/src/openssl111/crypto/dsa/dsa_ossl.c

Source (jump to first uncovered line)
/*
 * 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;
}

Coverage Report

Created: 2023-06-08 06:40

Line	Count	Source (jump to first uncovered line)
1		/*
2		* Copyright 1995-2019 The OpenSSL Project Authors. All Rights Reserved.
3		*
4		* Licensed under the OpenSSL license (the "License"). You may not use
5		* this file except in compliance with the License. You can obtain a copy
6		* in the file LICENSE in the source distribution or at
7		* https://www.openssl.org/source/license.html
8		*/
9
10		#include <stdio.h>
11		#include "internal/cryptlib.h"
12		#include "crypto/bn.h"
13		#include <openssl/bn.h>
14		#include <openssl/sha.h>
15		#include "dsa_local.h"
16		#include <openssl/asn1.h>
17
18		static DSA_SIG dsa_do_sign(const unsigned char dgst, int dlen, DSA *dsa);
19		static int dsa_sign_setup_no_digest(DSA dsa, BN_CTX ctx_in, BIGNUM **kinvp,
20		BIGNUM **rp);
21		static int dsa_sign_setup(DSA dsa, BN_CTX ctx_in, BIGNUM **kinvp,
22		BIGNUM *rp, const unsigned char dgst, int dlen);
23		static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
24		DSA_SIG sig, DSA dsa);
25		static int dsa_init(DSA *dsa);
26		static int dsa_finish(DSA *dsa);
27		static BIGNUM dsa_mod_inverse_fermat(const BIGNUM k, const BIGNUM *q,
28		BN_CTX *ctx);
29
30		static DSA_METHOD openssl_dsa_meth = {
31		"OpenSSL DSA method",
32		dsa_do_sign,
33		dsa_sign_setup_no_digest,
34		dsa_do_verify,
35		NULL, /* dsa_mod_exp, */
36		NULL, /* dsa_bn_mod_exp, */
37		dsa_init,
38		dsa_finish,
39		DSA_FLAG_FIPS_METHOD,
40		NULL,
41		NULL,
42		NULL
43		};
44
45		static const DSA_METHOD *default_DSA_method = &openssl_dsa_meth;
46
47		void DSA_set_default_method(const DSA_METHOD *meth)
48	0	{
49	0	default_DSA_method = meth;
50	0	}
51
52		const DSA_METHOD *DSA_get_default_method(void)
53	0	{
54	0	return default_DSA_method;
55	0	}
56
57		const DSA_METHOD *DSA_OpenSSL(void)
58	0	{
59	0	return &openssl_dsa_meth;
60	0	}
61
62		static DSA_SIG dsa_do_sign(const unsigned char dgst, int dlen, DSA *dsa)
63	0	{
64	0	BIGNUM *kinv = NULL;
65	0	BIGNUM m, blind, blindm, tmp;
66	0	BN_CTX *ctx = NULL;
67	0	int reason = ERR_R_BN_LIB;
68	0	DSA_SIG *ret = NULL;
69	0	int rv = 0;
70
71	0	if (dsa->p == NULL \|\| dsa->q == NULL \|\| dsa->g == NULL) {
72	0	reason = DSA_R_MISSING_PARAMETERS;
73	0	goto err;
74	0	}
75	0	if (dsa->priv_key == NULL) {
76	0	reason = DSA_R_MISSING_PRIVATE_KEY;
77	0	goto err;
78	0	}
79
80	0	ret = DSA_SIG_new();
81	0	if (ret == NULL)
82	0	goto err;
83	0	ret->r = BN_new();
84	0	ret->s = BN_new();
85	0	if (ret->r == NULL \|\| ret->s == NULL)
86	0	goto err;
87
88	0	ctx = BN_CTX_new();
89	0	if (ctx == NULL)
90	0	goto err;
91	0	m = BN_CTX_get(ctx);
92	0	blind = BN_CTX_get(ctx);
93	0	blindm = BN_CTX_get(ctx);
94	0	tmp = BN_CTX_get(ctx);
95	0	if (tmp == NULL)
96	0	goto err;
97
98	0	redo:
99	0	if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen))
100	0	goto err;
101
102	0	if (dlen > BN_num_bytes(dsa->q))
103		/*
104		* if the digest length is greater than the size of q use the
105		* BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
106		* 4.2
107		*/
108	0	dlen = BN_num_bytes(dsa->q);
109	0	if (BN_bin2bn(dgst, dlen, m) == NULL)
110	0	goto err;
111
112		/*
