1EVP_PKEY_CTX_SET_HKDF_MD(3)         OpenSSL        EVP_PKEY_CTX_SET_HKDF_MD(3)
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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_hkdf_mode - HMAC-based Extract-and-Expand key derivation
9       algorithm
10

SYNOPSIS

12        #include <openssl/kdf.h>
13
14        int EVP_PKEY_CTX_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_hkdf_mode() sets the mode for the HKDF operation. There
36       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.
44
45           In this mode the digest, key, salt and info values must be set
46           before a key is derived or an error occurs.
47
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.
70
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       All these functions are implemented as macros.
90
91       A context for HKDF can be obtained by calling:
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93        EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
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95       The total length of the info buffer cannot exceed 1024 bytes in length:
96       this should be more than enough for any normal use of HKDF.
97
98       The output length of an HKDF expand operation is specified via the
99       length parameter to the EVP_PKEY_derive(3) function.  Since the HKDF
100       output length is variable, passing a NULL buffer as a means to obtain
101       the requisite length is not meaningful with HKDF in any mode that
102       performs an expand operation. Instead, the caller must allocate a
103       buffer of the desired length, and pass that buffer to
104       EVP_PKEY_derive(3) along with (a pointer initialized to) the desired
105       length. Passing a NULL buffer to obtain the length is allowed when
106       using EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.
107
108       Optimised versions of HKDF can be implemented in an ENGINE.
109

RETURN VALUES

111       All these functions return 1 for success and 0 or a negative value for
112       failure.  In particular a return value of -2 indicates the operation is
113       not supported by the public key algorithm.
114

EXAMPLE

116       This example derives 10 bytes using SHA-256 with the secret key
117       "secret", salt value "salt" and info value "label":
118
119        EVP_PKEY_CTX *pctx;
120        unsigned char out[10];
121        size_t outlen = sizeof(out);
122        pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
123
124        if (EVP_PKEY_derive_init(pctx) <= 0)
125            /* Error */
126        if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
127            /* Error */
128        if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
129            /* Error */
130        if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
131            /* Error */
132        if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
133            /* Error */
134        if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
135            /* Error */
136

CONFORMING TO

138       RFC 5869
139

SEE ALSO

141       EVP_PKEY_CTX_new(3), EVP_PKEY_CTX_ctrl_str(3), EVP_PKEY_derive(3)
142
144       Copyright 2016-2018 The OpenSSL Project Authors. All Rights Reserved.
145
146       Licensed under the OpenSSL license (the "License").  You may not use
147       this file except in compliance with the License.  You can obtain a copy
148       in the file LICENSE in the source distribution or at
149       <https://www.openssl.org/source/license.html>.
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1531.1.1c                            2019-05-28       EVP_PKEY_CTX_SET_HKDF_MD(3)
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