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