113		* The normal signature calculation is:
114		*
115		* s := k^-1 * (m + r * priv_key) mod q
116		*
117		* We will blind this to protect against side channel attacks
118		*
119		* s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q
120		*/
121
122		/* Generate a blinding value */
123	0	do {
124	0	if (!BN_priv_rand(blind, BN_num_bits(dsa->q) - 1,
125	0	BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))
126	0	goto err;
127	0	} while (BN_is_zero(blind));
128	0	BN_set_flags(blind, BN_FLG_CONSTTIME);
129	0	BN_set_flags(blindm, BN_FLG_CONSTTIME);
130	0	BN_set_flags(tmp, BN_FLG_CONSTTIME);
131
132		/* tmp := blind * priv_key * r mod q */
133	0	if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->q, ctx))
134	0	goto err;
135	0	if (!BN_mod_mul(tmp, tmp, ret->r, dsa->q, ctx))
136	0	goto err;
137
138		/* blindm := blind * m mod q */
139	0	if (!BN_mod_mul(blindm, blind, m, dsa->q, ctx))
140	0	goto err;
141
142		/* s : = (blind * priv_key * r) + (blind * m) mod q */
143	0	if (!BN_mod_add_quick(ret->s, tmp, blindm, dsa->q))
144	0	goto err;
145
146		/* s := s * k^-1 mod q */
147	0	if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->q, ctx))
148	0	goto err;
149
150		/* s:= s * blind^-1 mod q */
151	0	if (BN_mod_inverse(blind, blind, dsa->q, ctx) == NULL)
152	0	goto err;
153	0	if (!BN_mod_mul(ret->s, ret->s, blind, dsa->q, ctx))
154	0	goto err;
155
156		/*
157		* Redo if r or s is zero as required by FIPS 186-3: this is very
158		* unlikely.
159		*/
160	0	if (BN_is_zero(ret->r) \|\| BN_is_zero(ret->s))
161	0	goto redo;
162
163	0	rv = 1;
164
165	0	err:
166	0	if (rv == 0) {
167	0	DSAerr(DSA_F_DSA_DO_SIGN, reason);
168	0	DSA_SIG_free(ret);
169	0	ret = NULL;
170	0	}
171	0	BN_CTX_free(ctx);
172	0	BN_clear_free(kinv);
173	0	return ret;
174	0	}
175
176		static int dsa_sign_setup_no_digest(DSA dsa, BN_CTX ctx_in,
177		BIGNUM kinvp, BIGNUM rp)
178	0	{
179	0	return dsa_sign_setup(dsa, ctx_in, kinvp, rp, NULL, 0);
180	0	}
181
182		static int dsa_sign_setup(DSA dsa, BN_CTX ctx_in,
183		BIGNUM kinvp, BIGNUM rp,
184		const unsigned char *dgst, int dlen)
185	0	{
186	0	BN_CTX *ctx = NULL;
187	0	BIGNUM k, kinv = NULL, r = rp;
188	0	BIGNUM *l;
189	0	int ret = 0;
190	0	int q_bits, q_words;
191
192	0	if (!dsa->p \|\| !dsa->q \|\| !dsa->g) {
193	0	DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
194	0	return 0;
195	0	}
196
197		/* Reject obviously invalid parameters */
198	0	if (BN_is_zero(dsa->p) \|\| BN_is_zero(dsa->q) \|\| BN_is_zero(dsa->g)) {
199	0	DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_INVALID_PARAMETERS);
200	0	return 0;
201	0	}
202	0	if (dsa->priv_key == NULL) {
203	0	DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PRIVATE_KEY);
204	0	return 0;
205	0	}
206
207	0	k = BN_new();
208	0	l = BN_new();
209	0	if (k == NULL \|\| l == NULL)
210	0	goto err;
211
212	0	if (ctx_in == NULL) {
213	0	if ((ctx = BN_CTX_new()) == NULL)
214	0	goto err;
215	0	} else
216	0	ctx = ctx_in;
217
218		/* Preallocate space */
219	0	q_bits = BN_num_bits(dsa->q);
220	0	q_words = bn_get_top(dsa->q);
221	0	if (!bn_wexpand(k, q_words + 2)
222	0	\|\| !bn_wexpand(l, q_words + 2))
223	0	goto err;
224
225		/* Get random k */
226	0	do {
227	0	if (dgst != NULL) {
228		/*
229		* We calculate k from SHA512(private_key + H(message) + random).
