1*b0d17251Schristos=pod 2*b0d17251Schristos 3*b0d17251Schristos=head1 NAME 4*b0d17251Schristos 5*b0d17251SchristosEVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support 6*b0d17251Schristos 7*b0d17251Schristos=head1 DESCRIPTION 8*b0d17251Schristos 9*b0d17251SchristosKey exchange support for the B<ECDH> key type. 10*b0d17251Schristos 11*b0d17251Schristos=head2 ECDH Key Exchange parameters 12*b0d17251Schristos 13*b0d17251Schristos=over 4 14*b0d17251Schristos 15*b0d17251Schristos=item "ecdh-cofactor-mode" (B<OSSL_EXCHANGE_PARAM_EC_ECDH_COFACTOR_MODE>) <integer> 16*b0d17251Schristos 17*b0d17251SchristosSets or gets the ECDH mode of operation for the associated key exchange ctx. 18*b0d17251Schristos 19*b0d17251SchristosIn the context of an Elliptic Curve Diffie-Hellman key exchange, this parameter 20*b0d17251Schristoscan be used to select between the plain Diffie-Hellman (DH) or Cofactor 21*b0d17251SchristosDiffie-Hellman (CDH) variants of the key exchange algorithm. 22*b0d17251Schristos 23*b0d17251SchristosWhen setting, the value should be 1, 0 or -1, respectively forcing cofactor mode 24*b0d17251Schristoson, off, or resetting it to the default for the private key associated with the 25*b0d17251Schristosgiven key exchange ctx. 26*b0d17251Schristos 27*b0d17251SchristosWhen getting, the value should be either 1 or 0, respectively signaling if the 28*b0d17251Schristoscofactor mode is on or off. 29*b0d17251Schristos 30*b0d17251SchristosSee also L<provider-keymgmt(7)> for the related 31*b0d17251SchristosB<OSSL_PKEY_PARAM_USE_COFACTOR_ECDH> parameter that can be set on a 32*b0d17251Schristosper-key basis. 33*b0d17251Schristos 34*b0d17251Schristos=item "kdf-type" (B<OSSL_EXCHANGE_PARAM_KDF_TYPE>) <UTF8 string> 35*b0d17251Schristos 36*b0d17251SchristosSee L<provider-keyexch(7)/Common Key Exchange parameters>. 37*b0d17251Schristos 38*b0d17251Schristos=item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string> 39*b0d17251Schristos 40*b0d17251SchristosSee L<provider-keyexch(7)/Common Key Exchange parameters>. 41*b0d17251Schristos 42*b0d17251Schristos=item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string> 43*b0d17251Schristos 44*b0d17251SchristosSee L<provider-keyexch(7)/Common Key Exchange parameters>. 45*b0d17251Schristos 46*b0d17251Schristos=item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer> 47*b0d17251Schristos 48*b0d17251SchristosSee L<provider-keyexch(7)/Common Key Exchange parameters>. 49*b0d17251Schristos 50*b0d17251Schristos=item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string> 51*b0d17251Schristos 52*b0d17251SchristosSee L<provider-keyexch(7)/Common Key Exchange parameters>. 53*b0d17251Schristos 54*b0d17251Schristos=back 55*b0d17251Schristos 56*b0d17251Schristos=head1 EXAMPLES 57*b0d17251Schristos 58*b0d17251SchristosKeys for the host and peer must be generated as shown in 59*b0d17251SchristosL<EVP_PKEY-EC(7)/Examples> using the same curve name. 60*b0d17251Schristos 61*b0d17251SchristosThe code to generate a shared secret for the normal case is identical to 62*b0d17251SchristosL<EVP_KEYEXCH-DH(7)/Examples>. 63*b0d17251Schristos 64*b0d17251SchristosTo derive a shared secret on the host using the host's key and the peer's public 65*b0d17251Schristoskey but also using X963KDF with a user key material: 66*b0d17251Schristos 67*b0d17251Schristos /* It is assumed that the host_key, peer_pub_key and ukm are set up */ 68*b0d17251Schristos void derive_secret(EVP_PKEY *host_key, EVP_PKEY *peer_key, 69*b0d17251Schristos unsigned char *ukm, size_t ukm_len) 70*b0d17251Schristos { 71*b0d17251Schristos unsigned char secret[64]; 72*b0d17251Schristos size_t out_len = sizeof(secret); 73*b0d17251Schristos size_t secret_len = out_len; 74*b0d17251Schristos unsigned int pad = 1; 75*b0d17251Schristos OSSL_PARAM params[6]; 76*b0d17251Schristos EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL); 77*b0d17251Schristos 78*b0d17251Schristos EVP_PKEY_derive_init(dctx); 79*b0d17251Schristos 80*b0d17251Schristos params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad); 81*b0d17251Schristos params[1] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_TYPE, 82*b0d17251Schristos "X963KDF", 0); 83*b0d17251Schristos params[2] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_DIGEST, 84*b0d17251Schristos "SHA1", 0); 85*b0d17251Schristos params[3] = OSSL_PARAM_construct_size_t(OSSL_EXCHANGE_PARAM_KDF_OUTLEN, 86*b0d17251Schristos &out_len); 87*b0d17251Schristos params[4] = OSSL_PARAM_construct_octet_string(OSSL_EXCHANGE_PARAM_KDF_UKM, 88*b0d17251Schristos ukm, ukm_len); 89*b0d17251Schristos params[5] = OSSL_PARAM_construct_end(); 90*b0d17251Schristos EVP_PKEY_CTX_set_params(dctx, params); 91*b0d17251Schristos 92*b0d17251Schristos EVP_PKEY_derive_set_peer(dctx, peer_pub_key); 93*b0d17251Schristos EVP_PKEY_derive(dctx, secret, &secret_len); 94*b0d17251Schristos ... 95*b0d17251Schristos OPENSSL_clear_free(secret, secret_len); 96*b0d17251Schristos EVP_PKEY_CTX_free(dctx); 97*b0d17251Schristos } 98*b0d17251Schristos 99*b0d17251Schristos=head1 SEE ALSO 100*b0d17251Schristos 101*b0d17251SchristosL<EVP_PKEY-EC(7)> 102*b0d17251SchristosL<EVP_PKEY(3)>, 103*b0d17251SchristosL<provider-keyexch(7)>, 104*b0d17251SchristosL<provider-keymgmt(7)>, 105*b0d17251SchristosL<OSSL_PROVIDER-default(7)>, 106*b0d17251SchristosL<OSSL_PROVIDER-FIPS(7)>, 107*b0d17251Schristos 108*b0d17251Schristos=head1 COPYRIGHT 109*b0d17251Schristos 110*b0d17251SchristosCopyright 2020-2022 The OpenSSL Project Authors. All Rights Reserved. 111*b0d17251Schristos 112*b0d17251SchristosLicensed under the Apache License 2.0 (the "License"). You may not use 113*b0d17251Schristosthis file except in compliance with the License. You can obtain a copy 114*b0d17251Schristosin the file LICENSE in the source distribution or at 115*b0d17251SchristosL<https://www.openssl.org/source/license.html>. 116*b0d17251Schristos 117*b0d17251Schristos=cut 118