crypto/dh/dh_key.c

   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  * DH 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 "dh_local.h"
  19 #include "crypto/bn.h"
  20 #include "crypto/dh.h"
  21 #include "crypto/security_bits.h"
  22
  23 #ifdef FIPS_MODULE
  24 # define MIN_STRENGTH 112
  25 #else
  26 # define MIN_STRENGTH 80
  27 #endif
  28
  29 static int generate_key(DH *dh);
  30 static int dh_bn_mod_exp(const DH *dh, BIGNUM *r,
  31                          const BIGNUM *a, const BIGNUM *p,
  32                          const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx);
  33 static int dh_init(DH *dh);
  34 static int dh_finish(DH *dh);
  35
  36 /*
  37  * See SP800-56Ar3 Section 5.7.1.1
  38  * Finite Field Cryptography Diffie-Hellman (FFC DH) Primitive
  39  */
  40 int ossl_dh_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
  41 {
  42     BN_CTX *ctx = NULL;
  43     BN_MONT_CTX *mont = NULL;
  44     BIGNUM *z = NULL, *pminus1;
  45     int ret = -1;
  46
  47     if (BN_num_bits(dh->params.p) > OPENSSL_DH_MAX_MODULUS_BITS) {
  48         ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_LARGE);
  49         goto err;
  50     }
  51
  52     if (dh->params.q != NULL
  53         && BN_num_bits(dh->params.q) > OPENSSL_DH_MAX_MODULUS_BITS) {
  54         ERR_raise(ERR_LIB_DH, DH_R_Q_TOO_LARGE);
  55         goto err;
  56     }
  57
  58     if (BN_num_bits(dh->params.p) < DH_MIN_MODULUS_BITS) {
  59         ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_SMALL);
  60         return 0;
  61     }
  62
  63     ctx = BN_CTX_new_ex(dh->libctx);
  64     if (ctx == NULL)
  65         goto err;
  66     BN_CTX_start(ctx);
  67     pminus1 = BN_CTX_get(ctx);
  68     z = BN_CTX_get(ctx);
  69     if (z == NULL)
  70         goto err;
  71
  72     if (dh->priv_key == NULL) {
  73         ERR_raise(ERR_LIB_DH, DH_R_NO_PRIVATE_VALUE);
  74         goto err;
  75     }
  76
  77     if (dh->flags & DH_FLAG_CACHE_MONT_P) {
  78         mont = BN_MONT_CTX_set_locked(&dh->method_mont_p,
  79                                       dh->lock, dh->params.p, ctx);
  80         BN_set_flags(dh->priv_key, BN_FLG_CONSTTIME);
  81         if (!mont)
  82             goto err;
  83     }
  84
  85     /* (Step 1) Z = pub_key^priv_key mod p */
  86     if (!dh->meth->bn_mod_exp(dh, z, pub_key, dh->priv_key, dh->params.p, ctx,
  87                               mont)) {
  88         ERR_raise(ERR_LIB_DH, ERR_R_BN_LIB);
  89         goto err;
  90     }
  91
  92     /* (Step 2) Error if z <= 1 or z = p - 1 */
  93     if (BN_copy(pminus1, dh->params.p) == NULL
  94         || !BN_sub_word(pminus1, 1)
  95         || BN_cmp(z, BN_value_one()) <= 0
  96         || BN_cmp(z, pminus1) == 0) {
  97         ERR_raise(ERR_LIB_DH, DH_R_INVALID_SECRET);
  98         goto err;
  99     }
 100
 101     /* return the padded key, i.e. same number of bytes as the modulus */
 102     ret = BN_bn2binpad(z, key, BN_num_bytes(dh->params.p));
 103  err:
 104     BN_clear(z); /* (Step 2) destroy intermediate values */
 105     BN_CTX_end(ctx);
 106     BN_CTX_free(ctx);
 107     return ret;
 108 }
 109
 110 /*-
 111  * NB: This function is inherently not constant time due to the
 112  * RFC 5246 (8.1.2) padding style that strips leading zero bytes.
