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      1 =pod
      2 
      3 =head1 NAME
      4 
      5 EVP_KDF-KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation
      6 
      7 =head1 DESCRIPTION
      8 
      9 Support for computing the B<KRB5KDF> KDF through the B<EVP_KDF> API.
     10 
     11 The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined
     12 in RFC 3961, section 5.1 and is used by Krb5 to derive session keys.
     13 Three inputs are required to perform key derivation: a cipher, (for example
     14 AES-128-CBC), the initial key, and a constant.
     15 
     16 =head2 Identity
     17 
     18 "KRB5KDF" is the name for this implementation;
     19 it can be used with the EVP_KDF_fetch() function.
     20 
     21 =head2 Supported parameters
     22 
     23 The supported parameters are:
     24 
     25 =over 4
     26 
     27 =item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
     28 
     29 =item "cipher" (B<OSSL_KDF_PARAM_CIPHER>) <UTF8 string>
     30 
     31 =item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>
     32 
     33 These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
     34 
     35 =item "constant" (B<OSSL_KDF_PARAM_CONSTANT>) <octet string>
     36 
     37 This parameter sets the constant value for the KDF.
     38 If a value is already set, the contents are replaced.
     39 
     40 =back
     41 
     42 =head1 NOTES
     43 
     44 A context for KRB5KDF can be obtained by calling:
     45 
     46  EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
     47  EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
     48 
     49 The output length of the KRB5KDF derivation is specified via the I<keylen>
     50 parameter to the L<EVP_KDF_derive(3)> function, and MUST match the key
     51 length for the chosen cipher or an error is returned. Moreover, the
     52 constant's length must not exceed the block size of the cipher.
     53 Since the KRB5KDF output length depends on the chosen cipher, calling
     54 L<EVP_KDF_CTX_get_kdf_size(3)> to obtain the requisite length returns the correct length
     55 only after the cipher is set. Prior to that B<EVP_MAX_KEY_LENGTH> is returned.
     56 The caller must allocate a buffer of the correct length for the chosen
     57 cipher, and pass that buffer to the L<EVP_KDF_derive(3)> function along
     58 with that length.
     59 
     60 =head1 EXAMPLES
     61 
     62 This example derives a key using the AES-128-CBC cipher:
     63 
     64  EVP_KDF *kdf;
     65  EVP_KDF_CTX *kctx;
     66  unsigned char key[16] = "01234...";
     67  unsigned char constant[] = "I'm a constant";
     68  unsigned char out[16];
     69  size_t outlen = sizeof(out);
     70  OSSL_PARAM params[4], *p = params;
     71 
     72  kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
     73  kctx = EVP_KDF_CTX_new(kdf);
     74  EVP_KDF_free(kdf);
     75 
     76  *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER,
     77                                          SN_aes_128_cbc,
     78                                          strlen(SN_aes_128_cbc));
     79  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
     80                                           key, (size_t)16);
     81  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT,
     82                                           constant, strlen(constant));
     83  *p = OSSL_PARAM_construct_end();
     84  if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
     85      /* Error */
     86 
     87  EVP_KDF_CTX_free(kctx);
     88 
     89 =head1 CONFORMING TO
     90 
     91 RFC 3961
     92 
     93 =head1 SEE ALSO
     94 
     95 L<EVP_KDF(3)>,
     96 L<EVP_KDF_CTX_free(3)>,
     97 L<EVP_KDF_CTX_get_kdf_size(3)>,
     98 L<EVP_KDF_derive(3)>,
     99 L<EVP_KDF(3)/PARAMETERS>
    100 
    101 =head1 HISTORY
    102 
    103 This functionality was added in OpenSSL 3.0.
    104 
    105 =head1 COPYRIGHT
    106 
    107 Copyright 2016-2021 The OpenSSL Project Authors. All Rights Reserved.
    108 
    109 Licensed under the Apache License 2.0 (the "License").  You may not use
    110 this file except in compliance with the License.  You can obtain a copy
    111 in the file LICENSE in the source distribution or at
    112 L<https://www.openssl.org/source/license.html>.
    113 
    114 =cut
    115 
    116