1=pod
2
3=head1 NAME
4
5EVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support
6
7=head1 DESCRIPTION
8
9Key exchange support for the B<ECDH> key type.
10
11=head2 ECDH Key Exchange parameters
12
13=over 4
14
15=item "ecdh-cofactor-mode" (B<OSSL_EXCHANGE_PARAM_EC_ECDH_COFACTOR_MODE>) <integer>
16
17Sets or gets the ECDH mode of operation for the associated key exchange ctx.
18
19In the context of an Elliptic Curve Diffie-Hellman key exchange, this parameter
20can be used to select between the plain Diffie-Hellman (DH) or Cofactor
21Diffie-Hellman (CDH) variants of the key exchange algorithm.
22
23When setting, the value should be 1, 0 or -1, respectively forcing cofactor mode
24on, off, or resetting it to the default for the private key associated with the
25given key exchange ctx.
26
27When getting, the value should be either 1 or 0, respectively signaling if the
28cofactor mode is on or off.
29
30See also L<provider-keymgmt(7)> for the related
31B<OSSL_PKEY_PARAM_USE_COFACTOR_ECDH> parameter that can be set on a
32per-key basis.
33
34=item "kdf-type" (B<OSSL_EXCHANGE_PARAM_KDF_TYPE>) <UTF8 string>
35
36Sets or gets the Key Derivation Function type to apply within the associated key
37exchange ctx.
38
39=item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string>
40
41Sets or gets the Digest algorithm to be used as part of the Key Derivation Function
42associated with the given key exchange ctx.
43
44=item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string>
45
46Sets properties to be used upon look up of the implementation for the selected
47Digest algorithm for the Key Derivation Function associated with the given key
48exchange ctx.
49
50=item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer>
51
52Sets or gets the desired size for the output of the chosen Key Derivation Function
53associated with the given key exchange ctx.
54The length of the "kdf-outlen" parameter should not exceed that of a B<size_t>.
55
56=item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string>
57
58Sets the User Key Material to be used as part of the selected Key Derivation
59Function associated with the given key exchange ctx.
60
61=item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string ptr>
62
63Gets a pointer to the User Key Material to be used as part of the selected
64Key Derivation Function associated with the given key exchange ctx. Providers
65usually do not need to support this gettable parameter as its sole purpose
66is to support functionality of the deprecated EVP_PKEY_CTX_get0_ecdh_kdf_ukm()
67function.
68
69=back
70
71=head1 EXAMPLES
72
73Keys for the host and peer must be generated as shown in
74L<EVP_PKEY-EC(7)/Examples> using the same curve name.
75
76The code to generate a shared secret for the normal case is identical to
77L<EVP_KEYEXCH-DH(7)/Examples>.
78
79To derive a shared secret on the host using the host's key and the peer's public
80key but also using X963KDF with a user key material:
81
82    /* It is assumed that the host_key, peer_pub_key and ukm are set up */
83    void derive_secret(EVP_PKEY *host_key, EVP_PKEY *peer_key,
84                       unsigned char *ukm, size_t ukm_len)
85    {
86        unsigned char secret[64];
87        size_t out_len = sizeof(secret);
88        size_t secret_len = out_len;
89        unsigned int pad = 1;
90        OSSL_PARAM params[6];
91        EVP_PKET_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);
92
93        EVP_PKEY_derive_init(dctx);
94
95        params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
96        params[1] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_TYPE,
97                                                     "X963KDF", 0);
98        params[2] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_DIGEST,
99                                                     "SHA1", 0);
100        params[3] = OSSL_PARAM_construct_size_t(OSSL_EXCHANGE_PARAM_KDF_OUTLEN,
101                                                &out_len);
102        params[4] = OSSL_PARAM_construct_octet_string(OSSL_EXCHANGE_PARAM_KDF_UKM,
103                                                      ukm, ukm_len);
104        params[5] = OSSL_PARAM_construct_end();
105        EVP_PKEY_CTX_set_params(dctx, params);
106
107        EVP_PKEY_derive_set_peer(dctx, peer_pub_key);
108        EVP_PKEY_derive(dctx, secret, &secret_len);
109        ...
110        OPENSSL_clear_free(secret, secret_len);
111        EVP_PKEY_CTX_free(dctx);
112    }
113
114=head1 SEE ALSO
115
116L<EVP_PKEY-EC(7)>
117L<EVP_PKEY(3)>,
118L<provider-keyexch(7)>,
119L<provider-keymgmt(7)>,
120L<OSSL_PROVIDER-default(7)>,
121L<OSSL_PROVIDER-FIPS(7)>,
122
123=head1 COPYRIGHT
124
125Copyright 2020-2021 The OpenSSL Project Authors. All Rights Reserved.
126
127Licensed under the Apache License 2.0 (the "License").  You may not use
128this file except in compliance with the License.  You can obtain a copy
129in the file LICENSE in the source distribution or at
130L<https://www.openssl.org/source/license.html>.
131
132=cut
133