1 /*
2 * Copyright 1995-2024 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 #include "internal/deterministic_nonce.h"
24
25 #define MIN_DSA_SIGN_QBITS 128
26 #define MAX_DSA_SIGN_RETRIES 8
27
28 static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa);
29 static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
30 BIGNUM **rp);
31 static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
32 BIGNUM **rp, const unsigned char *dgst, int dlen,
33 unsigned int nonce_type, const char *digestname,
34 OSSL_LIB_CTX *libctx, const char *propq);
35 static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
36 DSA_SIG *sig, DSA *dsa);
37 static int dsa_init(DSA *dsa);
38 static int dsa_finish(DSA *dsa);
39 static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
40 BN_CTX *ctx);
41
42 static DSA_METHOD openssl_dsa_meth = {
43 "OpenSSL DSA method",
44 dsa_do_sign,
45 dsa_sign_setup_no_digest,
46 dsa_do_verify,
47 NULL, /* dsa_mod_exp, */
48 NULL, /* dsa_bn_mod_exp, */
49 dsa_init,
50 dsa_finish,
51 DSA_FLAG_FIPS_METHOD,
52 NULL,
53 NULL,
54 NULL
55 };
56
57 static const DSA_METHOD *default_DSA_method = &openssl_dsa_meth;
58
59 #ifndef FIPS_MODULE
DSA_set_default_method(const DSA_METHOD * meth)60 void DSA_set_default_method(const DSA_METHOD *meth)
61 {
62 default_DSA_method = meth;
63 }
64 #endif /* FIPS_MODULE */
65
DSA_get_default_method(void)66 const DSA_METHOD *DSA_get_default_method(void)
67 {
68 return default_DSA_method;
69 }
70
DSA_OpenSSL(void)71 const DSA_METHOD *DSA_OpenSSL(void)
72 {
73 return &openssl_dsa_meth;
74 }
75
ossl_dsa_do_sign_int(const unsigned char * dgst,int dlen,DSA * dsa,unsigned int nonce_type,const char * digestname,OSSL_LIB_CTX * libctx,const char * propq)76 DSA_SIG *ossl_dsa_do_sign_int(const unsigned char *dgst, int dlen, DSA *dsa,
77 unsigned int nonce_type, const char *digestname,
78 OSSL_LIB_CTX *libctx, const char *propq)
79 {
80 BIGNUM *kinv = NULL;
81 BIGNUM *m, *blind, *blindm, *tmp;
82 BN_CTX *ctx = NULL;
83 int reason = ERR_R_BN_LIB;
84 DSA_SIG *ret = NULL;
85 int rv = 0;
86 int retries = 0;
87
88 if (dsa->params.p == NULL
89 || dsa->params.q == NULL
90 || dsa->params.g == NULL) {
91 reason = DSA_R_MISSING_PARAMETERS;
92 goto err;
93 }
94 if (dsa->priv_key == NULL) {
95 reason = DSA_R_MISSING_PRIVATE_KEY;
96 goto err;
97 }
98
99 ret = DSA_SIG_new();
100 if (ret == NULL)
101 goto err;
102 ret->r = BN_new();
103 ret->s = BN_new();
104 if (ret->r == NULL || ret->s == NULL)
105 goto err;
106
107 ctx = BN_CTX_new_ex(dsa->libctx);
108 if (ctx == NULL)
109 goto err;
110 m = BN_CTX_get(ctx);
111 blind = BN_CTX_get(ctx);
112 blindm = BN_CTX_get(ctx);
113 tmp = BN_CTX_get(ctx);
114 if (tmp == NULL)
115 goto err;
116
117 redo:
118 if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen,
119 nonce_type, digestname, libctx, propq))
120 goto err;
121
122 if (dlen > BN_num_bytes(dsa->params.q))
123 /*
124 * if the digest length is greater than the size of q use the
125 * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
126 * 4.2
127 */
128 dlen = BN_num_bytes(dsa->params.q);
129 if (BN_bin2bn(dgst, dlen, m) == NULL)
130 goto err;
131
132 /*
133 * The normal signature calculation is:
134 *
135 * s := k^-1 * (m + r * priv_key) mod q
136 *
137 * We will blind this to protect against side channel attacks
138 *
139 * s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q
140 */
141
142 /*
143 * Generate a blinding value
144 * The size of q is tested in dsa_sign_setup() so there should not be an infinite loop here.
