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 See L<provider-keyexch(7)/Common Key Exchange parameters>.
  37
  38 =item "kdf-digest" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST>) <UTF8 string>
  39
  40 See L<provider-keyexch(7)/Common Key Exchange parameters>.
  41
  42 =item "kdf-digest-props" (B<OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS>) <UTF8 string>
  43
  44 See L<provider-keyexch(7)/Common Key Exchange parameters>.
  45
  46 =item "kdf-outlen" (B<OSSL_EXCHANGE_PARAM_KDF_OUTLEN>) <unsigned integer>
  47
  48 See L<provider-keyexch(7)/Common Key Exchange parameters>.
  49
  50 =item "kdf-ukm" (B<OSSL_EXCHANGE_PARAM_KDF_UKM>) <octet string>
  51
  52 See L<provider-keyexch(7)/Common Key Exchange parameters>.
  53
  54 =back
  55
  56 =head1 EXAMPLES
  57
  58 Keys for the host and peer must be generated as shown in
  59 L<EVP_PKEY-EC(7)/Examples> using the same curve name.
  60
  61 The code to generate a shared secret for the normal case is identical to
  62 L<EVP_KEYEXCH-DH(7)/Examples>.
  63
  64 To derive a shared secret on the host using the host's key and the peer's public
  65 key but also using X963KDF with a user key material:
  66
  67     /* It is assumed that the host_key, peer_pub_key and ukm are set up */
  68     void derive_secret(EVP_PKEY *host_key, EVP_PKEY *peer_key,
  69                        unsigned char *ukm, size_t ukm_len)
  70     {
  71         unsigned char secret[64];
  72         size_t out_len = sizeof(secret);
  73         size_t secret_len = out_len;
  74         unsigned int pad = 1;
  75         OSSL_PARAM params[6];
  76         EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);
  77
  78         EVP_PKEY_derive_init(dctx);
  79
  80         params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
  81         params[1] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_TYPE,
  82                                                      "X963KDF", 0);
  83         params[2] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_DIGEST,
  84                                                      "SHA1", 0);
  85         params[3] = OSSL_PARAM_construct_size_t(OSSL_EXCHANGE_PARAM_KDF_OUTLEN,
  86                                                 &out_len);
  87         params[4] = OSSL_PARAM_construct_octet_string(OSSL_EXCHANGE_PARAM_KDF_UKM,
  88                                                       ukm, ukm_len);
  89         params[5] = OSSL_PARAM_construct_end();
  90         EVP_PKEY_CTX_set_params(dctx, params);
  91
  92         EVP_PKEY_derive_set_peer(dctx, peer_pub_key);
  93         EVP_PKEY_derive(dctx, secret, &secret_len);
  94         ...
  95         OPENSSL_clear_free(secret, secret_len);
  96         EVP_PKEY_CTX_free(dctx);
  97     }
  98
  99 =head1 SEE ALSO
 100
 101 L<EVP_PKEY-EC(7)>
 102 L<EVP_PKEY(3)>,
 103 L<provider-keyexch(7)>,
 104 L<provider-keymgmt(7)>,
 105 L<OSSL_PROVIDER-default(7)>,
 106 L<OSSL_PROVIDER-FIPS(7)>,
 107
 108 =head1 COPYRIGHT
 109
 110 Copyright 2020-2022 The OpenSSL Project Authors. All Rights Reserved.
 111
 112 Licensed under the Apache License 2.0 (the "License").  You may not use
 113 this file except in compliance with the License.  You can obtain a copy
 114 in the file LICENSE in the source distribution or at
 115 L<https://www.openssl.org/source/license.html>.
 116
 117 =cut