crypto/dsa/dsa_gen.c

   1 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
   2  * All rights reserved.
   3  *
   4  * This package is an SSL implementation written
   5  * by Eric Young (eay@cryptsoft.com).
   6  * The implementation was written so as to conform with Netscapes SSL.
   7  *
   8  * This library is free for commercial and non-commercial use as long as
   9  * the following conditions are aheared to.  The following conditions
  10  * apply to all code found in this distribution, be it the RC4, RSA,
  11  * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
  12  * included with this distribution is covered by the same copyright terms
  13  * except that the holder is Tim Hudson (tjh@cryptsoft.com).
  14  *
  15  * Copyright remains Eric Young's, and as such any Copyright notices in
  16  * the code are not to be removed.
  17  * If this package is used in a product, Eric Young should be given attribution
  18  * as the author of the parts of the library used.
  19  * This can be in the form of a textual message at program startup or
  20  * in documentation (online or textual) provided with the package.
  21  *
  22  * Redistribution and use in source and binary forms, with or without
  23  * modification, are permitted provided that the following conditions
  24  * are met:
  25  * 1. Redistributions of source code must retain the copyright
  26  *    notice, this list of conditions and the following disclaimer.
  27  * 2. Redistributions in binary form must reproduce the above copyright
  28  *    notice, this list of conditions and the following disclaimer in the
  29  *    documentation and/or other materials provided with the distribution.
  30  * 3. All advertising materials mentioning features or use of this software
  31  *    must display the following acknowledgement:
  32  *    "This product includes cryptographic software written by
  33  *     Eric Young (eay@cryptsoft.com)"
  34  *    The word 'cryptographic' can be left out if the rouines from the library
  35  *    being used are not cryptographic related :-).
  36  * 4. If you include any Windows specific code (or a derivative thereof) from
  37  *    the apps directory (application code) you must include an acknowledgement:
  38  *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
  39  *
  40  * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
  41  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  42  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  43  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  44  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  45  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  46  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  47  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  48  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  49  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  50  * SUCH DAMAGE.
  51  *
  52  * The licence and distribution terms for any publically available version or
  53  * derivative of this code cannot be changed.  i.e. this code cannot simply be
  54  * copied and put under another distribution licence
  55  * [including the GNU Public Licence.]
  56  */
  57
  58 /*
  59  * Parameter generation follows the updated Appendix 2.2 for FIPS PUB 186,
  60  * also Appendix 2.2 of FIPS PUB 186-1 (i.e. use SHA as defined in FIPS PUB
  61  * 180-1)
  62  */
  63 #define xxxHASH    EVP_sha1()
  64
  65 #include <openssl/opensslconf.h> /* To see if OPENSSL_NO_SHA is defined */
  66
  67 #include <stdio.h>
  68 #include "internal/cryptlib.h"
  69 #include <openssl/evp.h>
  70 #include <openssl/bn.h>
  71 #include <openssl/rand.h>
  72 #include <openssl/sha.h>
  73
  74 #include "dsa_locl.h"
  75
  76 int DSA_generate_parameters_ex(DSA *ret, int bits,
  77                                const unsigned char *seed_in, int seed_len,
  78                                int *counter_ret, unsigned long *h_ret,
  79                                BN_GENCB *cb)
  80 {
  81     if (ret->meth->dsa_paramgen)
  82         return ret->meth->dsa_paramgen(ret, bits, seed_in, seed_len,
  83                                        counter_ret, h_ret, cb);