 113  */
 114 int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
 115 {
 116     int ret = 0, i;
 117     volatile size_t npad = 0, mask = 1;
 118
 119     /* compute the key; ret is constant unless compute_key is external */
 120 #ifdef FIPS_MODULE
 121     ret = ossl_dh_compute_key(key, pub_key, dh);
 122 #else
 123     ret = dh->meth->compute_key(key, pub_key, dh);
 124 #endif
 125     if (ret <= 0)
 126         return ret;
 127
 128     /* count leading zero bytes, yet still touch all bytes */
 129     for (i = 0; i < ret; i++) {
 130         mask &= !key[i];
 131         npad += mask;
 132     }
 133
 134     /* unpad key */
 135     ret -= npad;
 136     /* key-dependent memory access, potentially leaking npad / ret */
 137     memmove(key, key + npad, ret);
 138     /* key-dependent memory access, potentially leaking npad / ret */
 139     memset(key + ret, 0, npad);
 140
 141     return ret;
 142 }
 143
 144 int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh)
 145 {
 146     int rv, pad;
 147
 148     /* rv is constant unless compute_key is external */
 149 #ifdef FIPS_MODULE
 150     rv = ossl_dh_compute_key(key, pub_key, dh);
 151 #else
 152     rv = dh->meth->compute_key(key, pub_key, dh);
 153 #endif
 154     if (rv <= 0)
 155         return rv;
 156     pad = BN_num_bytes(dh->params.p) - rv;
 157     /* pad is constant (zero) unless compute_key is external */
 158     if (pad > 0) {
 159         memmove(key + pad, key, rv);
 160         memset(key, 0, pad);
 161     }
 162     return rv + pad;
 163 }
 164
 165 static DH_METHOD dh_ossl = {
 166     "OpenSSL DH Method",
 167     generate_key,
 168     ossl_dh_compute_key,
 169     dh_bn_mod_exp,
 170     dh_init,
 171     dh_finish,
 172     DH_FLAG_FIPS_METHOD,
 173     NULL,
 174     NULL
 175 };
 176
 177 static const DH_METHOD *default_DH_method = &dh_ossl;
 178
 179 const DH_METHOD *DH_OpenSSL(void)
 180 {
 181     return &dh_ossl;
 182 }
 183
 184 const DH_METHOD *DH_get_default_method(void)
 185 {
 186     return default_DH_method;
 187 }
 188
 189 static int dh_bn_mod_exp(const DH *dh, BIGNUM *r,
 190                          const BIGNUM *a, const BIGNUM *p,
 191                          const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx)
 192 {
 193 #ifdef S390X_MOD_EXP
 194     return s390x_mod_exp(r, a, p, m, ctx, m_ctx);
 195 #else
 196     return BN_mod_exp_mont(r, a, p, m, ctx, m_ctx);
 197 #endif
 198 }
 199
 200 static int dh_init(DH *dh)
 201 {
 202     dh->flags |= DH_FLAG_CACHE_MONT_P;
 203     dh->dirty_cnt++;
 204     return 1;
 205 }
 206
 207 static int dh_finish(DH *dh)
 208 {
 209     BN_MONT_CTX_free(dh->method_mont_p);
 210     return 1;
 211 }
 212
 213 #ifndef FIPS_MODULE
 214 void DH_set_default_method(const DH_METHOD *meth)
 215 {
 216     default_DH_method = meth;
 217 }
 218 #endif /* FIPS_MODULE */
 219
 220 int DH_generate_key(DH *dh)
 221 {
 222 #ifdef FIPS_MODULE
 223     return generate_key(dh);
 224 #else
 225     return dh->meth->generate_key(dh);
 226 #endif
 227 }
 228
 229 int ossl_dh_generate_public_key(BN_CTX *ctx, const DH *dh,
 230                                 const BIGNUM *priv_key, BIGNUM *pub_key)
 231 {
 232     int ret = 0;
 233     BIGNUM *prk = BN_new();
 234     BN_MONT_CTX *mont = NULL;
 235
 236     if (prk == NULL)
 237         return 0;
 238
 239     if (dh->flags & DH_FLAG_CACHE_MONT_P) {
 240         /*
 241          * We take the input DH as const, but we lie, because in some cases we
 242          * want to get a hold of its Montgomery context.
 243          *
 244          * We cast to remove the const qualifier in this case, it should be
 245          * fine...
