1=pod 2 3=head1 NAME 4 5EVP_KEYEXCH-DH 6- DH Key Exchange algorithm support 7 8=head1 DESCRIPTION 9 10Key exchange support for the B<DH> key type. 11 12=head2 DH key exchange parameters 13 14=over 4 15 16=item "pad" (B<OSSL_EXCHANGE_PARAM_PAD>) <unsigned integer> 17 18Sets the padding mode for the associated key exchange ctx. 19Setting a value of 1 will turn padding on. 20Setting a value of 0 will turn padding off. 21If padding is off then the derived shared secret may be smaller than the 22largest possible secret size. 23If padding is on then the derived shared secret will have its first bytes 24filled with zeros where necessary to make the shared secret the same size as 25the largest possible secret size. 26The padding mode parameter is ignored (and padding implicitly enabled) when 27the KDF type is set to "X942KDF-ASN1" (B<OSSL_KDF_NAME_X942KDF_ASN1>). 28 29=item "kdf-type" (B<OSSL_EXCHANGE_PARAM_KDF_TYPE>) <UTF8 string> 30 31See L<provider-keyexch(7)/Common Key Exchange parameters>. 32 33=item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string> 34 35See L<provider-keyexch(7)/Common Key Exchange parameters>. 36 37=item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string> 38 39See L<provider-keyexch(7)/Common Key Exchange parameters>. 40 41=item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer> 42 43See L<provider-keyexch(7)/Common Key Exchange parameters>. 44 45=item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string> 46 47See L<provider-keyexch(7)/Common Key Exchange parameters>. 48 49=item "cekalg" (B<OSSL_KDF_PARAM_CEK_ALG>) <octet string ptr> 50 51See L<provider-kdf(7)/KDF Parameters>. 52 53=back 54 55=head1 EXAMPLES 56 57The examples assume a host and peer both generate keys using the same 58named group (or domain parameters). See L<EVP_PKEY-DH(7)/Examples>. 59Both the host and peer transfer their public key to each other. 60 61To convert the peer's generated key pair to a public key in DER format in order 62to transfer to the host: 63 64 EVP_PKEY *peer_key; /* It is assumed this contains the peers generated key */ 65 unsigned char *peer_pub_der = NULL; 66 int peer_pub_der_len; 67 68 peer_pub_der_len = i2d_PUBKEY(peer_key, &peer_pub_der); 69 ... 70 OPENSSL_free(peer_pub_der); 71 72To convert the received peer's public key from DER format on the host: 73 74 const unsigned char *pd = peer_pub_der; 75 EVP_PKEY *peer_pub_key = d2i_PUBKEY(NULL, &pd, peer_pub_der_len); 76 ... 77 EVP_PKEY_free(peer_pub_key); 78 79To derive a shared secret on the host using the host's key and the peer's public 80key: 81 82 /* It is assumed that the host_key and peer_pub_key are set up */ 83 void derive_secret(EVP_KEY *host_key, EVP_PKEY *peer_pub_key) 84 { 85 unsigned int pad = 1; 86 OSSL_PARAM params[2]; 87 unsigned char *secret = NULL; 88 size_t secret_len = 0; 89 EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL); 90 91 EVP_PKEY_derive_init(dctx); 92 93 /* Optionally set the padding */ 94 params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad); 95 params[1] = OSSL_PARAM_construct_end(); 96 EVP_PKEY_CTX_set_params(dctx, params); 97 98 EVP_PKEY_derive_set_peer(dctx, peer_pub_key); 99 100 /* Get the size by passing NULL as the buffer */ 101 EVP_PKEY_derive(dctx, NULL, &secret_len); 102 secret = OPENSSL_zalloc(secret_len); 103 104 EVP_PKEY_derive(dctx, secret, &secret_len); 105 ... 106 OPENSSL_clear_free(secret, secret_len); 107 EVP_PKEY_CTX_free(dctx); 108 } 109 110Very similar code can be used by the peer to derive the same shared secret 111using the host's public key and the peer's generated key pair. 112 113=head1 SEE ALSO 114 115L<EVP_PKEY-DH(7)>, 116L<EVP_PKEY-FFC(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 2020-2022 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