1EVP_KEYEXCH-DH(7ossl)               OpenSSL              EVP_KEYEXCH-DH(7ossl)
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NAME

6       EVP_KEYEXCH-DH - DH Key Exchange algorithm support
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DESCRIPTION

9       Key exchange support for the DH key type.
10
11   DH key exchange parameters
12       "pad" (OSSL_EXCHANGE_PARAM_PAD) <unsigned integer>
13           Sets the padding mode for the associated key exchange ctx.  Setting
14           a value of 1 will turn padding on.  Setting a value of 0 will turn
15           padding off.  If padding is off then the derived shared secret may
16           be smaller than the largest possible secret size.  If padding is on
17           then the derived shared secret will have its first bytes filled
18           with zeros where necessary to make the shared secret the same size
19           as the largest possible secret size.  The padding mode parameter is
20           ignored (and padding implicitly enabled) when the KDF type is set
21           to "X942KDF-ASN1" (OSSL_KDF_NAME_X942KDF_ASN1).
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23       "kdf-type" (OSSL_EXCHANGE_PARAM_KDF_TYPE) <UTF8 string>
24           See "Common Key Exchange parameters" in provider-keyexch(7).
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26       "kdf-digest" (OSSL_EXCHANGE_PARAM_KDF_DIGEST) <UTF8 string>
27           See "Common Key Exchange parameters" in provider-keyexch(7).
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29       "kdf-digest-props" (OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS) <UTF8 string>
30           See "Common Key Exchange parameters" in provider-keyexch(7).
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32       "kdf-outlen" (OSSL_EXCHANGE_PARAM_KDF_OUTLEN) <unsigned integer>
33           See "Common Key Exchange parameters" in provider-keyexch(7).
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35       "kdf-ukm" (OSSL_EXCHANGE_PARAM_KDF_UKM) <octet string>
36           See "Common Key Exchange parameters" in provider-keyexch(7).
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38       "cekalg" (OSSL_KDF_PARAM_CEK_ALG) <octet string ptr>
39           See "KDF Parameters" in provider-kdf(7).
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EXAMPLES

42       The examples assume a host and peer both generate keys using the same
43       named group (or domain parameters). See "Examples" in EVP_PKEY-DH(7).
44       Both the host and peer transfer their public key to each other.
45
46       To convert the peer's generated key pair to a public key in DER format
47       in order to transfer to the host:
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49           EVP_PKEY *peer_key; /* It is assumed this contains the peers generated key */
50           unsigned char *peer_pub_der = NULL;
51           int peer_pub_der_len;
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53           peer_pub_der_len = i2d_PUBKEY(peer_key, &peer_pub_der);
54           ...
55           OPENSSL_free(peer_pub_der);
56
57       To convert the received peer's public key from DER format on the host:
58
59           const unsigned char *pd = peer_pub_der;
60           EVP_PKEY *peer_pub_key = d2i_PUBKEY(NULL, &pd, peer_pub_der_len);
61           ...
62           EVP_PKEY_free(peer_pub_key);
63
64       To derive a shared secret on the host using the host's key and the
65       peer's public key:
66
67           /* It is assumed that the host_key and peer_pub_key are set up */
68           void derive_secret(EVP_KEY *host_key, EVP_PKEY *peer_pub_key)
69           {
70               unsigned int pad = 1;
71               OSSL_PARAM params[2];
72               unsigned char *secret = NULL;
73               size_t secret_len = 0;
74               EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);
75
76               EVP_PKEY_derive_init(dctx);
77
78               /* Optionally set the padding */
79               params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
80               params[1] = OSSL_PARAM_construct_end();
81               EVP_PKEY_CTX_set_params(dctx, params);
82
83               EVP_PKEY_derive_set_peer(dctx, peer_pub_key);
84
85               /* Get the size by passing NULL as the buffer */
86               EVP_PKEY_derive(dctx, NULL, &secret_len);
87               secret = OPENSSL_zalloc(secret_len);
88
89               EVP_PKEY_derive(dctx, secret, &secret_len);
90               ...
91               OPENSSL_clear_free(secret, secret_len);
92               EVP_PKEY_CTX_free(dctx);
93           }
94
95       Very similar code can be used by the peer to derive the same shared
96       secret using the host's public key and the peer's generated key pair.
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SEE ALSO

99       EVP_PKEY-DH(7), EVP_PKEY-FFC(7), EVP_PKEY(3), provider-keyexch(7),
100       provider-keymgmt(7), OSSL_PROVIDER-default(7), OSSL_PROVIDER-FIPS(7),
101

103       Copyright 2020-2022 The OpenSSL Project Authors. All Rights Reserved.
104
105       Licensed under the Apache License 2.0 (the "License").  You may not use
106       this file except in compliance with the License.  You can obtain a copy
107       in the file LICENSE in the source distribution or at
108       <https://www.openssl.org/source/license.html>.
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1123.1.1                             2023-08-31             EVP_KEYEXCH-DH(7ossl)
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