 246          */
 247         BN_MONT_CTX **pmont = (BN_MONT_CTX **)&dh->method_mont_p;
 248
 249         mont = BN_MONT_CTX_set_locked(pmont, dh->lock, dh->params.p, ctx);
 250         if (mont == NULL)
 251             goto err;
 252     }
 253     BN_with_flags(prk, priv_key, BN_FLG_CONSTTIME);
 254
 255     /* pub_key = g^priv_key mod p */
 256     if (!dh->meth->bn_mod_exp(dh, pub_key, dh->params.g, prk, dh->params.p,
 257                               ctx, mont))
 258         goto err;
 259     ret = 1;
 260 err:
 261     BN_clear_free(prk);
 262     return ret;
 263 }
 264
 265 static int generate_key(DH *dh)
 266 {
 267     int ok = 0;
 268     int generate_new_key = 0;
 269 #ifndef FIPS_MODULE
 270     unsigned l;
 271 #endif
 272     BN_CTX *ctx = NULL;
 273     BIGNUM *pub_key = NULL, *priv_key = NULL;
 274
 275     if (BN_num_bits(dh->params.p) > OPENSSL_DH_MAX_MODULUS_BITS) {
 276         ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_LARGE);
 277         return 0;
 278     }
 279
 280     if (dh->params.q != NULL
 281         && BN_num_bits(dh->params.q) > OPENSSL_DH_MAX_MODULUS_BITS) {
 282         ERR_raise(ERR_LIB_DH, DH_R_Q_TOO_LARGE);
 283         return 0;
 284     }
 285
 286     if (BN_num_bits(dh->params.p) < DH_MIN_MODULUS_BITS) {
 287         ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_SMALL);
 288         return 0;
 289     }
 290
 291     ctx = BN_CTX_new_ex(dh->libctx);
 292     if (ctx == NULL)
 293         goto err;
 294
 295     if (dh->priv_key == NULL) {
 296         priv_key = BN_secure_new();
 297         if (priv_key == NULL)
 298             goto err;
 299         generate_new_key = 1;
 300     } else {
 301         priv_key = dh->priv_key;
 302     }
 303
 304     if (dh->pub_key == NULL) {
 305         pub_key = BN_new();
 306         if (pub_key == NULL)
 307             goto err;
 308     } else {
 309         pub_key = dh->pub_key;
 310     }
 311     if (generate_new_key) {
 312         /* Is it an approved safe prime ?*/
 313         if (DH_get_nid(dh) != NID_undef) {
 314             int max_strength =
 315                     ossl_ifc_ffc_compute_security_bits(BN_num_bits(dh->params.p));
 316
 317             if (dh->params.q == NULL
 318                 || dh->length > BN_num_bits(dh->params.q))
 319                 goto err;
 320             /* dh->length = maximum bit length of generated private key */
 321             if (!ossl_ffc_generate_private_key(ctx, &dh->params, dh->length,
 322                                                max_strength, priv_key))
 323                 goto err;
 324         } else {
 325 #ifdef FIPS_MODULE
 326             if (dh->params.q == NULL)
 327                 goto err;
 328 #else
 329             if (dh->params.q == NULL) {
 330                 /* secret exponent length, must satisfy 2^(l-1) <= p */
 331                 if (dh->length != 0
 332                     && dh->length >= BN_num_bits(dh->params.p))
 333                     goto err;
 334                 l = dh->length ? dh->length : BN_num_bits(dh->params.p) - 1;
 335                 if (!BN_priv_rand_ex(priv_key, l, BN_RAND_TOP_ONE,
 336                                      BN_RAND_BOTTOM_ANY, 0, ctx))
 337                     goto err;
 338                 /*
 339                  * We handle just one known case where g is a quadratic non-residue:
 340                  * for g = 2: p % 8 == 3
 341                  */
 342                 if (BN_is_word(dh->params.g, DH_GENERATOR_2)
 343                     && !BN_is_bit_set(dh->params.p, 2)) {
 344                     /* clear bit 0, since it won't be a secret anyway */
 345                     if (!BN_clear_bit(priv_key, 0))
 346                         goto err;
 347                 }
