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
29static int generate_key(DH *dh);
30static 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);
33static int dh_init(DH *dh);
34static 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 */
40int 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 */
114int 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
144int 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
165static 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
177static const DH_METHOD *default_DH_method = &dh_ossl;
178
179const DH_METHOD *DH_OpenSSL(void)
180{
181    return &dh_ossl;
182}
183
184const DH_METHOD *DH_get_default_method(void)
185{
186    return default_DH_method;
187}
188
189static 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    return BN_mod_exp_mont(r, a, p, m, ctx, m_ctx);
194}
195
196static int dh_init(DH *dh)
197{
198    dh->flags |= DH_FLAG_CACHE_MONT_P;
199    dh->dirty_cnt++;
200    return 1;
201}
202
203static int dh_finish(DH *dh)
204{
205    BN_MONT_CTX_free(dh->method_mont_p);
206    return 1;
207}
208
209#ifndef FIPS_MODULE
210void DH_set_default_method(const DH_METHOD *meth)
211{
212    default_DH_method = meth;
213}
214#endif /* FIPS_MODULE */
215
216int DH_generate_key(DH *dh)
217{
218#ifdef FIPS_MODULE
219    return generate_key(dh);
220#else
221    return dh->meth->generate_key(dh);
222#endif
223}
224
225int ossl_dh_generate_public_key(BN_CTX *ctx, const DH *dh,
226                                const BIGNUM *priv_key, BIGNUM *pub_key)
227{
228    int ret = 0;
229    BIGNUM *prk = BN_new();
230    BN_MONT_CTX *mont = NULL;
231
232    if (prk == NULL)
233        return 0;
234
235    if (dh->flags & DH_FLAG_CACHE_MONT_P) {
236        /*
237         * We take the input DH as const, but we lie, because in some cases we
238         * want to get a hold of its Montgomery context.
239         *
240         * We cast to remove the const qualifier in this case, it should be
241         * fine...
242         */
243        BN_MONT_CTX **pmont = (BN_MONT_CTX **)&dh->method_mont_p;
244
245        mont = BN_MONT_CTX_set_locked(pmont, dh->lock, dh->params.p, ctx);
246        if (mont == NULL)
247            goto err;
248    }
249    BN_with_flags(prk, priv_key, BN_FLG_CONSTTIME);
250
251    /* pub_key = g^priv_key mod p */
252    if (!dh->meth->bn_mod_exp(dh, pub_key, dh->params.g, prk, dh->params.p,
253                              ctx, mont))
254        goto err;
255    ret = 1;
256err:
257    BN_clear_free(prk);
258    return ret;
259}
260
261static int generate_key(DH *dh)
262{
263    int ok = 0;
264    int generate_new_key = 0;
265#ifndef FIPS_MODULE
266    unsigned l;
267#endif
268    BN_CTX *ctx = NULL;
269    BIGNUM *pub_key = NULL, *priv_key = NULL;
270
271    if (BN_num_bits(dh->params.p) > OPENSSL_DH_MAX_MODULUS_BITS) {
272        ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_LARGE);
273        return 0;
274    }
275
276    if (dh->params.q != NULL
277        && BN_num_bits(dh->params.q) > OPENSSL_DH_MAX_MODULUS_BITS) {
278        ERR_raise(ERR_LIB_DH, DH_R_Q_TOO_LARGE);
279        return 0;
280    }
281
282    if (BN_num_bits(dh->params.p) < DH_MIN_MODULUS_BITS) {
283        ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_SMALL);
284        return 0;
285    }
286
287    ctx = BN_CTX_new_ex(dh->libctx);
288    if (ctx == NULL)
289        goto err;
290
291    if (dh->priv_key == NULL) {
292        priv_key = BN_secure_new();
293        if (priv_key == NULL)
294            goto err;
295        generate_new_key = 1;
296    } else {
297        priv_key = dh->priv_key;
298    }
299
300    if (dh->pub_key == NULL) {
301        pub_key = BN_new();
302        if (pub_key == NULL)
303            goto err;
304    } else {
305        pub_key = dh->pub_key;
306    }
307    if (generate_new_key) {
308        /* Is it an approved safe prime ?*/
309        if (DH_get_nid(dh) != NID_undef) {
310            int max_strength =
311                    ossl_ifc_ffc_compute_security_bits(BN_num_bits(dh->params.p));
312
313            if (dh->params.q == NULL
314                || dh->length > BN_num_bits(dh->params.q))
315                goto err;
316            /* dh->length = maximum bit length of generated private key */
317            if (!ossl_ffc_generate_private_key(ctx, &dh->params, dh->length,
318                                               max_strength, priv_key))
319                goto err;
320        } else {
321#ifdef FIPS_MODULE
322            if (dh->params.q == NULL)