  84     else {
  85         const EVP_MD *evpmd = bits >= 2048 ? EVP_sha256() : EVP_sha1();
  86         size_t qbits = EVP_MD_size(evpmd) * 8;
  87
  88         return dsa_builtin_paramgen(ret, bits, qbits, evpmd,
  89                                     seed_in, seed_len, NULL, counter_ret,
  90                                     h_ret, cb);
  91     }
  92 }
  93
  94 int dsa_builtin_paramgen(DSA *ret, size_t bits, size_t qbits,
  95                          const EVP_MD *evpmd, const unsigned char *seed_in,
  96                          size_t seed_len, unsigned char *seed_out,
  97                          int *counter_ret, unsigned long *h_ret, BN_GENCB *cb)
  98 {
  99     int ok = 0;
 100     unsigned char seed[SHA256_DIGEST_LENGTH];
 101     unsigned char md[SHA256_DIGEST_LENGTH];
 102     unsigned char buf[SHA256_DIGEST_LENGTH], buf2[SHA256_DIGEST_LENGTH];
 103     BIGNUM *r0, *W, *X, *c, *test;
 104     BIGNUM *g = NULL, *q = NULL, *p = NULL;
 105     BN_MONT_CTX *mont = NULL;
 106     int i, k, n = 0, m = 0, qsize = qbits >> 3;
 107     int counter = 0;
 108     int r = 0;
 109     BN_CTX *ctx = NULL;
 110     unsigned int h = 2;
 111
 112     if (qsize != SHA_DIGEST_LENGTH && qsize != SHA224_DIGEST_LENGTH &&
 113         qsize != SHA256_DIGEST_LENGTH)
 114         /* invalid q size */
 115         return 0;
 116
 117     if (evpmd == NULL)
 118         /* use SHA1 as default */
 119         evpmd = EVP_sha1();
 120
 121     if (bits < 512)
 122         bits = 512;
 123
 124     bits = (bits + 63) / 64 * 64;
 125
 126     if (seed_in != NULL) {
 127         if (seed_len < (size_t)qsize)
 128             return 0;
 129         if (seed_len > (size_t)qsize) {
 130             /* Only consume as much seed as is expected. */
 131             seed_len = qsize;
 132         }
 133         memcpy(seed, seed_in, seed_len);
 134     }
 135
 136     if ((mont = BN_MONT_CTX_new()) == NULL)
 137         goto err;
 138
 139     if ((ctx = BN_CTX_new()) == NULL)
 140         goto err;
 141
 142     BN_CTX_start(ctx);
 143
 144     r0 = BN_CTX_get(ctx);
 145     g = BN_CTX_get(ctx);
 146     W = BN_CTX_get(ctx);
 147     q = BN_CTX_get(ctx);
 148     X = BN_CTX_get(ctx);
 149     c = BN_CTX_get(ctx);
 150     p = BN_CTX_get(ctx);
 151     test = BN_CTX_get(ctx);
 152
 153     if (!BN_lshift(test, BN_value_one(), bits - 1))
 154         goto err;
 155
 156     for (;;) {
 157         for (;;) {              /* find q */
 158             int use_random_seed = (seed_in == NULL);
 159
 160             /* step 1 */
 161             if (!BN_GENCB_call(cb, 0, m++))
 162                 goto err;
 163
 164             if (use_random_seed) {
 165                 if (RAND_bytes(seed, qsize) <= 0)
 166                     goto err;
 167             } else {
 168                 /* If we come back through, use random seed next time. */
 169                 seed_in = NULL;
 170             }
 171             memcpy(buf, seed, qsize);
 172             memcpy(buf2, seed, qsize);
 173             /* precompute "SEED + 1" for step 7: */
 174             for (i = qsize - 1; i >= 0; i--) {
 175                 buf[i]++;
 176                 if (buf[i] != 0)
 177                     break;
 178             }
 179
 180             /* step 2 */
 181             if (!EVP_Digest(seed, qsize, md, NULL, evpmd, NULL))
 182                 goto err;
 183             if (!EVP_Digest(buf, qsize, buf2, NULL, evpmd, NULL))
 184                 goto err;
 185             for (i = 0; i < qsize; i++)
 186                 md[i] ^= buf2[i];
 187
 188             /* step 3 */
 189             md[0] |= 0x80;
 190             md[qsize - 1] |= 0x01;
 191             if (!BN_bin2bn(md, qsize, q))
 192                 goto err;
 193
 194             /* step 4 */
 195             r = BN_is_prime_fasttest_ex(q, DSS_prime_checks, ctx,
 196                                         use_random_seed, cb);
 197             if (r > 0)
 198                 break;
 199             if (r != 0)
 200                 goto err;
 201
 202             /* do a callback call */
 203             /* step 5 */
 204         }
 205
