crypto/dsa/dsa_ossl.c

   1 /*
   2  * Copyright 1995-2018 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 #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 #ifndef FIPS_MODE
  48 void DSA_set_default_method(const DSA_METHOD *meth)
  49 {
  50     default_DSA_method = meth;
  51 }
  52 #endif /* FIPS_MODE */
  53
  54 const DSA_METHOD *DSA_get_default_method(void)
  55 {
  56     return default_DSA_method;
  57 }
  58
  59 const DSA_METHOD *DSA_OpenSSL(void)
  60 {
  61     return &openssl_dsa_meth;
  62 }
  63
  64 DSA_SIG *dsa_do_sign_int(OPENSSL_CTX *libctx, const unsigned char *dgst,
  65                          int dlen, DSA *dsa)
  66 {
  67     BIGNUM *kinv = NULL;
  68     BIGNUM *m, *blind, *blindm, *tmp;
  69     BN_CTX *ctx = NULL;
  70     int reason = ERR_R_BN_LIB;
  71     DSA_SIG *ret = NULL;
  72     int rv = 0;
  73
  74     if (dsa->params.p == NULL
  75         || dsa->params.q == NULL
  76         || dsa->params.g == NULL) {
  77         reason = DSA_R_MISSING_PARAMETERS;
  78         goto err;
  79     }
  80     if (dsa->priv_key == NULL) {
  81         reason = DSA_R_MISSING_PRIVATE_KEY;
  82         goto err;
  83     }
  84
  85     ret = DSA_SIG_new();
  86     if (ret == NULL)
  87         goto err;
  88     ret->r = BN_new();
  89     ret->s = BN_new();
  90     if (ret->r == NULL || ret->s == NULL)
  91         goto err;
  92
  93     ctx = BN_CTX_new_ex(libctx);
  94     if (ctx == NULL)
  95         goto err;
  96     m = BN_CTX_get(ctx);
  97     blind = BN_CTX_get(ctx);
  98     blindm = BN_CTX_get(ctx);
  99     tmp = BN_CTX_get(ctx);
 100     if (tmp == NULL)
 101         goto err;
 102
 103  redo:
 104     if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen))
 105         goto err;
 106
 107     if (dlen > BN_num_bytes(dsa->params.q))
 108         /*
 109          * if the digest length is greater than the size of q use the
 110          * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
 111          * 4.2
 112          */
 113         dlen = BN_num_bytes(dsa->params.q);
 114     if (BN_bin2bn(dgst, dlen, m) == NULL)
 115         goto err;
 116
 117     /*
 118      * The normal signature calculation is:
 119      *
 120      *   s := k^-1 * (m + r * priv_key) mod q
 121      *
 122      * We will blind this to protect against side channel attacks
 123      *
 124      *   s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q
 125      */
 126
 127     /* Generate a blinding value */
 128     do {
 129         if (!BN_priv_rand_ex(blind, BN_num_bits(dsa->params.q) - 1,
 130                              BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY, ctx))
 131             goto err;
 132     } while (BN_is_zero(blind));
 133     BN_set_flags(blind, BN_FLG_CONSTTIME);
 134     BN_set_flags(blindm, BN_FLG_CONSTTIME);
 135     BN_set_flags(tmp, BN_FLG_CONSTTIME);
 136
 137     /* tmp := blind * priv_key * r mod q */
 138     if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->params.q, ctx))
 139         goto err;
 140     if (!BN_mod_mul(tmp, tmp, ret->r, dsa->params.q, ctx))
 141         goto err;
 142
 143     /* blindm := blind * m mod q */
 144     if (!BN_mod_mul(blindm, blind, m, dsa->params.q, ctx))
 145         goto err;
 146
 147     /* s : = (blind * priv_key * r) + (blind * m) mod q */
 148     if (!BN_mod_add_quick(ret->s, tmp, blindm, dsa->params.q))
 149         goto err;
 150
 151     /* s := s * k^-1 mod q */
 152     if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->params.q, ctx))
 153         goto err;
 154
 155     /* s:= s * blind^-1 mod q */
 156     if (BN_mod_inverse(blind, blind, dsa->params.q, ctx) == NULL)
 157         goto err;
 158     if (!BN_mod_mul(ret->s, ret->s, blind, dsa->params.q, ctx))
 159         goto err;
 160
 161     /*
 162      * Redo if r or s is zero as required by FIPS 186-3: this is very
 163      * unlikely.
