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      1 =pod
      2 
      3 =head1 NAME
      4 
      5 EVP_KDF-HKDF - The HKDF EVP_KDF implementation
      6 
      7 =head1 DESCRIPTION
      8 
      9 Support for computing the B<HKDF> KDF through the B<EVP_KDF> API.
     10 
     11 The EVP_KDF-HKDF algorithm implements the HKDF key derivation function.
     12 HKDF follows the "extract-then-expand" paradigm, where the KDF logically
     13 consists of two modules. The first stage takes the input keying material
     14 and "extracts" from it a fixed-length pseudorandom key K. The second stage
     15 "expands" the key K into several additional pseudorandom keys (the output
     16 of the KDF).
     17 
     18 The output is considered to be keying material.
     19 
     20 =head2 Identity
     21 
     22 "HKDF" is the name for this implementation; it
     23 can be used with the EVP_KDF_fetch() function.
     24 
     25 =head2 Supported parameters
     26 
     27 The supported parameters are:
     28 
     29 =over 4
     30 
     31 =item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
     32 
     33 =item "digest" (B<OSSL_KDF_PARAM_DIGEST>) <UTF8 string>
     34 
     35 =item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>
     36 
     37 =item "salt" (B<OSSL_KDF_PARAM_SALT>) <octet string>
     38 
     39 These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
     40 
     41 =item "info" (B<OSSL_KDF_PARAM_INFO>) <octet string>
     42 
     43 This parameter sets the info value.
     44 The length of the context info buffer cannot exceed 1024 bytes;
     45 this should be more than enough for any normal use of HKDF.
     46 
     47 =item "mode" (B<OSSL_KDF_PARAM_MODE>) <UTF8 string> or <integer>
     48 
     49 This parameter sets the mode for the HKDF operation.
     50 There are three modes that are currently defined:
     51 
     52 =over 4
     53 
     54 =item "EXTRACT_AND_EXPAND" or B<EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND>
     55 
     56 This is the default mode.  Calling L<EVP_KDF_derive(3)> on an EVP_KDF_CTX set
     57 up for HKDF will perform an extract followed by an expand operation in one go.
     58 The derived key returned will be the result after the expand operation. The
     59 intermediate fixed-length pseudorandom key K is not returned.
     60 
     61 In this mode the digest, key, salt and info values must be set before a key is
     62 derived otherwise an error will occur.
     63 
     64 =item "EXTRACT_ONLY" or B<EVP_KDF_HKDF_MODE_EXTRACT_ONLY>
     65 
     66 In this mode calling L<EVP_KDF_derive(3)> will just perform the extract
     67 operation. The value returned will be the intermediate fixed-length pseudorandom
     68 key K.  The I<keylen> parameter must match the size of K, which can be looked
     69 up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest.
     70 
     71 The digest, key and salt values must be set before a key is derived otherwise
     72 an error will occur.
     73 
     74 =item "EXPAND_ONLY" or B<EVP_KDF_HKDF_MODE_EXPAND_ONLY>
     75 
     76 In this mode calling L<EVP_KDF_derive(3)> will just perform the expand
     77 operation. The input key should be set to the intermediate fixed-length
     78 pseudorandom key K returned from a previous extract operation.
     79 
     80 The digest, key and info values must be set before a key is derived otherwise
     81 an error will occur.
     82 
     83 =back
     84 
     85 =back
     86 
     87 The OpenSSL FIPS provider also supports the following parameters:
     88 
     89 =over 4
     90 
     91 =item "fips-indicator" (B<OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR>) <integer>
     92 
     93 A getter that returns 1 if the operation is FIPS approved, or 0 otherwise.
     94 This may be used after calling EVP_KDF_derive. It returns 0 if "key-check"
     95 is set to 0 and the check fails.
     96 
     97 =item "key-check" (B<OSSL_KDF_PARAM_FIPS_KEY_CHECK>) <integer>
     98 
     99 The default value of 1 causes an error during EVP_KDF_CTX_set_params() if the
    100 length of used key-derivation key (B<OSSL_KDF_PARAM_KEY>) is shorter than 112
    101 bits.
    102 Setting this to zero will ignore the error and set the approved
    103 "fips-indicator" to 0.
    104 This option breaks FIPS compliance if it causes the approved "fips-indicator"
    105 to return 0.
    106 
    107 =back
    108 
    109 =head1 NOTES
    110 
    111 A context for HKDF can be obtained by calling:
    112 
    113  EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
    114  EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
    115 
    116 The output length of an HKDF expand operation is specified via the I<keylen>
    117 parameter to the L<EVP_KDF_derive(3)> function.  When using
    118 EVP_KDF_HKDF_MODE_EXTRACT_ONLY the I<keylen> parameter must equal the size of
    119 the intermediate fixed-length pseudorandom key otherwise an error will occur.
    120 For that mode, the fixed output size can be looked up by calling EVP_KDF_CTX_get_kdf_size()
    121 after setting the mode and digest on the B<EVP_KDF_CTX>.
    122 
    123 =head1 EXAMPLES
    124 
    125 This example derives 10 bytes using SHA-256 with the secret key "secret",
    126 salt value "salt" and info value "label":
    127 
    128  EVP_KDF *kdf;
    129  EVP_KDF_CTX *kctx;
    130  unsigned char out[10];
    131  OSSL_PARAM params[5], *p = params;
    132 
    133  kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
    134  kctx = EVP_KDF_CTX_new(kdf);
    135  EVP_KDF_free(kdf);
    136 
    137  *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
    138                                          SN_sha256, strlen(SN_sha256));
    139  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
    140                                           "secret", (size_t)6);
    141  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
    142                                           "label", (size_t)5);
    143  *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
    144                                           "salt", (size_t)4);
    145  *p = OSSL_PARAM_construct_end();
    146  if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    147      error("EVP_KDF_derive");
    148  }
    149 
    150  EVP_KDF_CTX_free(kctx);
    151 
    152 =head1 CONFORMING TO
    153 
    154 RFC 5869
    155 
    156 =head1 SEE ALSO
    157 
    158 L<EVP_KDF(3)>,
    159 L<EVP_KDF_CTX_new(3)>,
    160 L<EVP_KDF_CTX_free(3)>,
    161 L<EVP_KDF_CTX_get_kdf_size(3)>,
    162 L<EVP_KDF_CTX_set_params(3)>,
    163 L<EVP_KDF_derive(3)>,
    164 L<EVP_KDF(3)/PARAMETERS>,
    165 L<EVP_KDF-TLS13_KDF(7)>
    166 
    167 =head1 HISTORY
    168 
    169 This functionality was added in OpenSSL 3.0.
    170 
    171 =head1 COPYRIGHT
    172 
    173 Copyright 2016-2024 The OpenSSL Project Authors. All Rights Reserved.
    174 
    175 Licensed under the Apache License 2.0 (the "License").  You may not use
    176 this file except in compliance with the License.  You can obtain a copy
    177 in the file LICENSE in the source distribution or at
    178 L<https://www.openssl.org/source/license.html>.
    179 
    180 =cut
    181