 206         if (!BN_GENCB_call(cb, 2, 0))
 207             goto err;
 208         if (!BN_GENCB_call(cb, 3, 0))
 209             goto err;
 210
 211         /* step 6 */
 212         counter = 0;
 213         /* "offset = 2" */
 214
 215         n = (bits - 1) / 160;
 216
 217         for (;;) {
 218             if ((counter != 0) && !BN_GENCB_call(cb, 0, counter))
 219                 goto err;
 220
 221             /* step 7 */
 222             BN_zero(W);
 223             /* now 'buf' contains "SEED + offset - 1" */
 224             for (k = 0; k <= n; k++) {
 225                 /*
 226                  * obtain "SEED + offset + k" by incrementing:
 227                  */
 228                 for (i = qsize - 1; i >= 0; i--) {
 229                     buf[i]++;
 230                     if (buf[i] != 0)
 231                         break;
 232                 }
 233
 234                 if (!EVP_Digest(buf, qsize, md, NULL, evpmd, NULL))
 235                     goto err;
 236
 237                 /* step 8 */
 238                 if (!BN_bin2bn(md, qsize, r0))
 239                     goto err;
 240                 if (!BN_lshift(r0, r0, (qsize << 3) * k))
 241                     goto err;
 242                 if (!BN_add(W, W, r0))
 243                     goto err;
 244             }
 245
 246             /* more of step 8 */
 247             if (!BN_mask_bits(W, bits - 1))
 248                 goto err;
 249             if (!BN_copy(X, W))
 250                 goto err;
 251             if (!BN_add(X, X, test))
 252                 goto err;
 253
 254             /* step 9 */
 255             if (!BN_lshift1(r0, q))
 256                 goto err;
 257             if (!BN_mod(c, X, r0, ctx))
 258                 goto err;
 259             if (!BN_sub(r0, c, BN_value_one()))
 260                 goto err;
 261             if (!BN_sub(p, X, r0))
 262                 goto err;
 263
 264             /* step 10 */
 265             if (BN_cmp(p, test) >= 0) {
 266                 /* step 11 */
 267                 r = BN_is_prime_fasttest_ex(p, DSS_prime_checks, ctx, 1, cb);
 268                 if (r > 0)
 269                     goto end;   /* found it */
 270                 if (r != 0)
 271                     goto err;
 272             }
 273
 274             /* step 13 */
 275             counter++;
 276             /* "offset = offset + n + 1" */
 277
 278             /* step 14 */
 279             if (counter >= 4096)
 280                 break;
 281         }
 282     }
 283  end:
 284     if (!BN_GENCB_call(cb, 2, 1))
 285         goto err;
 286
 287     /* We now need to generate g */
 288     /* Set r0=(p-1)/q */
 289     if (!BN_sub(test, p, BN_value_one()))
 290         goto err;
 291     if (!BN_div(r0, NULL, test, q, ctx))
 292         goto err;
 293
 294     if (!BN_set_word(test, h))
 295         goto err;
 296     if (!BN_MONT_CTX_set(mont, p, ctx))
 297         goto err;
 298
 299     for (;;) {
 300         /* g=test^r0%p */
 301         if (!BN_mod_exp_mont(g, test, r0, p, ctx, mont))
 302             goto err;
 303         if (!BN_is_one(g))
 304             break;
 305         if (!BN_add(test, test, BN_value_one()))
 306             goto err;
 307         h++;
 308     }
 309
 310     if (!BN_GENCB_call(cb, 3, 1))
 311         goto err;
 312
 313     ok = 1;
 314  err:
 315     if (ok) {
 316         BN_free(ret->p);
 317         BN_free(ret->q);
 318         BN_free(ret->g);
 319         ret->p = BN_dup(p);
 320         ret->q = BN_dup(q);
 321         ret->g = BN_dup(g);
 322         if (ret->p == NULL || ret->q == NULL || ret->g == NULL) {
 323             ok = 0;
 324             goto err;
 325         }
 326         if (counter_ret != NULL)
 327             *counter_ret = counter;
 328         if (h_ret != NULL)
 329             *h_ret = h;
 330         if (seed_out)
 331             memcpy(seed_out, seed, qsize);
 332     }
 333     if (ctx)
 334         BN_CTX_end(ctx);
 335     BN_CTX_free(ctx);
 336     BN_MONT_CTX_free(mont);
 337     return ok;
 338 }
 339
 340 /*
 341  * This is a parameter generation algorithm for the DSA2 algorithm as
 342  * described in FIPS 186-3.