 164      */
 165     if (BN_is_zero(ret->r) || BN_is_zero(ret->s))
 166         goto redo;
 167
 168     rv = 1;
 169
 170  err:
 171     if (rv == 0) {
 172         DSAerr(0, reason);
 173         DSA_SIG_free(ret);
 174         ret = NULL;
 175     }
 176     BN_CTX_free(ctx);
 177     BN_clear_free(kinv);
 178     return ret;
 179 }
 180
 181 static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
 182 {
 183     return dsa_do_sign_int(NULL, dgst, dlen, dsa);
 184 }
 185
 186 static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in,
 187                                     BIGNUM **kinvp, BIGNUM **rp)
 188 {
 189     return dsa_sign_setup(dsa, ctx_in, kinvp, rp, NULL, 0);
 190 }
 191
 192 static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
 193                           BIGNUM **kinvp, BIGNUM **rp,
 194                           const unsigned char *dgst, int dlen)
 195 {
 196     BN_CTX *ctx = NULL;
 197     BIGNUM *k, *kinv = NULL, *r = *rp;
 198     BIGNUM *l;
 199     int ret = 0;
 200     int q_bits, q_words;
 201
 202     if (!dsa->params.p || !dsa->params.q || !dsa->params.g) {
 203         DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
 204         return 0;
 205     }
 206
 207     /* Reject obviously invalid parameters */
 208     if (BN_is_zero(dsa->params.p)
 209         || BN_is_zero(dsa->params.q)
 210         || BN_is_zero(dsa->params.g)) {
 211         DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_INVALID_PARAMETERS);
 212         return 0;
 213     }
 214     if (dsa->priv_key == NULL) {
 215         DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PRIVATE_KEY);
 216         return 0;
 217     }
 218
 219     k = BN_new();
 220     l = BN_new();
 221     if (k == NULL || l == NULL)
 222         goto err;
 223
 224     if (ctx_in == NULL) {
 225         /* if you don't pass in ctx_in you get a default libctx */
 226         if ((ctx = BN_CTX_new_ex(NULL)) == NULL)
 227             goto err;
 228     } else
 229         ctx = ctx_in;
 230
 231     /* Preallocate space */
 232     q_bits = BN_num_bits(dsa->params.q);
 233     q_words = bn_get_top(dsa->params.q);
 234     if (!bn_wexpand(k, q_words + 2)
 235         || !bn_wexpand(l, q_words + 2))
 236         goto err;
 237
 238     /* Get random k */
 239     do {
 240         if (dgst != NULL) {
 241             /*
 242              * We calculate k from SHA512(private_key + H(message) + random).
 243              * This protects the private key from a weak PRNG.
 244              */
 245             if (!BN_generate_dsa_nonce(k, dsa->params.q, dsa->priv_key, dgst,
 246                                        dlen, ctx))
 247                 goto err;
 248         } else if (!BN_priv_rand_range_ex(k, dsa->params.q, ctx))
 249             goto err;
 250     } while (BN_is_zero(k));
 251
 252     BN_set_flags(k, BN_FLG_CONSTTIME);
 253     BN_set_flags(l, BN_FLG_CONSTTIME);
 254
 255     if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
 256         if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
 257                                     dsa->lock, dsa->params.p, ctx))
 258             goto err;
 259     }
 260
 261     /* Compute r = (g^k mod p) mod q */
 262
 263     /*
 264      * We do not want timing information to leak the length of k, so we
 265      * compute G^k using an equivalent scalar of fixed bit-length.
 266      *
 267      * We unconditionally perform both of these additions to prevent a
 268      * small timing information leakage.  We then choose the sum that is
 269      * one bit longer than the modulus.
 270      *
 271      * There are some concerns about the efficacy of doing this.  More
 272      * specifically refer to the discussion starting with:
 273      *     https://github.com/openssl/openssl/pull/7486#discussion_r228323705
 274      * The fix is to rework BN so these gymnastics aren't required.
 275      */
 276     if (!BN_add(l, k, dsa->params.q)
 277         || !BN_add(k, l, dsa->params.q))
 278         goto err;
 279
 280     BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
 281
 282     if ((dsa)->meth->bn_mod_exp != NULL) {
 283             if (!dsa->meth->bn_mod_exp(dsa, r, dsa->params.g, k, dsa->params.p,
 284                                        ctx, dsa->method_mont_p))
 285                 goto err;
 286     } else {
 287             if (!BN_mod_exp_mont(r, dsa->params.g, k, dsa->params.p, ctx,
 288                                  dsa->method_mont_p))
 289                 goto err;
 290     }
 291
 292     if (!BN_mod(r, r, dsa->params.q, ctx))
 293         goto err;
 294
 295     /* Compute part of 's = inv(k) (m + xr) mod q' */
 296     if ((kinv = dsa_mod_inverse_fermat(k, dsa->params.q, ctx)) == NULL)
 297         goto err;
 298
 299     BN_clear_free(*kinvp);
 300     *kinvp = kinv;
 301     kinv = NULL;
 302     ret = 1;
 303  err:
 304     if (!ret)
