1/*
2 * Copyright 1995-2023 The OpenSSL Project Authors. All Rights Reserved.
3 *
4 * Licensed under the Apache License 2.0 (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/*
11 * DSA low level APIs are deprecated for public use, but still ok for
12 * internal use.
13 */
14#include "internal/deprecated.h"
15
16#include <stdio.h>
17#include "internal/cryptlib.h"
18#include "crypto/bn.h"
19#include <openssl/bn.h>
20#include <openssl/sha.h>
21#include "dsa_local.h"
22#include <openssl/asn1.h>
23
24#define MIN_DSA_SIGN_QBITS   128
25#define MAX_DSA_SIGN_RETRIES 8
26
27static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa);
28static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
29                                    BIGNUM **rp);
30static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
31                          BIGNUM **rp, const unsigned char *dgst, int dlen);
32static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
33                         DSA_SIG *sig, DSA *dsa);
34static int dsa_init(DSA *dsa);
35static int dsa_finish(DSA *dsa);
36static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
37                                      BN_CTX *ctx);
38
39static DSA_METHOD openssl_dsa_meth = {
40    "OpenSSL DSA method",
41    dsa_do_sign,
42    dsa_sign_setup_no_digest,
43    dsa_do_verify,
44    NULL,                       /* dsa_mod_exp, */
45    NULL,                       /* dsa_bn_mod_exp, */
46    dsa_init,
47    dsa_finish,
48    DSA_FLAG_FIPS_METHOD,
49    NULL,
50    NULL,
51    NULL
52};
53
54static const DSA_METHOD *default_DSA_method = &openssl_dsa_meth;
55
56#ifndef FIPS_MODULE
57void DSA_set_default_method(const DSA_METHOD *meth)
58{
59    default_DSA_method = meth;
60}
61#endif /* FIPS_MODULE */
62
63const DSA_METHOD *DSA_get_default_method(void)
64{
65    return default_DSA_method;
66}
67
68const DSA_METHOD *DSA_OpenSSL(void)
69{
70    return &openssl_dsa_meth;
71}
72
73DSA_SIG *ossl_dsa_do_sign_int(const unsigned char *dgst, int dlen, DSA *dsa)
74{
75    BIGNUM *kinv = NULL;
76    BIGNUM *m, *blind, *blindm, *tmp;
77    BN_CTX *ctx = NULL;
78    int reason = ERR_R_BN_LIB;
79    DSA_SIG *ret = NULL;
80    int rv = 0;
81    int retries = 0;
82
83    if (dsa->params.p == NULL
84        || dsa->params.q == NULL
85        || dsa->params.g == NULL) {
86        reason = DSA_R_MISSING_PARAMETERS;
87        goto err;
88    }
89    if (dsa->priv_key == NULL) {
90        reason = DSA_R_MISSING_PRIVATE_KEY;
91        goto err;
92    }
93
94    ret = DSA_SIG_new();
95    if (ret == NULL)
96        goto err;
97    ret->r = BN_new();
98    ret->s = BN_new();
99    if (ret->r == NULL || ret->s == NULL)
100        goto err;
101
102    ctx = BN_CTX_new_ex(dsa->libctx);
103    if (ctx == NULL)
104        goto err;
105    m = BN_CTX_get(ctx);
106    blind = BN_CTX_get(ctx);
107    blindm = BN_CTX_get(ctx);
108    tmp = BN_CTX_get(ctx);
109    if (tmp == NULL)
110        goto err;
111
112 redo:
113    if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen))
114        goto err;
115
116    if (dlen > BN_num_bytes(dsa->params.q))
117        /*
118         * if the digest length is greater than the size of q use the
119         * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
120         * 4.2
121         */
122        dlen = BN_num_bytes(dsa->params.q);
123    if (BN_bin2bn(dgst, dlen, m) == NULL)
124        goto err;
125
126    /*
127     * The normal signature calculation is:
128     *
129     *   s := k^-1 * (m + r * priv_key) mod q
130     *
131     * We will blind this to protect against side channel attacks
132     *
133     *   s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q
134     */
135
136    /*
137     * Generate a blinding value
138     * The size of q is tested in dsa_sign_setup() so there should not be an infinite loop here.
