1EVP_PKEY_CTX_SET_HKDF_MD(3ossl)     OpenSSL    EVP_PKEY_CTX_SET_HKDF_MD(3ossl)
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NAME

6       EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt,
7       EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info,
8       EVP_PKEY_CTX_set_hkdf_mode - HMAC-based Extract-and-Expand key
9       derivation algorithm
10

SYNOPSIS

12        #include <openssl/kdf.h>
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14        int EVP_PKEY_CTX_set_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);
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16        int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
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18        int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
19                                        int saltlen);
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21        int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
22                                       int keylen);
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24        int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
25                                        int infolen);
26

DESCRIPTION

28       The EVP_PKEY_HKDF algorithm implements the HKDF key derivation
29       function.  HKDF follows the "extract-then-expand" paradigm, where the
30       KDF logically consists of two modules. The first stage takes the input
31       keying material and "extracts" from it a fixed-length pseudorandom key
32       K. The second stage "expands" the key K into several additional
33       pseudorandom keys (the output of the KDF).
34
35       EVP_PKEY_CTX_set_hkdf_mode() sets the mode for the HKDF operation.
36       There are three modes that are currently defined:
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38       EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND
39           This is the default mode. Calling EVP_PKEY_derive(3) on an
40           EVP_PKEY_CTX set up for HKDF will perform an extract followed by an
41           expand operation in one go.  The derived key returned will be the
42           result after the expand operation. The intermediate fixed-length
43           pseudorandom key K is not returned.
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45           In this mode the digest, key, salt and info values must be set
46           before a key is derived or an error occurs.
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48       EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY
49           In this mode calling EVP_PKEY_derive(3) will just perform the
50           extract operation. The value returned will be the intermediate
51           fixed-length pseudorandom key K.
52
53           The digest, key and salt values must be set before a key is derived
54           or an error occurs.
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56       EVP_PKEY_HKDEF_MODE_EXPAND_ONLY
57           In this mode calling EVP_PKEY_derive(3) will just perform the
58           expand operation. The input key should be set to the intermediate
59           fixed-length pseudorandom key K returned from a previous extract
60           operation.
61
62           The digest, key and info values must be set before a key is derived
63           or an error occurs.
64
65       EVP_PKEY_CTX_set_hkdf_md() sets the message digest associated with the
66       HKDF.
67
68       EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to saltlen bytes of the
69       buffer salt. Any existing value is replaced.
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71       EVP_PKEY_CTX_set1_hkdf_key() sets the key to keylen bytes of the buffer
72       key. Any existing value is replaced.
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74       EVP_PKEY_CTX_add1_hkdf_info() sets the info value to infolen bytes of
75       the buffer info. If a value is already set, it is appended to the
76       existing value.
77

STRING CTRLS

79       HKDF also supports string based control operations via
80       EVP_PKEY_CTX_ctrl_str(3).  The type parameter "md" uses the supplied
81       value as the name of the digest algorithm to use.  The type parameter
82       "mode" uses the values "EXTRACT_AND_EXPAND", "EXTRACT_ONLY" and
83       "EXPAND_ONLY" to determine the mode to use.  The type parameters
84       "salt", "key" and "info" use the supplied value parameter as a seed,
85       key or info value.  The names "hexsalt", "hexkey" and "hexinfo" are
86       similar except they take a hex string which is converted to binary.
87

NOTES

89       A context for HKDF can be obtained by calling:
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91        EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
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93       The total length of the info buffer cannot exceed 2048 bytes in length:
94       this should be more than enough for any normal use of HKDF.
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96       The output length of an HKDF expand operation is specified via the
97       length parameter to the EVP_PKEY_derive(3) function.  Since the HKDF
98       output length is variable, passing a NULL buffer as a means to obtain
99       the requisite length is not meaningful with HKDF in any mode that
100       performs an expand operation. Instead, the caller must allocate a
101       buffer of the desired length, and pass that buffer to
102       EVP_PKEY_derive(3) along with (a pointer initialized to) the desired
103       length. Passing a NULL buffer to obtain the length is allowed when
104       using EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.
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106       Optimised versions of HKDF can be implemented in an ENGINE.
107

RETURN VALUES

109       All these functions return 1 for success and 0 or a negative value for
110       failure.  In particular a return value of -2 indicates the operation is
111       not supported by the public key algorithm.
112

EXAMPLES

114       This example derives 10 bytes using SHA-256 with the secret key
115       "secret", salt value "salt" and info value "label":
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117        EVP_PKEY_CTX *pctx;
118        unsigned char out[10];
119        size_t outlen = sizeof(out);
120        pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
121
122        if (EVP_PKEY_derive_init(pctx) <= 0)
123            /* Error */
124        if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
125            /* Error */
126        if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
127            /* Error */
128        if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
129            /* Error */
130        if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
131            /* Error */
132        if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
133            /* Error */
134

CONFORMING TO

136       RFC 5869
137

SEE ALSO

139       EVP_PKEY_CTX_new(3), EVP_PKEY_CTX_ctrl_str(3), EVP_PKEY_derive(3)
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HISTORY

142       All of the functions described here were converted from macros to
143       functions in OpenSSL 3.0.
144
146       Copyright 2016-2022 The OpenSSL Project Authors. All Rights Reserved.
147
148       Licensed under the Apache License 2.0 (the "License").  You may not use
149       this file except in compliance with the License.  You can obtain a copy
150       in the file LICENSE in the source distribution or at
151       <https://www.openssl.org/source/license.html>.
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1553.0.9                             2023-07-27   EVP_PKEY_CTX_SET_HKDF_MD(3ossl)
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