145 */
146 do {
147 if (!BN_priv_rand_ex(blind, BN_num_bits(dsa->params.q) - 1,
148 BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY, 0, ctx))
149 goto err;
150 } while (BN_is_zero(blind));
151 BN_set_flags(blind, BN_FLG_CONSTTIME);
152 BN_set_flags(blindm, BN_FLG_CONSTTIME);
153 BN_set_flags(tmp, BN_FLG_CONSTTIME);
154
155 /* tmp := blind * priv_key * r mod q */
156 if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->params.q, ctx))
157 goto err;
158 if (!BN_mod_mul(tmp, tmp, ret->r, dsa->params.q, ctx))
159 goto err;
160
161 /* blindm := blind * m mod q */
162 if (!BN_mod_mul(blindm, blind, m, dsa->params.q, ctx))
163 goto err;
164
165 /* s : = (blind * priv_key * r) + (blind * m) mod q */
166 if (!BN_mod_add_quick(ret->s, tmp, blindm, dsa->params.q))
167 goto err;
168
169 /* s := s * k^-1 mod q */
170 if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->params.q, ctx))
171 goto err;
172
173 /* s:= s * blind^-1 mod q */
174 if (BN_mod_inverse(blind, blind, dsa->params.q, ctx) == NULL)
175 goto err;
176 if (!BN_mod_mul(ret->s, ret->s, blind, dsa->params.q, ctx))
177 goto err;
178
179 /*
180 * Redo if r or s is zero as required by FIPS 186-4: Section 4.6
181 * This is very unlikely.
182 * Limit the retries so there is no possibility of an infinite
183 * loop for bad domain parameter values.
184 */
185 if (BN_is_zero(ret->r) || BN_is_zero(ret->s)) {
186 if (retries++ > MAX_DSA_SIGN_RETRIES) {
187 reason = DSA_R_TOO_MANY_RETRIES;
188 goto err;
189 }
190 goto redo;
191 }
192 rv = 1;
193 err:
194 if (rv == 0) {
195 ERR_raise(ERR_LIB_DSA, reason);
196 DSA_SIG_free(ret);
197 ret = NULL;
198 }
199 BN_CTX_free(ctx);
200 BN_clear_free(kinv);
201 return ret;
202 }
203
dsa_do_sign(const unsigned char * dgst,int dlen,DSA * dsa)204 static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
205 {
206 return ossl_dsa_do_sign_int(dgst, dlen, dsa,
207 0, NULL, NULL, NULL);
208 }
209
dsa_sign_setup_no_digest(DSA * dsa,BN_CTX * ctx_in,BIGNUM ** kinvp,BIGNUM ** rp)210 static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in,
211 BIGNUM **kinvp, BIGNUM **rp)
212 {
213 return dsa_sign_setup(dsa, ctx_in, kinvp, rp, NULL, 0,
214 0, NULL, NULL, NULL);
215 }
216
dsa_sign_setup(DSA * dsa,BN_CTX * ctx_in,BIGNUM ** kinvp,BIGNUM ** rp,const unsigned char * dgst,int dlen,unsigned int nonce_type,const char * digestname,OSSL_LIB_CTX * libctx,const char * propq)217 static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
218 BIGNUM **kinvp, BIGNUM **rp,
219 const unsigned char *dgst, int dlen,
220 unsigned int nonce_type, const char *digestname,
221 OSSL_LIB_CTX *libctx, const char *propq)
222 {
223 BN_CTX *ctx = NULL;
224 BIGNUM *k, *kinv = NULL, *r = *rp;
225 BIGNUM *l;
226 int ret = 0;
227 int q_bits, q_words;
228
229 if (!dsa->params.p || !dsa->params.q || !dsa->params.g) {
230 ERR_raise(ERR_LIB_DSA, DSA_R_MISSING_PARAMETERS);
231 return 0;
232 }
233
234 /* Reject obviously invalid parameters */
235 if (BN_is_zero(dsa->params.p)
236 || BN_is_zero(dsa->params.q)
237 || BN_is_zero(dsa->params.g)
238 || BN_is_negative(dsa->params.p)
239 || BN_is_negative(dsa->params.q)
240 || BN_is_negative(dsa->params.g)) {
241 ERR_raise(ERR_LIB_DSA, DSA_R_INVALID_PARAMETERS);
242 return 0;
243 }
244 if (dsa->priv_key == NULL) {
245 ERR_raise(ERR_LIB_DSA, DSA_R_MISSING_PRIVATE_KEY);
246 return 0;
247 }
248 k = BN_new();
249 l = BN_new();
250 if (k == NULL || l == NULL)
251 goto err;
252
253 if (ctx_in == NULL) {
254 /* if you don't pass in ctx_in you get a default libctx */
255 if ((ctx = BN_CTX_new_ex(NULL)) == NULL)
256 goto err;
257 } else
258 ctx = ctx_in;
259
260 /* Preallocate space */
261 q_bits = BN_num_bits(dsa->params.q);
262 q_words = bn_get_top(dsa->params.q);
263 if (q_bits < MIN_DSA_SIGN_QBITS
264 || !bn_wexpand(k, q_words + 2)
265 || !bn_wexpand(l, q_words + 2))
266 goto err;
267
268 /* Get random k */
269 do {
270 if (dgst != NULL) {
271 if (nonce_type == 1) {
272 #ifndef FIPS_MODULE
273 if (!ossl_gen_deterministic_nonce_rfc6979(k, dsa->params.q,
274 dsa->priv_key,
275 dgst, dlen,
276 digestname,
277 libctx, propq))
278 #endif
279 goto err;
280 } else {
281 /*
282 * We calculate k from SHA512(private_key + H(message) + random).