 305         DSAerr(DSA_F_DSA_SIGN_SETUP, ERR_R_BN_LIB);
 306     if (ctx != ctx_in)
 307         BN_CTX_free(ctx);
 308     BN_clear_free(k);
 309     BN_clear_free(l);
 310     return ret;
 311 }
 312
 313 static int dsa_do_verify(const unsigned char *dgst, int dgst_len,
 314                          DSA_SIG *sig, DSA *dsa)
 315 {
 316     BN_CTX *ctx;
 317     BIGNUM *u1, *u2, *t1;
 318     BN_MONT_CTX *mont = NULL;
 319     const BIGNUM *r, *s;
 320     int ret = -1, i;
 321     if (dsa->params.p == NULL
 322         || dsa->params.q == NULL
 323         || dsa->params.g == NULL) {
 324         DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MISSING_PARAMETERS);
 325         return -1;
 326     }
 327
 328     i = BN_num_bits(dsa->params.q);
 329     /* fips 186-3 allows only different sizes for q */
 330     if (i != 160 && i != 224 && i != 256) {
 331         DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_BAD_Q_VALUE);
 332         return -1;
 333     }
 334
 335     if (BN_num_bits(dsa->params.p) > OPENSSL_DSA_MAX_MODULUS_BITS) {
 336         DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MODULUS_TOO_LARGE);
 337         return -1;
 338     }
 339     u1 = BN_new();
 340     u2 = BN_new();
 341     t1 = BN_new();
 342     ctx = BN_CTX_new_ex(NULL); /* verify does not need a libctx */
 343     if (u1 == NULL || u2 == NULL || t1 == NULL || ctx == NULL)
 344         goto err;
 345
 346     DSA_SIG_get0(sig, &r, &s);
 347
 348     if (BN_is_zero(r) || BN_is_negative(r) ||
 349         BN_ucmp(r, dsa->params.q) >= 0) {
 350         ret = 0;
 351         goto err;
 352     }
 353     if (BN_is_zero(s) || BN_is_negative(s) ||
 354         BN_ucmp(s, dsa->params.q) >= 0) {
 355         ret = 0;
 356         goto err;
 357     }
 358
 359     /*
 360      * Calculate W = inv(S) mod Q save W in u2
 361      */
 362     if ((BN_mod_inverse(u2, s, dsa->params.q, ctx)) == NULL)
 363         goto err;
 364
 365     /* save M in u1 */
 366     if (dgst_len > (i >> 3))
 367         /*
 368          * if the digest length is greater than the size of q use the
 369          * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,
 370          * 4.2
 371          */
 372         dgst_len = (i >> 3);
 373     if (BN_bin2bn(dgst, dgst_len, u1) == NULL)
 374         goto err;
 375
 376     /* u1 = M * w mod q */
 377     if (!BN_mod_mul(u1, u1, u2, dsa->params.q, ctx))
 378         goto err;
 379
 380     /* u2 = r * w mod q */
 381     if (!BN_mod_mul(u2, r, u2, dsa->params.q, ctx))
 382         goto err;
 383
 384     if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
 385         mont = BN_MONT_CTX_set_locked(&dsa->method_mont_p,
 386                                       dsa->lock, dsa->params.p, ctx);
 387         if (!mont)
 388             goto err;
 389     }
 390
 391     if (dsa->meth->dsa_mod_exp != NULL) {
 392         if (!dsa->meth->dsa_mod_exp(dsa, t1, dsa->params.g, u1, dsa->pub_key, u2,
 393                                     dsa->params.p, ctx, mont))
 394             goto err;
 395     } else {
 396         if (!BN_mod_exp2_mont(t1, dsa->params.g, u1, dsa->pub_key, u2,
 397                               dsa->params.p, ctx, mont))
 398             goto err;
 399     }
 400
 401     /* let u1 = u1 mod q */
 402     if (!BN_mod(u1, t1, dsa->params.q, ctx))
 403         goto err;
 404
 405     /*
 406      * V is now in u1.  If the signature is correct, it will be equal to R.
 407      */
 408     ret = (BN_ucmp(u1, r) == 0);
 409
 410  err:
 411     if (ret < 0)
 412         DSAerr(DSA_F_DSA_DO_VERIFY, ERR_R_BN_LIB);
 413     BN_CTX_free(ctx);
 414     BN_free(u1);
 415     BN_free(u2);
 416     BN_free(t1);
 417     return ret;
 418 }
 419
 420 static int dsa_init(DSA *dsa)
 421 {
 422     dsa->flags |= DSA_FLAG_CACHE_MONT_P;
 423     ffc_params_init(&dsa->params);
 424     return 1;
 425 }
 426
 427 static int dsa_finish(DSA *dsa)
 428 {
 429     BN_MONT_CTX_free(dsa->method_mont_p);
 430     return 1;
 431 }
 432
 433 /*
 434  * Compute the inverse of k modulo q.
 435  * Since q is prime, Fermat's Little Theorem applies, which reduces this to
 436  * mod-exp operation.  Both the exponent and modulus are public information
 437  * so a mod-exp that doesn't leak the base is sufficient.  A newly allocated
 438  * BIGNUM is returned which the caller must free.
 439  */
 440 static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
 441                                       BN_CTX *ctx)
 442 {
 443     BIGNUM *res = NULL;
 444     BIGNUM *r, *e;
 445
 446     if ((r = BN_new()) == NULL)
 447         return NULL;
 448
 449     BN_CTX_start(ctx);
 450     if ((e = BN_CTX_get(ctx)) != NULL
 451             && BN_set_word(r, 2)
 452             && BN_sub(e, q, r)
 453             && BN_mod_exp_mont(r, k, e, q, ctx, NULL))
 454         res = r;
 455     else
 456         BN_free(r);
 457     BN_CTX_end(ctx);
 458     return res;
 459 }