 343  */
 344
 345 int dsa_builtin_paramgen2(DSA *ret, size_t L, size_t N,
 346                           const EVP_MD *evpmd, const unsigned char *seed_in,
 347                           size_t seed_len, int idx, unsigned char *seed_out,
 348                           int *counter_ret, unsigned long *h_ret,
 349                           BN_GENCB *cb)
 350 {
 351     int ok = -1;
 352     unsigned char *seed = NULL, *seed_tmp = NULL;
 353     unsigned char md[EVP_MAX_MD_SIZE];
 354     int mdsize;
 355     BIGNUM *r0, *W, *X, *c, *test;
 356     BIGNUM *g = NULL, *q = NULL, *p = NULL;
 357     BN_MONT_CTX *mont = NULL;
 358     int i, k, n = 0, m = 0, qsize = N >> 3;
 359     int counter = 0;
 360     int r = 0;
 361     BN_CTX *ctx = NULL;
 362     EVP_MD_CTX *mctx = EVP_MD_CTX_new();
 363     unsigned int h = 2;
 364
 365     if (mctx == NULL)
 366         goto err;
 367
 368     if (evpmd == NULL) {
 369         if (N == 160)
 370             evpmd = EVP_sha1();
 371         else if (N == 224)
 372             evpmd = EVP_sha224();
 373         else
 374             evpmd = EVP_sha256();
 375     }
 376
 377     mdsize = EVP_MD_size(evpmd);
 378     /* If unverificable g generation only don't need seed */
 379     if (!ret->p || !ret->q || idx >= 0) {
 380         if (seed_len == 0)
 381             seed_len = mdsize;
 382
 383         seed = OPENSSL_malloc(seed_len);
 384
 385         if (seed_out)
 386             seed_tmp = seed_out;
 387         else
 388             seed_tmp = OPENSSL_malloc(seed_len);
 389
 390         if (seed == NULL || seed_tmp == NULL)
 391             goto err;
 392
 393         if (seed_in)
 394             memcpy(seed, seed_in, seed_len);
 395
 396     }
 397
 398     if ((ctx = BN_CTX_new()) == NULL)
 399         goto err;
 400
 401     if ((mont = BN_MONT_CTX_new()) == NULL)
 402         goto err;
 403
 404     BN_CTX_start(ctx);
 405     r0 = BN_CTX_get(ctx);
 406     g = BN_CTX_get(ctx);
 407     W = BN_CTX_get(ctx);
 408     X = BN_CTX_get(ctx);
 409     c = BN_CTX_get(ctx);
 410     test = BN_CTX_get(ctx);
 411
 412     /* if p, q already supplied generate g only */
 413     if (ret->p && ret->q) {
 414         p = ret->p;
 415         q = ret->q;
 416         if (idx >= 0)
 417             memcpy(seed_tmp, seed, seed_len);
 418         goto g_only;
 419     } else {
 420         p = BN_CTX_get(ctx);
 421         q = BN_CTX_get(ctx);
 422     }
 423
 424     if (!BN_lshift(test, BN_value_one(), L - 1))
 425         goto err;
 426     for (;;) {
 427         for (;;) {              /* find q */
 428             unsigned char *pmd;
 429             /* step 1 */
 430             if (!BN_GENCB_call(cb, 0, m++))
 431                 goto err;
 432
 433             if (!seed_in) {
 434                 if (RAND_bytes(seed, seed_len) <= 0)
 435                     goto err;
 436             }
 437             /* step 2 */
 438             if (!EVP_Digest(seed, seed_len, md, NULL, evpmd, NULL))
 439                 goto err;
 440             /* Take least significant bits of md */
 441             if (mdsize > qsize)
 442                 pmd = md + mdsize - qsize;
 443             else
 444                 pmd = md;
 445
 446             if (mdsize < qsize)
 447                 memset(md + mdsize, 0, qsize - mdsize);
 448
 449             /* step 3 */
 450             pmd[0] |= 0x80;
 451             pmd[qsize - 1] |= 0x01;
 452             if (!BN_bin2bn(pmd, qsize, q))
 453                 goto err;
 454
 455             /* step 4 */