139     */
140    do {
141        if (!BN_priv_rand_ex(blind, BN_num_bits(dsa->params.q) - 1,
142                             BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY, 0, ctx))
143            goto err;
144    } while (BN_is_zero(blind));
145    BN_set_flags(blind, BN_FLG_CONSTTIME);
146    BN_set_flags(blindm, BN_FLG_CONSTTIME);
147    BN_set_flags(tmp, BN_FLG_CONSTTIME);
148
149    /* tmp := blind * priv_key * r mod q */
150    if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->params.q, ctx))
151        goto err;
152    if (!BN_mod_mul(tmp, tmp, ret->r, dsa->params.q, ctx))
153        goto err;
154
155    /* blindm := blind * m mod q */
156    if (!BN_mod_mul(blindm, blind, m, dsa->params.q, ctx))
157        goto err;
158
159    /* s : = (blind * priv_key * r) + (blind * m) mod q */
160    if (!BN_mod_add_quick(ret->s, tmp, blindm, dsa->params.q))
161        goto err;
162
163    /* s := s * k^-1 mod q */
164    if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->params.q, ctx))
165        goto err;
166
167    /* s:= s * blind^-1 mod q */
168    if (BN_mod_inverse(blind, blind, dsa->params.q, ctx) == NULL)
169        goto err;
170    if (!BN_mod_mul(ret->s, ret->s, blind, dsa->params.q, ctx))
171        goto err;
172
173    /*
174     * Redo if r or s is zero as required by FIPS 186-4: Section 4.6
175     * This is very unlikely.
176     * Limit the retries so there is no possibility of an infinite
177     * loop for bad domain parameter values.
178     */
179    if (BN_is_zero(ret->r) || BN_is_zero(ret->s)) {
180        if (retries++ > MAX_DSA_SIGN_RETRIES) {
181            reason = DSA_R_TOO_MANY_RETRIES;
182            goto err;
183        }
184        goto redo;
185    }
186    rv = 1;
187 err:
188    if (rv == 0) {
189        ERR_raise(ERR_LIB_DSA, reason);
190        DSA_SIG_free(ret);
191        ret = NULL;
192    }
193    BN_CTX_free(ctx);
194    BN_clear_free(kinv);
195    return ret;
196}
197
198static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
199{
200    return ossl_dsa_do_sign_int(dgst, dlen, dsa);
201}
202
203static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in,
204                                    BIGNUM **kinvp, BIGNUM **rp)
205{
206    return dsa_sign_setup(dsa, ctx_in, kinvp, rp, NULL, 0);
207}
208
209static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
210                          BIGNUM **kinvp, BIGNUM **rp,
211                          const unsigned char *dgst, int dlen)
212{
213    BN_CTX *ctx = NULL;
214    BIGNUM *k, *kinv = NULL, *r = *rp;
215    BIGNUM *l;
216    int ret = 0;
217    int q_bits, q_words;
218
219    if (!dsa->params.p || !dsa->params.q || !dsa->params.g) {
220        ERR_raise(ERR_LIB_DSA, DSA_R_MISSING_PARAMETERS);
221        return 0;
222    }
223
224    /* Reject obviously invalid parameters */
225    if (BN_is_zero(dsa->params.p)
226        || BN_is_zero(dsa->params.q)
227        || BN_is_zero(dsa->params.g)
228        || BN_is_negative(dsa->params.p)
229        || BN_is_negative(dsa->params.q)
230        || BN_is_negative(dsa->params.g)) {
231        ERR_raise(ERR_LIB_DSA, DSA_R_INVALID_PARAMETERS);
232        return 0;
233    }
234    if (dsa->priv_key == NULL) {
235        ERR_raise(ERR_LIB_DSA, DSA_R_MISSING_PRIVATE_KEY);
236        return 0;
237    }
238    k = BN_new();
239    l = BN_new();
240    if (k == NULL || l == NULL)
241        goto err;
242
243    if (ctx_in == NULL) {
244        /* if you don't pass in ctx_in you get a default libctx */
245        if ((ctx = BN_CTX_new_ex(NULL)) == NULL)
246            goto err;
247    } else
248        ctx = ctx_in;
249
250    /* Preallocate space */
251    q_bits = BN_num_bits(dsa->params.q);
252    q_words = bn_get_top(dsa->params.q);
253    if (q_bits < MIN_DSA_SIGN_QBITS
254        || !bn_wexpand(k, q_words + 2)
255        || !bn_wexpand(l, q_words + 2))
256        goto err;
257
258    /* Get random k */
259    do {
260        if (dgst != NULL) {
261            /*
262             * We calculate k from SHA512(private_key + H(message) + random).