 348             } else
 349 #endif
 350             {
 351                 /* Do a partial check for invalid p, q, g */
 352                 if (!ossl_ffc_params_simple_validate(dh->libctx, &dh->params,
 353                                                      FFC_PARAM_TYPE_DH, NULL))
 354                     goto err;
 355                 /*
 356                  * For FFC FIPS 186-4 keygen
 357                  * security strength s = 112,
 358                  * Max Private key size N = len(q)
 359                  */
 360                 if (!ossl_ffc_generate_private_key(ctx, &dh->params,
 361                                                    BN_num_bits(dh->params.q),
 362                                                    MIN_STRENGTH,
 363                                                    priv_key))
 364                     goto err;
 365             }
 366         }
 367     }
 368
 369     if (!ossl_dh_generate_public_key(ctx, dh, priv_key, pub_key))
 370         goto err;
 371
 372     dh->pub_key = pub_key;
 373     dh->priv_key = priv_key;
 374     dh->dirty_cnt++;
 375     ok = 1;
 376  err:
 377     if (ok != 1)
 378         ERR_raise(ERR_LIB_DH, ERR_R_BN_LIB);
 379
 380     if (pub_key != dh->pub_key)
 381         BN_free(pub_key);
 382     if (priv_key != dh->priv_key)
 383         BN_free(priv_key);
 384     BN_CTX_free(ctx);
 385     return ok;
 386 }
 387
 388 int ossl_dh_buf2key(DH *dh, const unsigned char *buf, size_t len)
 389 {
 390     int err_reason = DH_R_BN_ERROR;
 391     BIGNUM *pubkey = NULL;
 392     const BIGNUM *p;
 393     int ret;
 394
 395     if ((pubkey = BN_bin2bn(buf, len, NULL)) == NULL)
 396         goto err;
 397     DH_get0_pqg(dh, &p, NULL, NULL);
 398     if (p == NULL || BN_num_bytes(p) == 0) {
 399         err_reason = DH_R_NO_PARAMETERS_SET;
 400         goto err;
 401     }
 402     /* Prevent small subgroup attacks per RFC 8446 Section 4.2.8.1 */
 403     if (!ossl_dh_check_pub_key_partial(dh, pubkey, &ret)) {
 404         err_reason = DH_R_INVALID_PUBKEY;
 405         goto err;
 406     }
 407     if (DH_set0_key(dh, pubkey, NULL) != 1)
 408         goto err;
 409     return 1;
 410 err:
 411     ERR_raise(ERR_LIB_DH, err_reason);
 412     BN_free(pubkey);
 413     return 0;
 414 }
 415
 416 size_t ossl_dh_key2buf(const DH *dh, unsigned char **pbuf_out, size_t size,
 417                        int alloc)
 418 {
 419     const BIGNUM *pubkey;
 420     unsigned char *pbuf = NULL;
 421     const BIGNUM *p;
 422     int p_size;
 423
 424     DH_get0_pqg(dh, &p, NULL, NULL);
 425     DH_get0_key(dh, &pubkey, NULL);
 426     if (p == NULL || pubkey == NULL
 427             || (p_size = BN_num_bytes(p)) == 0
 428             || BN_num_bytes(pubkey) == 0) {
 429         ERR_raise(ERR_LIB_DH, DH_R_INVALID_PUBKEY);
 430         return 0;
 431     }
 432     if (pbuf_out != NULL && (alloc || *pbuf_out != NULL)) {
 433         if (!alloc) {
 434             if (size >= (size_t)p_size)
 435                 pbuf = *pbuf_out;
 436             if (pbuf == NULL)
 437                 ERR_raise(ERR_LIB_DH, DH_R_INVALID_SIZE);
 438         } else {
 439             pbuf = OPENSSL_malloc(p_size);
 440         }
 441
 442         /* Errors raised above */
 443         if (pbuf == NULL)
 444             return 0;
 445         /*
 446          * As per Section 4.2.8.1 of RFC 8446 left pad public
 447          * key with zeros to the size of p
 448          */
 449         if (BN_bn2binpad(pubkey, pbuf, p_size) < 0) {
 450             if (alloc)
 451                 OPENSSL_free(pbuf);
 452             ERR_raise(ERR_LIB_DH, DH_R_BN_ERROR);
 453             return 0;
 454         }
 455         *pbuf_out = pbuf;
 456     }
 457     return p_size;
 458 }