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      1 =pod
      2 
      3 =head1 NAME
      4 
      5 EVP_KEYEXCH-ECDH - ECDH Key Exchange algorithm support
      6 
      7 =head1 DESCRIPTION
      8 
      9 Key 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 
     17 Sets or gets the ECDH mode of operation for the associated key exchange ctx.
     18 
     19 In the context of an Elliptic Curve Diffie-Hellman key exchange, this parameter
     20 can be used to select between the plain Diffie-Hellman (DH) or Cofactor
     21 Diffie-Hellman (CDH) variants of the key exchange algorithm.
     22 
     23 When setting, the value should be 1, 0 or -1, respectively forcing cofactor mode
     24 on, off, or resetting it to the default for the private key associated with the
     25 given key exchange ctx.
     26 
     27 When getting, the value should be either 1 or 0, respectively signaling if the
     28 cofactor mode is on or off.
     29 
     30 See also L<provider-keymgmt(7)> for the related
     31 B<OSSL_PKEY_PARAM_USE_COFACTOR_ECDH> parameter that can be set on a
     32 per-key basis.
     33 
     34 =item "kdf-type" (B<OSSL_EXCHANGE_PARAM_KDF_TYPE>) <UTF8 string>
     35 
     36 =item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string>
     37 
     38 =item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string>
     39 
     40 =item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer>
     41 
     42 =item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string>
     43 
     44 =back
     45 
     46 The OpenSSL FIPS provider also supports the following parameters:
     47 
     48 =over 4
     49 
     50 =item "fips-indicator" (B<OSSL_EXCHANGE_PARAM_FIPS_APPROVED_INDICATOR>) <integer>
     51 
     52 =item "key-check" (B<OSSL_EXCHANGE_PARAM_FIPS_KEY_CHECK>) <integer>
     53 
     54 =item "digest-check" (B<OSSL_EXCHANGE_PARAM_FIPS_DIGEST_CHECK>) <integer>
     55 
     56 See L<provider-keyexch(7)/Common Key Exchange parameters>.
     57 
     58 =item "ecdh-cofactor-check" (B<OSSL_EXCHANGE_PARAM_FIPS_ECDH_COFACTOR_CHECK>) <integer>
     59 
     60 If required this parameter should before OSSL_FUNC_keyexch_derive().
     61 The default value of 1 causes an error during the OSSL_FUNC_keyexch_derive if
     62 the EC curve has a cofactor that is not 1, and the cofactor is not used.
     63 Setting this to 0 will ignore the error and set the approved
     64 "fips-indicator" to 0.
     65 This option breaks FIPS compliance if it causes the approved "fips-indicator"
     66 to return 0.
     67 
     68 =back
     69 
     70 =head1 EXAMPLES
     71 
     72 Examples of key agreement can be found in demos/keyexch.
     73 
     74 Keys for the host and peer must be generated as shown in
     75 L<EVP_PKEY-EC(7)/Examples> using the same curve name.
     76 
     77 The code to generate a shared secret for the normal case is identical to
     78 L<EVP_KEYEXCH-DH(7)/Examples>.
     79 
     80 To derive a shared secret on the host using the host's key and the peer's public
     81 key but also using X963KDF with a user key material:
     82 
     83     /* It is assumed that the host_key, peer_pub_key and ukm are set up */
     84     void derive_secret(EVP_PKEY *host_key, EVP_PKEY *peer_key,
     85                        unsigned char *ukm, size_t ukm_len)
     86     {
     87         unsigned char secret[64];
     88         size_t out_len = sizeof(secret);
     89         size_t secret_len = out_len;
     90         unsigned int pad = 1;
     91         OSSL_PARAM params[6];
     92         EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);
     93 
     94         EVP_PKEY_derive_init(dctx);
     95 
     96         params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
     97         params[1] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_TYPE,
     98                                                      "X963KDF", 0);
     99         params[2] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_DIGEST,
    100                                                      "SHA1", 0);
    101         params[3] = OSSL_PARAM_construct_size_t(OSSL_EXCHANGE_PARAM_KDF_OUTLEN,
    102                                                 &out_len);
    103         params[4] = OSSL_PARAM_construct_octet_string(OSSL_EXCHANGE_PARAM_KDF_UKM,
    104                                                       ukm, ukm_len);
    105         params[5] = OSSL_PARAM_construct_end();
    106         EVP_PKEY_CTX_set_params(dctx, params);
    107 
    108         EVP_PKEY_derive_set_peer(dctx, peer_pub_key);
    109         EVP_PKEY_derive(dctx, secret, &secret_len);
    110         ...
    111         OPENSSL_clear_free(secret, secret_len);
    112         EVP_PKEY_CTX_free(dctx);
    113     }
    114 
    115 =head1 SEE ALSO
    116 
    117 L<EVP_PKEY-EC(7)>
    118 L<EVP_PKEY(3)>,
    119 L<provider-keyexch(7)>,
    120 L<provider-keymgmt(7)>,
    121 L<OSSL_PROVIDER-default(7)>,
    122 L<OSSL_PROVIDER-FIPS(7)>,
    123 
    124 =head1 COPYRIGHT
    125 
    126 Copyright 2020-2024 The OpenSSL Project Authors. All Rights Reserved.
    127 
    128 Licensed under the Apache License 2.0 (the "License").  You may not use
    129 this file except in compliance with the License.  You can obtain a copy
    130 in the file LICENSE in the source distribution or at
    131 L<https://www.openssl.org/source/license.html>.
    132 
    133 =cut
    134