283 * This protects the private key from a weak PRNG.
284 */
285 if (!ossl_bn_gen_dsa_nonce_fixed_top(k, dsa->params.q,
286 dsa->priv_key, dgst,
287 dlen, ctx))
288 goto err;
289 }
290 } else if (!ossl_bn_priv_rand_range_fixed_top(k, dsa->params.q, 0, ctx))
291 goto err;
292 } while (ossl_bn_is_word_fixed_top(k, 0));
293
294 BN_set_flags(k, BN_FLG_CONSTTIME);
295 BN_set_flags(l, BN_FLG_CONSTTIME);
296
297 if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
298 if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
299 dsa->lock, dsa->params.p, ctx))
300 goto err;
301 }
302
303 /* Compute r = (g^k mod p) mod q */
304
305 /*
306 * We do not want timing information to leak the length of k, so we
307 * compute G^k using an equivalent scalar of fixed bit-length.
308 *
309 * We unconditionally perform both of these additions to prevent a
310 * small timing information leakage. We then choose the sum that is
311 * one bit longer than the modulus.
312 *
313 * There are some concerns about the efficacy of doing this. More
314 * specifically refer to the discussion starting with:
315 * https://github.com/openssl/openssl/pull/7486#discussion_r228323705
316 * The fix is to rework BN so these gymnastics aren't required.
317 */
318 if (!BN_add(l, k, dsa->params.q)
319 || !BN_add(k, l, dsa->params.q))
320 goto err;
321
322 BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
323
324 if ((dsa)->meth->bn_mod_exp != NULL) {
325 if (!dsa->meth->bn_mod_exp(dsa, r, dsa->params.g, k, dsa->params.p,
326 ctx, dsa->method_mont_p))
327 goto err;
328 } else {
329 if (!BN_mod_exp_mont(r, dsa->params.g, k, dsa->params.p, ctx,
330 dsa->method_mont_p))
331 goto err;
332 }
333
334 if (!BN_mod(r, r, dsa->params.q, ctx))
335 goto err;
336
337 /* Compute part of 's = inv(k) (m + xr) mod q' */
338 if ((kinv = dsa_mod_inverse_fermat(k, dsa->params.q, ctx)) == NULL)
339 goto err;
340
341 BN_clear_free(*kinvp);
342 *kinvp = kinv;
343 kinv = NULL;
344 ret = 1;
345 err:
346 if (!ret)
347 ERR_raise(ERR_LIB_DSA, ERR_R_BN_LIB);
348 if (ctx != ctx_in)
349 BN_CTX_free(ctx);
350 BN_clear_free(k);
351 BN_clear_free(l);
352 return ret;
353 }
354
dsa_do_verify(const unsigned char * dgst,int dgst_len,DSA_SIG * sig,DSA * dsa)355 static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
356 DSA_SIG *sig, DSA *dsa)
357 {
358 BN_CTX *ctx;
359 BIGNUM *u1, *u2, *t1;
360 BN_MONT_CTX *mont = NULL;
361 const BIGNUM *r, *s;
362 int ret = -1, i;
363
364 if (dsa->params.p == NULL
365 || dsa->params.q == NULL
366 || dsa->params.g == NULL) {
367 ERR_raise(ERR_LIB_DSA, DSA_R_MISSING_PARAMETERS);
368 return -1;
369 }
370
371 i = BN_num_bits(dsa->params.q);
372 /* fips 186-3 allows only different sizes for q */
373 if (i != 160 && i != 224 && i != 256) {
374 ERR_raise(ERR_LIB_DSA, DSA_R_BAD_Q_VALUE);
375 return -1;
376 }
377
378 if (BN_num_bits(dsa->params.p) > OPENSSL_DSA_MAX_MODULUS_BITS) {
379 ERR_raise(ERR_LIB_DSA, DSA_R_MODULUS_TOO_LARGE);
380 return -1;
381 }
382 u1 = BN_new();
383 u2 = BN_new();
384 t1 = BN_new();
