doc/man7/EVP_KEYEXCH-ECDH.pod

   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 Sets or gets the Key Derivation Function type to apply within the associated key
  37 exchange ctx.
  38
  39 =item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string>
  40
  41 Sets or gets the Digest algorithm to be used as part of the Key Derivation Function
  42 associated with the given key exchange ctx.
  43
  44 =item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string>
  45
  46 Sets properties to be used upon look up of the implementation for the selected
  47 Digest algorithm for the Key Derivation Function associated with the given key
  48 exchange ctx.
  49
  50 =item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer>
  51
  52 Sets or gets the desired size for the output of the chosen Key Derivation Function
  53 associated with the given key exchange ctx.
  54 The 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
  58 Sets the User Key Material to be used as part of the selected Key Derivation
  59 Function associated with the given key exchange ctx.
  60
  61 =item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string ptr>
  62
  63 Gets a pointer to the User Key Material to be used as part of the selected
  64 Key Derivation Function associated with the given key exchange ctx. Providers
  65 usually do not need to support this gettable parameter as its sole purpose
  66 is to support functionality of the deprecated EVP_PKEY_CTX_get0_ecdh_kdf_ukm()
  67 function.
  68
  69 =back
  70
  71 =head1 EXAMPLES
  72
  73 Keys for the host and peer must be generated as shown in
  74 L<EVP_PKEY-EC(7)/Examples> using the same curve name.
  75
  76 The code to generate a shared secret for the normal case is identical to
  77 L<EVP_KEYEXCH-DH(7)/Examples>.
  78
  79 To derive a shared secret on the host using the host's key and the peer's public
  80 key 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
 116 L<EVP_PKEY-EC(7)>
 117 L<EVP_PKEY(3)>,
 118 L<provider-keyexch(7)>,
 119 L<provider-keymgmt(7)>,
 120 L<OSSL_PROVIDER-default(7)>,
 121 L<OSSL_PROVIDER-FIPS(7)>,
 122
 123 =head1 COPYRIGHT
 124
 125 Copyright 2020-2021 The OpenSSL Project Authors. All Rights Reserved.
 126
 127 Licensed under the Apache License 2.0 (the "License").  You may not use
 128 this file except in compliance with the License.  You can obtain a copy
 129 in the file LICENSE in the source distribution or at
 130 L<https://www.openssl.org/source/license.html>.
 131
 132 =cut