263             * This protects the private key from a weak PRNG.
264             */
265            if (!BN_generate_dsa_nonce(k, dsa->params.q, dsa->priv_key, dgst,
266                                       dlen, ctx))
267                goto err;
268        } else if (!BN_priv_rand_range_ex(k, dsa->params.q, 0, ctx))
269            goto err;
270    } while (BN_is_zero(k));
271
272    BN_set_flags(k, BN_FLG_CONSTTIME);
273    BN_set_flags(l, BN_FLG_CONSTTIME);
274
275    if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
276        if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
277                                    dsa->lock, dsa->params.p, ctx))
278            goto err;
279    }
280
281    /* Compute r = (g^k mod p) mod q */
282
283    /*
284     * We do not want timing information to leak the length of k, so we
285     * compute G^k using an equivalent scalar of fixed bit-length.
286     *
287     * We unconditionally perform both of these additions to prevent a
288     * small timing information leakage.  We then choose the sum that is
289     * one bit longer than the modulus.
290     *
291     * There are some concerns about the efficacy of doing this.  More
292     * specifically refer to the discussion starting with:
293     *     https://github.com/openssl/openssl/pull/7486#discussion_r228323705
294     * The fix is to rework BN so these gymnastics aren't required.
295     */
296    if (!BN_add(l, k, dsa->params.q)
297        || !BN_add(k, l, dsa->params.q))
298        goto err;
299
300    BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
301
302    if ((dsa)->meth->bn_mod_exp != NULL) {
303            if (!dsa->meth->bn_mod_exp(dsa, r, dsa->params.g, k, dsa->params.p,
304                                       ctx, dsa->method_mont_p))
305                goto err;
306    } else {
307            if (!BN_mod_exp_mont(r, dsa->params.g, k, dsa->params.p, ctx,
308                                 dsa->method_mont_p))
309                goto err;
310    }
311
312    if (!BN_mod(r, r, dsa->params.q, ctx))
313        goto err;
314
315    /* Compute part of 's = inv(k) (m + xr) mod q' */
316    if ((kinv = dsa_mod_inverse_fermat(k, dsa->params.q, ctx)) == NULL)
317        goto err;
318
319    BN_clear_free(*kinvp);
320    *kinvp = kinv;
321    kinv = NULL;
322    ret = 1;
323 err:
324    if (!ret)
325        ERR_raise(ERR_LIB_DSA, ERR_R_BN_LIB);
326    if (ctx != ctx_in)
327        BN_CTX_free(ctx);
328    BN_clear_free(k);
329    BN_clear_free(l);
330    return ret;
331}
332
333static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
334                         DSA_SIG *sig, DSA *dsa)
335{
336    BN_CTX *ctx;
337    BIGNUM *u1, *u2, *t1;
338    BN_MONT_CTX *mont = NULL;
339    const BIGNUM *r, *s;
340    int ret = -1, i;
341
342    if (dsa->params.p == NULL
343        || dsa->params.q == NULL
344        || dsa->params.g == NULL) {
345        ERR_raise(ERR_LIB_DSA, DSA_R_MISSING_PARAMETERS);
346        return -1;
347    }
348
349    i = BN_num_bits(dsa->params.q);
350    /* fips 186-3 allows only different sizes for q */
351    if (i != 160 && i != 224 && i != 256) {
352        ERR_raise(ERR_LIB_DSA, DSA_R_BAD_Q_VALUE);
353        return -1;
354    }
355
356    if (BN_num_bits(dsa->params.p) > OPENSSL_DSA_MAX_MODULUS_BITS) {
357        ERR_raise(ERR_LIB_DSA, DSA_R_MODULUS_TOO_LARGE);
358        return -1;
359    }
360    u1 = BN_new();
361    u2 = BN_new();
362    t1 = BN_new();