385 ctx = BN_CTX_new_ex(NULL); /* verify does not need a libctx */
386 if (u1 == NULL || u2 == NULL || t1 == NULL || ctx == NULL)
387 goto err;
388
389 DSA_SIG_get0(sig, &r, &s);
390
391 if (BN_is_zero(r) || BN_is_negative(r) ||
392 BN_ucmp(r, dsa->params.q) >= 0) {
393 ret = 0;
394 goto err;
395 }
396 if (BN_is_zero(s) || BN_is_negative(s) ||
397 BN_ucmp(s, dsa->params.q) >= 0) {
398 ret = 0;
399 goto err;
400 }
401
402 /*
403 * Calculate W = inv(S) mod Q save W in u2
404 */
405 if ((BN_mod_inverse(u2, s, dsa->params.q, ctx)) == NULL)
406 goto err;
407
408 /* save M in u1 */
409 if (dgst_len > (i >> 3))
410 /*
411 * if the digest length is greater than the size of q use the
412 * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
413 * 4.2
414 */
415 dgst_len = (i >> 3);
416 if (BN_bin2bn(dgst, dgst_len, u1) == NULL)
417 goto err;
418
419 /* u1 = M * w mod q */
420 if (!BN_mod_mul(u1, u1, u2, dsa->params.q, ctx))
421 goto err;
422
423 /* u2 = r * w mod q */
424 if (!BN_mod_mul(u2, r, u2, dsa->params.q, ctx))
425 goto err;
426
427 if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
428 mont = BN_MONT_CTX_set_locked(&dsa->method_mont_p,
429 dsa->lock, dsa->params.p, ctx);
430 if (!mont)
431 goto err;
432 }
433
434 if (dsa->meth->dsa_mod_exp != NULL) {
435 if (!dsa->meth->dsa_mod_exp(dsa, t1, dsa->params.g, u1, dsa->pub_key, u2,
436 dsa->params.p, ctx, mont))
437 goto err;
438 } else {
439 if (!BN_mod_exp2_mont(t1, dsa->params.g, u1, dsa->pub_key, u2,
440 dsa->params.p, ctx, mont))
441 goto err;
442 }
443
444 /* let u1 = u1 mod q */
445 if (!BN_mod(u1, t1, dsa->params.q, ctx))
446 goto err;
447
448 /*
449 * V is now in u1. If the signature is correct, it will be equal to R.
450 */
451 ret = (BN_ucmp(u1, r) == 0);
452
453 err:
454 if (ret < 0)
455 ERR_raise(ERR_LIB_DSA, ERR_R_BN_LIB);
456 BN_CTX_free(ctx);
457 BN_free(u1);
458 BN_free(u2);
459 BN_free(t1);
460 return ret;
461 }
462
dsa_init(DSA * dsa)463 static int dsa_init(DSA *dsa)
464 {
465 dsa->flags |= DSA_FLAG_CACHE_MONT_P;
466 dsa->dirty_cnt++;
467 return 1;
468 }
469
dsa_finish(DSA * dsa)470 static int dsa_finish(DSA *dsa)
471 {
472 BN_MONT_CTX_free(dsa->method_mont_p);
473 return 1;
474 }
475
476 /*
477 * Compute the inverse of k modulo q.
478 * Since q is prime, Fermat's Little Theorem applies, which reduces this to
479 * mod-exp operation. Both the exponent and modulus are public information
480 * so a mod-exp that doesn't leak the base is sufficient. A newly allocated
481 * BIGNUM is returned which the caller must free.
482 */
dsa_mod_inverse_fermat(const BIGNUM * k,const BIGNUM * q,BN_CTX * ctx)483 static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
484 BN_CTX *ctx)
485 {
486 BIGNUM *res = NULL;
487 BIGNUM *r, *e;
488
489 if ((r = BN_new()) == NULL)
490 return NULL;
491
492 BN_CTX_start(ctx);
493 if ((e = BN_CTX_get(ctx)) != NULL
494 && BN_set_word(r, 2)
495 && BN_sub(e, q, r)
496 && BN_mod_exp_mont(r, k, e, q, ctx, NULL))
497 res = r;
498 else
499 BN_free(r);
500 BN_CTX_end(ctx);
501 return res;
502 }
503