323                goto err;
324#else
325            if (dh->params.q == NULL) {
326                /* secret exponent length, must satisfy 2^(l-1) <= p */
327                if (dh->length != 0
328                    && dh->length >= BN_num_bits(dh->params.p))
329                    goto err;
330                l = dh->length ? dh->length : BN_num_bits(dh->params.p) - 1;
331                if (!BN_priv_rand_ex(priv_key, l, BN_RAND_TOP_ONE,
332                                     BN_RAND_BOTTOM_ANY, 0, ctx))
333                    goto err;
334                /*
335                 * We handle just one known case where g is a quadratic non-residue:
336                 * for g = 2: p % 8 == 3
337                 */
338                if (BN_is_word(dh->params.g, DH_GENERATOR_2)
339                    && !BN_is_bit_set(dh->params.p, 2)) {
340                    /* clear bit 0, since it won't be a secret anyway */
341                    if (!BN_clear_bit(priv_key, 0))
342                        goto err;
343                }
344            } else
345#endif
346            {
347                /* Do a partial check for invalid p, q, g */
348                if (!ossl_ffc_params_simple_validate(dh->libctx, &dh->params,
349                                                     FFC_PARAM_TYPE_DH, NULL))
350                    goto err;
351                /*
352                 * For FFC FIPS 186-4 keygen
353                 * security strength s = 112,
354                 * Max Private key size N = len(q)
355                 */
356                if (!ossl_ffc_generate_private_key(ctx, &dh->params,
357                                                   BN_num_bits(dh->params.q),
358                                                   MIN_STRENGTH,
359                                                   priv_key))
360                    goto err;
361            }
362        }
363    }
364
365    if (!ossl_dh_generate_public_key(ctx, dh, priv_key, pub_key))
366        goto err;
367
368    dh->pub_key = pub_key;
369    dh->priv_key = priv_key;
370    dh->dirty_cnt++;
371    ok = 1;
372 err:
373    if (ok != 1)
374        ERR_raise(ERR_LIB_DH, ERR_R_BN_LIB);
375
376    if (pub_key != dh->pub_key)
377        BN_free(pub_key);
378    if (priv_key != dh->priv_key)
379        BN_free(priv_key);
380    BN_CTX_free(ctx);
381    return ok;
382}
383
384int ossl_dh_buf2key(DH *dh, const unsigned char *buf, size_t len)
385{
386    int err_reason = DH_R_BN_ERROR;
387    BIGNUM *pubkey = NULL;
388    const BIGNUM *p;
389    int ret;
390
391    if ((pubkey = BN_bin2bn(buf, len, NULL)) == NULL)
392        goto err;
393    DH_get0_pqg(dh, &p, NULL, NULL);
394    if (p == NULL || BN_num_bytes(p) == 0) {
395        err_reason = DH_R_NO_PARAMETERS_SET;
396        goto err;
397    }
398    /* Prevent small subgroup attacks per RFC 8446 Section 4.2.8.1 */
399    if (!ossl_dh_check_pub_key_partial(dh, pubkey, &ret)) {
400        err_reason = DH_R_INVALID_PUBKEY;
401        goto err;
402    }
403    if (DH_set0_key(dh, pubkey, NULL) != 1)
404        goto err;
405    return 1;
406err:
407    ERR_raise(ERR_LIB_DH, err_reason);
408    BN_free(pubkey);
409    return 0;
410}
411
412size_t ossl_dh_key2buf(const DH *dh, unsigned char **pbuf_out, size_t size,
413                       int alloc)
414{
415    const BIGNUM *pubkey;
416    unsigned char *pbuf = NULL;
417    const BIGNUM *p;
418    int p_size;
419
420    DH_get0_pqg(dh, &p, NULL, NULL);
421    DH_get0_key(dh, &pubkey, NULL);
422    if (p == NULL || pubkey == NULL
423            || (p_size = BN_num_bytes(p)) == 0
424            || BN_num_bytes(pubkey) == 0) {
425        ERR_raise(ERR_LIB_DH, DH_R_INVALID_PUBKEY);
426        return 0;
427    }
428    if (pbuf_out != NULL && (alloc || *pbuf_out != NULL)) {
429        if (!alloc) {
430            if (size >= (size_t)p_size)
431                pbuf = *pbuf_out;
432        } else {
433            pbuf = OPENSSL_malloc(p_size);
434        }
435
436        if (pbuf == NULL) {
437            ERR_raise(ERR_LIB_DH, ERR_R_MALLOC_FAILURE);
438            return 0;
439        }
440        /*
441         * As per Section 4.2.8.1 of RFC 8446 left pad public
442         * key with zeros to the size of p
443         */
444        if (BN_bn2binpad(pubkey, pbuf, p_size) < 0) {
445            if (alloc)
446                OPENSSL_free(pbuf);
447            ERR_raise(ERR_LIB_DH, DH_R_BN_ERROR);
448            return 0;
449        }
450        *pbuf_out = pbuf;
451    }
452    return p_size;
453}
454