 456             r = BN_is_prime_fasttest_ex(q, DSS_prime_checks, ctx,
 457                                         seed_in ? 1 : 0, cb);
 458             if (r > 0)
 459                 break;
 460             if (r != 0)
 461                 goto err;
 462             /* Provided seed didn't produce a prime: error */
 463             if (seed_in) {
 464                 ok = 0;
 465                 DSAerr(DSA_F_DSA_BUILTIN_PARAMGEN2, DSA_R_Q_NOT_PRIME);
 466                 goto err;
 467             }
 468
 469             /* do a callback call */
 470             /* step 5 */
 471         }
 472         /* Copy seed to seed_out before we mess with it */
 473         if (seed_out)
 474             memcpy(seed_out, seed, seed_len);
 475
 476         if (!BN_GENCB_call(cb, 2, 0))
 477             goto err;
 478         if (!BN_GENCB_call(cb, 3, 0))
 479             goto err;
 480
 481         /* step 6 */
 482         counter = 0;
 483         /* "offset = 1" */
 484
 485         n = (L - 1) / (mdsize << 3);
 486
 487         for (;;) {
 488             if ((counter != 0) && !BN_GENCB_call(cb, 0, counter))
 489                 goto err;
 490
 491             /* step 7 */
 492             BN_zero(W);
 493             /* now 'buf' contains "SEED + offset - 1" */
 494             for (k = 0; k <= n; k++) {
 495                 /*
 496                  * obtain "SEED + offset + k" by incrementing:
 497                  */
 498                 for (i = seed_len - 1; i >= 0; i--) {
 499                     seed[i]++;
 500                     if (seed[i] != 0)
 501                         break;
 502                 }
 503
 504                 if (!EVP_Digest(seed, seed_len, md, NULL, evpmd, NULL))
 505                     goto err;
 506
 507                 /* step 8 */
 508                 if (!BN_bin2bn(md, mdsize, r0))
 509                     goto err;
 510                 if (!BN_lshift(r0, r0, (mdsize << 3) * k))
 511                     goto err;
 512                 if (!BN_add(W, W, r0))
 513                     goto err;
 514             }
 515
 516             /* more of step 8 */
 517             if (!BN_mask_bits(W, L - 1))
 518                 goto err;
 519             if (!BN_copy(X, W))
 520                 goto err;
 521             if (!BN_add(X, X, test))
 522                 goto err;
 523
 524             /* step 9 */
 525             if (!BN_lshift1(r0, q))
 526                 goto err;
 527             if (!BN_mod(c, X, r0, ctx))
 528                 goto err;
 529             if (!BN_sub(r0, c, BN_value_one()))
 530                 goto err;
 531             if (!BN_sub(p, X, r0))
 532                 goto err;
 533
 534             /* step 10 */
 535             if (BN_cmp(p, test) >= 0) {
 536                 /* step 11 */
 537                 r = BN_is_prime_fasttest_ex(p, DSS_prime_checks, ctx, 1, cb);
 538                 if (r > 0)
 539                     goto end;   /* found it */
 540                 if (r != 0)
 541                     goto err;
 542             }
 543
 544             /* step 13 */
 545             counter++;
 546             /* "offset = offset + n + 1" */
 547
 548             /* step 14 */
 549             if (counter >= (int)(4 * L))
 550                 break;
 551         }
 552         if (seed_in) {
 553             ok = 0;
 554             DSAerr(DSA_F_DSA_BUILTIN_PARAMGEN2, DSA_R_INVALID_PARAMETERS);
 555             goto err;
 556         }
 557     }
 558  end:
 559     if (!BN_GENCB_call(cb, 2, 1))
 560         goto err;
 561
