xref: /openssl/crypto/ec/ecdh_ossl.c (revision e077455e)
1 /*
2  * Copyright 2002-2021 The OpenSSL Project Authors. All Rights Reserved.
3  * Copyright (c) 2002, Oracle and/or its affiliates. All rights reserved
4  *
5  * Licensed under the Apache License 2.0 (the "License").  You may not use
6  * this file except in compliance with the License.  You can obtain a copy
7  * in the file LICENSE in the source distribution or at
8  * https://www.openssl.org/source/license.html
9  */
10 
11 /*
12  * ECDH low level APIs are deprecated for public use, but still ok for
13  * internal use.
14  */
15 #include "internal/deprecated.h"
16 
17 #include <string.h>
18 #include <limits.h>
19 
20 #include "internal/cryptlib.h"
21 
22 #include <openssl/err.h>
23 #include <openssl/bn.h>
24 #include <openssl/objects.h>
25 #include <openssl/ec.h>
26 #include "ec_local.h"
27 
ossl_ecdh_compute_key(unsigned char ** psec,size_t * pseclen,const EC_POINT * pub_key,const EC_KEY * ecdh)28 int ossl_ecdh_compute_key(unsigned char **psec, size_t *pseclen,
29                           const EC_POINT *pub_key, const EC_KEY *ecdh)
30 {
31     if (ecdh->group->meth->ecdh_compute_key == NULL) {
32         ERR_raise(ERR_LIB_EC, EC_R_CURVE_DOES_NOT_SUPPORT_ECDH);
33         return 0;
34     }
35 
36     return ecdh->group->meth->ecdh_compute_key(psec, pseclen, pub_key, ecdh);
37 }
38 
39 /*-
40  * This implementation is based on the following primitives in the
41  * IEEE 1363 standard:
42  *  - ECKAS-DH1
43  *  - ECSVDP-DH
44  *
45  * It also conforms to SP800-56A r3
46  * See Section 5.7.1.2 "Elliptic Curve Cryptography Cofactor Diffie-Hellman
47  * (ECC CDH) Primitive:". The steps listed below refer to SP800-56A.
48  */
ossl_ecdh_simple_compute_key(unsigned char ** pout,size_t * poutlen,const EC_POINT * pub_key,const EC_KEY * ecdh)49 int ossl_ecdh_simple_compute_key(unsigned char **pout, size_t *poutlen,
50                                  const EC_POINT *pub_key, const EC_KEY *ecdh)
51 {
52     BN_CTX *ctx;
53     EC_POINT *tmp = NULL;
54     BIGNUM *x = NULL;
55     const BIGNUM *priv_key;
56     const EC_GROUP *group;
57     int ret = 0;
58     size_t buflen, len;
59     unsigned char *buf = NULL;
60 
61     if ((ctx = BN_CTX_new_ex(ecdh->libctx)) == NULL)
62         goto err;
63     BN_CTX_start(ctx);
64     x = BN_CTX_get(ctx);
65     if (x == NULL) {
66         ERR_raise(ERR_LIB_EC, ERR_R_BN_LIB);
67         goto err;
68     }
69 
70     priv_key = EC_KEY_get0_private_key(ecdh);
71     if (priv_key == NULL) {
72         ERR_raise(ERR_LIB_EC, EC_R_MISSING_PRIVATE_KEY);
73         goto err;
74     }
75 
76     group = EC_KEY_get0_group(ecdh);
77 
78     /*
79      * Step(1) - Compute the point tmp = cofactor * owners_private_key
80      *                                   * peer_public_key.
81      */
82     if (EC_KEY_get_flags(ecdh) & EC_FLAG_COFACTOR_ECDH) {
83         if (!EC_GROUP_get_cofactor(group, x, NULL)) {
84             ERR_raise(ERR_LIB_EC, ERR_R_EC_LIB);
85             goto err;
86         }
87         if (!BN_mul(x, x, priv_key, ctx)) {
88             ERR_raise(ERR_LIB_EC, ERR_R_BN_LIB);
89             goto err;
90         }
91         priv_key = x;
92     }
93 
94     if ((tmp = EC_POINT_new(group)) == NULL) {
95         ERR_raise(ERR_LIB_EC, ERR_R_EC_LIB);
96         goto err;
97     }
98 
99     if (!EC_POINT_mul(group, tmp, NULL, pub_key, priv_key, ctx)) {
100         ERR_raise(ERR_LIB_EC, EC_R_POINT_ARITHMETIC_FAILURE);
101         goto err;
102     }
103 
104     /*
105      * Step(2) : If point tmp is at infinity then clear intermediate values and
106      * exit. Note: getting affine coordinates returns 0 if point is at infinity.
107      * Step(3a) : Get x-coordinate of point x = tmp.x
108      */
109     if (!EC_POINT_get_affine_coordinates(group, tmp, x, NULL, ctx)) {
110         ERR_raise(ERR_LIB_EC, EC_R_POINT_ARITHMETIC_FAILURE);
111         goto err;
112     }
113 
114     /*
115      * Step(3b) : convert x to a byte string, using the field-element-to-byte
116      * string conversion routine defined in Appendix C.2
117      */
118     buflen = (EC_GROUP_get_degree(group) + 7) / 8;
119     len = BN_num_bytes(x);
120     if (len > buflen) {
121         ERR_raise(ERR_LIB_EC, ERR_R_INTERNAL_ERROR);
122         goto err;
123     }
124     if ((buf = OPENSSL_malloc(buflen)) == NULL)
125         goto err;
126 
127     memset(buf, 0, buflen - len);
128     if (len != (size_t)BN_bn2bin(x, buf + buflen - len)) {
129         ERR_raise(ERR_LIB_EC, ERR_R_BN_LIB);
130         goto err;
131     }
132 
133     *pout = buf;
134     *poutlen = buflen;
135     buf = NULL;
136 
137     ret = 1;
138 
139  err:
140     /* Step(4) : Destroy all intermediate calculations */
141     BN_clear(x);
142     EC_POINT_clear_free(tmp);
143     BN_CTX_end(ctx);
144     BN_CTX_free(ctx);
145     OPENSSL_free(buf);
146     return ret;
147 }
148