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
27 static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa);
28 static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
29 BIGNUM **rp);
30 static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
31 BIGNUM **rp, const unsigned char *dgst, int dlen);
32 static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
33 DSA_SIG *sig, DSA *dsa);
34 static int dsa_init(DSA *dsa);
35 static int dsa_finish(DSA *dsa);
36 static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
37 BN_CTX *ctx);
38
39 static 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
54 static const DSA_METHOD *default_DSA_method = &openssl_dsa_meth;
55
56 #ifndef FIPS_MODULE
DSA_set_default_method(const DSA_METHOD * meth)57 void DSA_set_default_method(const DSA_METHOD *meth)
58 {
59 default_DSA_method = meth;
60 }
61 #endif /* FIPS_MODULE */
62
DSA_get_default_method(void)63 const DSA_METHOD *DSA_get_default_method(void)
64 {
65 return default_DSA_method;
66 }
67
DSA_OpenSSL(void)68 const DSA_METHOD *DSA_OpenSSL(void)
69 {
70 return &openssl_dsa_meth;
71 }
72
ossl_dsa_do_sign_int(const unsigned char * dgst,int dlen,DSA * dsa)73 DSA_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
dsa_do_sign(const unsigned char * dgst,int dlen,DSA * dsa)198 static 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
dsa_sign_setup_no_digest(DSA * dsa,BN_CTX * ctx_in,BIGNUM ** kinvp,BIGNUM ** rp)203 static 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
dsa_sign_setup(DSA * dsa,BN_CTX * ctx_in,BIGNUM ** kinvp,BIGNUM ** rp,const unsigned char * dgst,int dlen)209 static 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
dsa_do_verify(const unsigned char * dgst,int dgst_len,DSA_SIG * sig,DSA * dsa)333 static 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
dsa_init(DSA * dsa)441 static int dsa_init(DSA *dsa)
442 {
443 dsa->flags |= DSA_FLAG_CACHE_MONT_P;
444 ossl_ffc_params_init(&dsa->params);
445 dsa->dirty_cnt++;
446 return 1;
447 }
448
dsa_finish(DSA * dsa)449 static int dsa_finish(DSA *dsa)
450 {
451 BN_MONT_CTX_free(dsa->method_mont_p);
452 return 1;
453 }
454
455 /*
456 * Compute the inverse of k modulo q.
457 * Since q is prime, Fermat's Little Theorem applies, which reduces this to
458 * mod-exp operation. Both the exponent and modulus are public information
459 * so a mod-exp that doesn't leak the base is sufficient. A newly allocated
460 * BIGNUM is returned which the caller must free.
461 */
dsa_mod_inverse_fermat(const BIGNUM * k,const BIGNUM * q,BN_CTX * ctx)462 static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
463 BN_CTX *ctx)
464 {
465 BIGNUM *res = NULL;
466 BIGNUM *r, *e;
467
468 if ((r = BN_new()) == NULL)
469 return NULL;
470
471 BN_CTX_start(ctx);
472 if ((e = BN_CTX_get(ctx)) != NULL
473 && BN_set_word(r, 2)
474 && BN_sub(e, q, r)
475 && BN_mod_exp_mont(r, k, e, q, ctx, NULL))
476 res = r;
477 else
478 BN_free(r);
479 BN_CTX_end(ctx);
480 return res;
481 }
482