230		* This protects the private key from a weak PRNG.
231		*/
232	0	if (!BN_generate_dsa_nonce(k, dsa->q, dsa->priv_key, dgst,
233	0	dlen, ctx))
234	0	goto err;
235	0	} else if (!BN_priv_rand_range(k, dsa->q))
236	0	goto err;
237	0	} while (BN_is_zero(k));
238
239	0	BN_set_flags(k, BN_FLG_CONSTTIME);
240	0	BN_set_flags(l, BN_FLG_CONSTTIME);
241
242	0	if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
243	0	if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
244	0	dsa->lock, dsa->p, ctx))
245	0	goto err;
246	0	}
247
248		/* Compute r = (g^k mod p) mod q */
249
250		/*
251		* We do not want timing information to leak the length of k, so we
252		* compute G^k using an equivalent scalar of fixed bit-length.
253		*
254		* We unconditionally perform both of these additions to prevent a
255		* small timing information leakage. We then choose the sum that is
256		* one bit longer than the modulus.
257		*
258		* There are some concerns about the efficacy of doing this. More
259		* specifically refer to the discussion starting with:
260		* https://github.com/openssl/openssl/pull/7486#discussion_r228323705
261		* The fix is to rework BN so these gymnastics aren't required.
262		*/
263	0	if (!BN_add(l, k, dsa->q)
264	0	\|\| !BN_add(k, l, dsa->q))
265	0	goto err;
266
267	0	BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
268
269	0	if ((dsa)->meth->bn_mod_exp != NULL) {
270	0	if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,
271	0	dsa->method_mont_p))
272	0	goto err;
273	0	} else {
274	0	if (!BN_mod_exp_mont(r, dsa->g, k, dsa->p, ctx, dsa->method_mont_p))
275	0	goto err;
276	0	}
277
278	0	if (!BN_mod(r, r, dsa->q, ctx))
279	0	goto err;
280
281		/* Compute part of 's = inv(k) (m + xr) mod q' */
282	0	if ((kinv = dsa_mod_inverse_fermat(k, dsa->q, ctx)) == NULL)
283	0	goto err;
284
285	0	BN_clear_free(*kinvp);
286	0	*kinvp = kinv;
287	0	kinv = NULL;
288	0	ret = 1;
289	0	err:
290	0	if (!ret)
291	0	DSAerr(DSA_F_DSA_SIGN_SETUP, ERR_R_BN_LIB);
292	0	if (ctx != ctx_in)
293	0	BN_CTX_free(ctx);
294	0	BN_clear_free(k);
295	0	BN_clear_free(l);
296	0	return ret;
297	0	}
298
299		static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
300		DSA_SIG sig, DSA dsa)
301	0	{
302	0	BN_CTX *ctx;
303	0	BIGNUM u1, u2, *t1;
304	0	BN_MONT_CTX *mont = NULL;
305	0	const BIGNUM r, s;
306	0	int ret = -1, i;
307	0	if (!dsa->p \|\| !dsa->q \|\| !dsa->g) {
308	0	DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MISSING_PARAMETERS);
309	0	return -1;
310	0	}
311
312	0	i = BN_num_bits(dsa->q);
313		/* fips 186-3 allows only different sizes for q */
314	0	if (i != 160 && i != 224 && i != 256) {
315	0	DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_BAD_Q_VALUE);
316	0	return -1;
317	0	}
318
319	0	if (BN_num_bits(dsa->p) > OPENSSL_DSA_MAX_MODULUS_BITS) {
320	0	DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MODULUS_TOO_LARGE);
321	0	return -1;
322	0	}
323	0	u1 = BN_new();
324	0	u2 = BN_new();
325	0	t1 = BN_new();
