doc/man7/EVP_KDF-SSHKDF.pod

   1 =pod
   2
   3 =head1 NAME
   4
   5 EVP_KDF-SSHKDF - The SSHKDF EVP_KDF implementation
   6
   7 =head1 DESCRIPTION
   8
   9 Support for computing the B<SSHKDF> KDF through the B<EVP_KDF> API.
  10
  11 The EVP_KDF-SSHKDF algorithm implements the SSHKDF key derivation function.
  12 It is defined in RFC 4253, section 7.2 and is used by SSH to derive IVs,
  13 encryption keys and integrity keys.
  14 Five inputs are required to perform key derivation: The hashing function
  15 (for example SHA256), the Initial Key, the Exchange Hash, the Session ID,
  16 and the derivation key type.
  17
  18 =head2 Identity
  19
  20 "SSHKDF" is the name for this implementation; it
  21 can be used with the EVP_KDF_fetch() function.
  22
  23 =head2 Supported parameters
  24
  25 The supported parameters are:
  26
  27 =over 4
  28
  29 =item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
  30
  31 =item "digest" (B<OSSL_KDF_PARAM_DIGEST>) <UTF8 string>
  32
  33 =item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>
  34
  35 These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
  36
  37 =item "xcghash" (B<OSSL_KDF_PARAM_SSHKDF_XCGHASH>) <octet string>
  38
  39 =item "session_id" (B<OSSL_KDF_PARAM_SSHKDF_SESSION_ID>) <octet string>
  40
  41 These parameters set the respective values for the KDF.
  42 If a value is already set, the contents are replaced.
  43
  44 =item "type" (B<OSSL_KDF_PARAM_SSHKDF_TYPE>) <UTF8 string>
  45
  46 This parameter sets the type for the SSHKDF operation.
  47 There are six supported types:
  48
  49 =over 4
  50
  51 =item EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV
  52
  53 The Initial IV from client to server.
  54 A single char of value 65 (ASCII char 'A').
  55
  56 =item EVP_KDF_SSHKDF_TYPE_INITIAL_IV_SRV_TO_CLI
  57
  58 The Initial IV from server to client
  59 A single char of value 66 (ASCII char 'B').
  60
  61 =item EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_CLI_TO_SRV
  62
  63 The Encryption Key from client to server
  64 A single char of value 67 (ASCII char 'C').
  65
  66 =item EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_SRV_TO_CLI
  67
  68 The Encryption Key from server to client
  69 A single char of value 68 (ASCII char 'D').
  70
  71 =item EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_CLI_TO_SRV
  72
  73 The Integrity Key from client to server
  74 A single char of value 69 (ASCII char 'E').
  75
  76 =item EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_SRV_TO_CLI
  77
  78 The Integrity Key from client to server
  79 A single char of value 70 (ASCII char 'F').
  80
  81 =back
  82
  83 =back
  84
  85 =head1 NOTES
  86
  87 A context for SSHKDF can be obtained by calling:
  88
  89  EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL);
  90  EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
  91
  92 The output length of the SSHKDF derivation is specified via the I<keylen>
  93 parameter to the L<EVP_KDF_derive(3)> function.
  94 Since the SSHKDF output length is variable, calling L<EVP_KDF_CTX_get_kdf_size(3)>
  95 to obtain the requisite length is not meaningful. The caller must
  96 allocate a buffer of the desired length, and pass that buffer to the
  97 L<EVP_KDF_derive(3)> function along with the desired length.
  98
  99 =head1 EXAMPLES
 100
 101 This example derives an 8 byte IV using SHA-256 with a 1K "key" and appropriate
 102 "xcghash" and "session_id" values:
 103
 104  EVP_KDF *kdf;
 105  EVP_KDF_CTX *kctx;
 106  char type = EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV;
 107  unsigned char key[1024] = "01234...";
 108  unsigned char xcghash[32] = "012345...";
 109  unsigned char session_id[32] = "012345...";
 110  unsigned char out[8];
 111  size_t outlen = sizeof(out);
 112  OSSL_PARAM params[6], *p = params;
 113
 114  kdf = EVP_KDF_fetch(NULL, "SSHKDF", NULL);
 115  kctx = EVP_KDF_CTX_new(kdf);
 116  EVP_KDF_free(kdf);
 117
 118  *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
 119                                          SN_sha256, strlen(SN_sha256));
 120  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
 121                                           key, (size_t)1024);
 122  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_XCGHASH,
 123                                           xcghash, (size_t)32);
 124  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SSHKDF_SESSION_ID,
 125                                           session_id, (size_t)32);
 126  *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_SSHKDF_TYPE,
 127                                          &type, sizeof(type));
 128  *p = OSSL_PARAM_construct_end();
 129  if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
 130      /* Error */
 131
 132
 133 =head1 CONFORMING TO
 134
 135 RFC 4253
 136
 137 =head1 SEE ALSO
 138
 139 L<EVP_KDF(3)>,
 140 L<EVP_KDF_CTX_new(3)>,
 141 L<EVP_KDF_CTX_free(3)>,
 142 L<EVP_KDF_CTX_set_params(3)>,
 143 L<EVP_KDF_CTX_get_kdf_size(3)>,
 144 L<EVP_KDF_derive(3)>,
 145 L<EVP_KDF(3)/PARAMETERS>
 146
 147 =head1 COPYRIGHT
 148
 149 Copyright 2016-2022 The OpenSSL Project Authors. All Rights Reserved.
 150
 151 Licensed under the Apache License 2.0 (the "License").  You may not use
 152 this file except in compliance with the License.  You can obtain a copy
 153 in the file LICENSE in the source distribution or at
 154 L<https://www.openssl.org/source/license.html>.
 155
 156 =cut
 157