in;
/* Preallocate space */
q_bits = BN_num_bits(dsa->q);
q_words = bn_get_top(dsa->q);
if (!bn_wexpand(k, q_words + 2)
|| !bn_wexpand(l, q_words + 2))
goto err;
/* Get random k */
do {
if (dgst != NULL) {
/*
* We calculate k from SHA512(private_key + H(message) + random).
* This protects the private key from a weak PRNG.
*/
if (!BN_generate_dsa_nonce(k, dsa->q, dsa->priv_key, dgst,
dlen, ctx))
goto err;
} else if (!BN_priv_rand_range(k, dsa->q))
goto err;
} while (BN_is_zero(k));
BN_set_flags(k, BN_FLG_CONSTTIME);
BN_set_flags(l, BN_FLG_CONSTTIME);
if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
dsa->lock, dsa->p, ctx))
goto err;
}
/* Compute r = (g^k mod p) mod q */
/*
* We do not want timing information to leak the length of k, so we
* compute G^k using an equivalent scalar of fixed bit-length.
*
* We unconditionally perform both of these additions to prevent a
* small timing information leakage. We then choose the sum that is
* one bit longer than the modulus.
*
* There are some concerns about the efficacy of doing this. More
* specifically refer to the discussion starting with:
* https://github.com/openssl/openssl/pull/7486#discussion_r228323705
* The fix is to rework BN so these gymnastics aren't required.
*/
if (!BN_add(l, k, dsa->q)
|| !BN_add(k, l, dsa->q))
goto err;
BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
if ((dsa)->meth->bn_mod_exp != NULL) {
if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,
dsa->method_mont_p))
goto err;
} else {
if (!BN_mod_exp_mont(r, dsa->g, k, dsa->p, ctx, dsa->method_mont_p))
goto err;
}
if (!BN_mod(r, r, dsa->q, ctx))
goto err;
/* Compute part of 's = inv(k) (m + xr) mod q' */
if ((kinv = dsa_mod_inverse_fermat(k, dsa->q, ctx)) == NULL)
goto err;
BN_clear_free(*kinvp);
*kinvp = kinv;
kinv = NULL;
ret = 1;
err:
if (!ret)
DSAerr(DSA_F_DSA_SIGN_SETUP, ERR_R_BN_LIB);
if (ctx != ctx_in)
BN_CTX_free(ctx);
BN_clear_free(k);
BN_clear_free(l);
return ret;
}
static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
DSA_SIG *sig, DSA *dsa)
{
BN_CTX *ctx;
BIGNUM *u1, *u2, *t1;
BN_MONT_CTX *mont = NULL;
const BIGNUM *r, *s;
int ret = -1, i;
if (!dsa->p || !dsa->q || !dsa->g) {
DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MISSING_PARAMETERS);
return -1;
}
i = BN_num_bits(dsa->q);
/* fips 186-3 allows only different sizes for q */
if (i != 160 && i != 224 && i != 256) {
DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_BAD_Q_VALUE);
return -1;
}
if (BN_num_bits(dsa->p) > OPENSSL_DSA_MAX_MODULUS_BITS) {
DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MODULUS_TOO_LARGE);
return -1;
}
u1 = BN_new();
u2 = BN_new();
t1 = BN_new();
ctx = BN_CTX_new();
if (u1 == NULL || u2 == NULL || t1 == NULL || ctx == NULL)
goto err;
DSA_SIG_get0(sig, &r, &s);
if (BN_is_zero(r) || BN_is_negative(r) ||
BN_ucmp(r, dsa->q) >= 0) {
ret = 0;
goto err;
}
if (BN_is_zero(s) || BN_is_negative(s) ||
BN_ucmp(s, dsa->q) >= 0) {
ret = 0;
goto err;
}
/*
* Calculate W = inv(S) mod Q save W in u2
*/
if ((BN_mod_inverse(u2, s, dsa->q, ctx)) == NULL)
goto err;
/* save M in u1 */
if (dgst_len > (i >> 3))
/*
* if the digest length is greater than the size of q use the
* BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
* 4.2
*/
dgst_len = (i >> 3);
if (BN_bin2bn(dgst, dgst_len, u1) == NULL)
goto err;
/* u1 = M * w mod q */
if (!BN_mod_mul(u1, u1, u2, dsa->q, ctx))
goto err;
/* u2 = r * w mod q */
if (!BN_mod_mul(u2, r, u2, dsa->q, ctx))
goto err;
if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
mont = BN_MONT_CTX_set_locked(&dsa->method_mont_p,
dsa->lock, dsa->p, ctx);
if (!mont)
goto err;
}
if (dsa->meth->dsa_mod_exp != NULL) {
if (!dsa->meth->dsa_mod_exp(dsa, t1, dsa->g, u1, dsa->pub_key, u2,
dsa->p, ctx, mont))
goto err;
} else {
if (!BN_mod_exp2_mont(t1, dsa->g, u1, dsa->pub_key, u2, dsa->p, ctx,
mont))
goto err;
}
/* let u1 = u1 mod q */
if (!BN_mod(u1, t1, dsa->q, ctx))
goto err;
/*
* V is now in u1. If the signature is correct, it will be equal to R.
*/
ret = (BN_ucmp(u1, r) == 0);
err:
if (ret < 0)
DSAerr(DSA_F_DSA_DO_VERIFY, ERR_R_BN_LIB);
BN_CTX_free(ctx);
BN_free(u1);
BN_free(u2);
BN_free(t1);
return ret;
}
static int dsa_init(DSA *dsa)
{
dsa->flags |= DSA_FLAG_CACHE_MONT_P;
return 1;
}
static int dsa_finish(DSA *dsa)
{
BN_MONT_CTX_free(dsa->method_mont_p);
return 1;
}
/*
* Compute the inverse of k modulo q.
* Since q is prime, Fermat's Little Theorem applies, which reduces this to
* mod-exp operation. Both the exponent and modulus are public information
* so a mod-exp that doesn't leak the base is sufficient. A newly allocated
* BIGNUM is returned which the caller must free.
*/
static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
BN_CTX *ctx)
{
BIGNUM *res = NULL;
BIGNUM *r, *e;
if ((r = BN_new()) == NULL)
return NULL;
BN_CTX_start(ctx);
if ((e = BN_CTX_get(ctx)) != NULL
&& BN_set_word(r, 2)
&& BN_sub(e, q, r)
&& BN_mod_exp_mont(r, k, e, q, ctx, NULL))
res = r;
else
BN_free(r);
BN_CTX_end(ctx);
return res;
}
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