326	0	ctx = BN_CTX_new();
327	0	if (u1 == NULL \|\| u2 == NULL \|\| t1 == NULL \|\| ctx == NULL)
328	0	goto err;
329
330	0	DSA_SIG_get0(sig, &r, &s);
331
332	0	if (BN_is_zero(r) \|\| BN_is_negative(r) \|\|
333	0	BN_ucmp(r, dsa->q) >= 0) {
334	0	ret = 0;
335	0	goto err;
336	0	}
337	0	if (BN_is_zero(s) \|\| BN_is_negative(s) \|\|
338	0	BN_ucmp(s, dsa->q) >= 0) {
339	0	ret = 0;
340	0	goto err;
341	0	}
342
343		/*
344		* Calculate W = inv(S) mod Q save W in u2
345		*/
346	0	if ((BN_mod_inverse(u2, s, dsa->q, ctx)) == NULL)
347	0	goto err;
348
349		/* save M in u1 */
350	0	if (dgst_len > (i >> 3))
351		/*
352		* if the digest length is greater than the size of q use the
353		* BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
354		* 4.2
355		*/
356	0	dgst_len = (i >> 3);
357	0	if (BN_bin2bn(dgst, dgst_len, u1) == NULL)
358	0	goto err;
359
360		/* u1 = M * w mod q */
361	0	if (!BN_mod_mul(u1, u1, u2, dsa->q, ctx))
362	0	goto err;
363
364		/* u2 = r * w mod q */
365	0	if (!BN_mod_mul(u2, r, u2, dsa->q, ctx))
366	0	goto err;
367
368	0	if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
369	0	mont = BN_MONT_CTX_set_locked(&dsa->method_mont_p,
370	0	dsa->lock, dsa->p, ctx);
371	0	if (!mont)
372	0	goto err;
373	0	}
374
375	0	if (dsa->meth->dsa_mod_exp != NULL) {
376	0	if (!dsa->meth->dsa_mod_exp(dsa, t1, dsa->g, u1, dsa->pub_key, u2,
377	0	dsa->p, ctx, mont))
378	0	goto err;
379	0	} else {
380	0	if (!BN_mod_exp2_mont(t1, dsa->g, u1, dsa->pub_key, u2, dsa->p, ctx,
381	0	mont))
382	0	goto err;
383	0	}
384
385		/* let u1 = u1 mod q */
386	0	if (!BN_mod(u1, t1, dsa->q, ctx))
387	0	goto err;
388
389		/*
390		* V is now in u1. If the signature is correct, it will be equal to R.
391		*/
392	0	ret = (BN_ucmp(u1, r) == 0);
393
394	0	err:
395	0	if (ret < 0)
396	0	DSAerr(DSA_F_DSA_DO_VERIFY, ERR_R_BN_LIB);
397	0	BN_CTX_free(ctx);
398	0	BN_free(u1);
399	0	BN_free(u2);
400	0	BN_free(t1);
401	0	return ret;
402	0	}
403
404		static int dsa_init(DSA *dsa)
405	0	{
406	0	dsa->flags \|= DSA_FLAG_CACHE_MONT_P;
407	0	return 1;
408	0	}
409
410		static int dsa_finish(DSA *dsa)
411	0	{
412	0	BN_MONT_CTX_free(dsa->method_mont_p);
413	0	return 1;
414	0	}
415
416		/*
417		* Compute the inverse of k modulo q.
418		* Since q is prime, Fermat's Little Theorem applies, which reduces this to
419		* mod-exp operation. Both the exponent and modulus are public information
420		* so a mod-exp that doesn't leak the base is sufficient. A newly allocated
421		* BIGNUM is returned which the caller must free.
422		*/
423		static BIGNUM dsa_mod_inverse_fermat(const BIGNUM k, const BIGNUM *q,
424		BN_CTX *ctx)
425	0	{
426	0	BIGNUM *res = NULL;
427	0	BIGNUM r, e;
428
429	0	if ((r = BN_new()) == NULL)
430	0	return NULL;
431
432	0	BN_CTX_start(ctx);
433	0	if ((e = BN_CTX_get(ctx)) != NULL
434	0	&& BN_set_word(r, 2)
435	0	&& BN_sub(e, q, r)
436	0	&& BN_mod_exp_mont(r, k, e, q, ctx, NULL))
437	0	res = r;
438	0	else
439	0	BN_free(r);
440	0	BN_CTX_end(ctx);
441	0	return res;
442	0	}