 562  g_only:
 563
 564     /* We now need to generate g */
 565     /* Set r0=(p-1)/q */
 566     if (!BN_sub(test, p, BN_value_one()))
 567         goto err;
 568     if (!BN_div(r0, NULL, test, q, ctx))
 569         goto err;
 570
 571     if (idx < 0) {
 572         if (!BN_set_word(test, h))
 573             goto err;
 574     } else
 575         h = 1;
 576     if (!BN_MONT_CTX_set(mont, p, ctx))
 577         goto err;
 578
 579     for (;;) {
 580         static const unsigned char ggen[4] = { 0x67, 0x67, 0x65, 0x6e };
 581         if (idx >= 0) {
 582             md[0] = idx & 0xff;
 583             md[1] = (h >> 8) & 0xff;
 584             md[2] = h & 0xff;
 585             if (!EVP_DigestInit_ex(mctx, evpmd, NULL))
 586                 goto err;
 587             if (!EVP_DigestUpdate(mctx, seed_tmp, seed_len))
 588                 goto err;
 589             if (!EVP_DigestUpdate(mctx, ggen, sizeof(ggen)))
 590                 goto err;
 591             if (!EVP_DigestUpdate(mctx, md, 3))
 592                 goto err;
 593             if (!EVP_DigestFinal_ex(mctx, md, NULL))
 594                 goto err;
 595             if (!BN_bin2bn(md, mdsize, test))
 596                 goto err;
 597         }
 598         /* g=test^r0%p */
 599         if (!BN_mod_exp_mont(g, test, r0, p, ctx, mont))
 600             goto err;
 601         if (!BN_is_one(g))
 602             break;
 603         if (idx < 0 && !BN_add(test, test, BN_value_one()))
 604             goto err;
 605         h++;
 606         if (idx >= 0 && h > 0xffff)
 607             goto err;
 608     }
 609
 610     if (!BN_GENCB_call(cb, 3, 1))
 611         goto err;
 612
 613     ok = 1;
 614  err:
 615     if (ok == 1) {
 616         if (p != ret->p) {
 617             BN_free(ret->p);
 618             ret->p = BN_dup(p);
 619         }
 620         if (q != ret->q) {
 621             BN_free(ret->q);
 622             ret->q = BN_dup(q);
 623         }
 624         BN_free(ret->g);
 625         ret->g = BN_dup(g);
 626         if (ret->p == NULL || ret->q == NULL || ret->g == NULL) {
 627             ok = -1;
 628             goto err;
 629         }
 630         if (counter_ret != NULL)
 631             *counter_ret = counter;
 632         if (h_ret != NULL)
 633             *h_ret = h;
 634     }
 635     OPENSSL_free(seed);
 636     if (seed_out != seed_tmp)
 637         OPENSSL_free(seed_tmp);
 638     if (ctx)
 639         BN_CTX_end(ctx);
 640     BN_CTX_free(ctx);
 641     BN_MONT_CTX_free(mont);
 642     EVP_MD_CTX_free(mctx);
 643     return ok;
 644 }
 645
 646 int dsa_paramgen_check_g(DSA *dsa)
 647 {
 648     BN_CTX *ctx;
 649     BIGNUM *tmp;
 650     BN_MONT_CTX *mont = NULL;
 651     int rv = -1;
 652     ctx = BN_CTX_new();
 653     if (ctx == NULL)
 654         return -1;
 655     BN_CTX_start(ctx);
 656     if (BN_cmp(dsa->g, BN_value_one()) <= 0)
 657         return 0;
 658     if (BN_cmp(dsa->g, dsa->p) >= 0)
 659         return 0;
 660     tmp = BN_CTX_get(ctx);
 661     if (!tmp)
 662         goto err;
 663     if ((mont = BN_MONT_CTX_new()) == NULL)
 664         goto err;
 665     if (!BN_MONT_CTX_set(mont, dsa->p, ctx))
 666         goto err;
 667     /* Work out g^q mod p */
 668     if (!BN_mod_exp_mont(tmp, dsa->g, dsa->q, dsa->p, ctx, mont))
 669         goto err;
 670     if (!BN_cmp(tmp, BN_value_one()))
 671         rv = 1;
 672     else
 673         rv = 0;
 674  err:
 675     BN_CTX_end(ctx);
 676     BN_MONT_CTX_free(mont);
 677     BN_CTX_free(ctx);
 678     return rv;
 679
 680 }