363    ctx = BN_CTX_new_ex(NULL); /* verify does not need a libctx */
364    if (u1 == NULL || u2 == NULL || t1 == NULL || ctx == NULL)
365        goto err;
366
367    DSA_SIG_get0(sig, &r, &s);
368
369    if (BN_is_zero(r) || BN_is_negative(r) ||
370        BN_ucmp(r, dsa->params.q) >= 0) {
371        ret = 0;
372        goto err;
373    }
374    if (BN_is_zero(s) || BN_is_negative(s) ||
375        BN_ucmp(s, dsa->params.q) >= 0) {
376        ret = 0;
377        goto err;
378    }
379
380    /*
381     * Calculate W = inv(S) mod Q save W in u2
382     */
383    if ((BN_mod_inverse(u2, s, dsa->params.q, ctx)) == NULL)
384        goto err;
385
386    /* save M in u1 */
387    if (dgst_len > (i >> 3))
388        /*
389         * if the digest length is greater than the size of q use the
390         * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
391         * 4.2
392         */
393        dgst_len = (i >> 3);
394    if (BN_bin2bn(dgst, dgst_len, u1) == NULL)
395        goto err;
396
397    /* u1 = M * w mod q */
398    if (!BN_mod_mul(u1, u1, u2, dsa->params.q, ctx))
399        goto err;
400
401    /* u2 = r * w mod q */
402    if (!BN_mod_mul(u2, r, u2, dsa->params.q, ctx))
403        goto err;
404
405    if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
406        mont = BN_MONT_CTX_set_locked(&dsa->method_mont_p,
407                                      dsa->lock, dsa->params.p, ctx);
408        if (!mont)
409            goto err;
410    }
411
412    if (dsa->meth->dsa_mod_exp != NULL) {
413        if (!dsa->meth->dsa_mod_exp(dsa, t1, dsa->params.g, u1, dsa->pub_key, u2,
414                                    dsa->params.p, ctx, mont))
415            goto err;
416    } else {
417        if (!BN_mod_exp2_mont(t1, dsa->params.g, u1, dsa->pub_key, u2,
418                              dsa->params.p, ctx, mont))
419            goto err;
420    }
421
422    /* let u1 = u1 mod q */
423    if (!BN_mod(u1, t1, dsa->params.q, ctx))
424        goto err;
425
426    /*
427     * V is now in u1.  If the signature is correct, it will be equal to R.
428     */
429    ret = (BN_ucmp(u1, r) == 0);
430
431 err:
432    if (ret < 0)
433        ERR_raise(ERR_LIB_DSA, ERR_R_BN_LIB);
434    BN_CTX_free(ctx);
435    BN_free(u1);
436    BN_free(u2);
437    BN_free(t1);
438    return ret;
439}
440
441static int dsa_init(DSA *dsa)
442{
443    dsa->flags |= DSA_FLAG_CACHE_MONT_P;
444    dsa->dirty_cnt++;
445    return 1;
446}
447
448static int dsa_finish(DSA *dsa)
449{
450    BN_MONT_CTX_free(dsa->method_mont_p);
451    return 1;
452}
453
454/*
455 * Compute the inverse of k modulo q.
456 * Since q is prime, Fermat's Little Theorem applies, which reduces this to
457 * mod-exp operation.  Both the exponent and modulus are public information
458 * so a mod-exp that doesn't leak the base is sufficient.  A newly allocated
459 * BIGNUM is returned which the caller must free.
460 */
461static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
462                                      BN_CTX *ctx)
463{
464    BIGNUM *res = NULL;
465    BIGNUM *r, *e;
466
467    if ((r = BN_new()) == NULL)
468        return NULL;
469
470    BN_CTX_start(ctx);
471    if ((e = BN_CTX_get(ctx)) != NULL
472            && BN_set_word(r, 2)
473            && BN_sub(e, q, r)
474            && BN_mod_exp_mont(r, k, e, q, ctx, NULL))
475        res = r;
476    else
477        BN_free(r);
478    BN_CTX_end(ctx);
479    return res;
480}
481