1EVP_KDF-HKDF(7ossl)                 OpenSSL                EVP_KDF-HKDF(7ossl)
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NAME

6       EVP_KDF-HKDF - The HKDF EVP_KDF implementation
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DESCRIPTION

9       Support for computing the HKDF KDF through the 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
13       logically consists of two modules. The first stage takes the input
14       keying material and "extracts" from it a fixed-length pseudorandom key
15       K. The second stage "expands" the key K into several additional
16       pseudorandom keys (the output of the KDF).
17
18   Identity
19       "HKDF" is the name for this implementation; it can be used with the
20       EVP_KDF_fetch() function.
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22   Supported parameters
23       The supported parameters are:
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25       "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
26       "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
27       "key" (OSSL_KDF_PARAM_KEY) <octet string>
28       "salt" (OSSL_KDF_PARAM_SALT) <octet string>
29           These parameters work as described in "PARAMETERS" in EVP_KDF(3).
30
31       "info" (OSSL_KDF_PARAM_INFO) <octet string>
32           This parameter sets the info value.  The length of the context info
33           buffer cannot exceed 1024 bytes; this should be more than enough
34           for any normal use of HKDF.
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36       "mode" (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>
37           This parameter sets the mode for the HKDF operation.  There are
38           three modes that are currently defined:
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40           "EXTRACT_AND_EXPAND" or EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND
41               This is the default mode.  Calling EVP_KDF_derive(3) on an
42               EVP_KDF_CTX set up for HKDF will perform an extract followed by
43               an expand operation in one go.  The derived key returned will
44               be the result after the expand operation. The intermediate
45               fixed-length pseudorandom key K is not returned.
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47               In this mode the digest, key, salt and info values must be set
48               before a key is derived otherwise an error will occur.
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50           "EXTRACT_ONLY" or EVP_KDF_HKDF_MODE_EXTRACT_ONLY
51               In this mode calling EVP_KDF_derive(3) will just perform the
52               extract operation. The value returned will be the intermediate
53               fixed-length pseudorandom key K.  The keylen parameter must
54               match the size of K, which can be looked up by calling
55               EVP_KDF_CTX_get_kdf_size() after setting the mode and digest.
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57               The digest, key and salt values must be set before a key is
58               derived otherwise an error will occur.
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60           "EXPAND_ONLY" or EVP_KDF_HKDF_MODE_EXPAND_ONLY
61               In this mode calling EVP_KDF_derive(3) will just perform the
62               expand operation. The input key should be set to the
63               intermediate fixed-length pseudorandom key K returned from a
64               previous extract operation.
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66               The digest, key and info values must be set before a key is
67               derived otherwise an error will occur.
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NOTES

70       A context for HKDF can be obtained by calling:
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72        EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
73        EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
74
75       The output length of an HKDF expand operation is specified via the
76       keylen parameter to the EVP_KDF_derive(3) function.  When using
77       EVP_KDF_HKDF_MODE_EXTRACT_ONLY the keylen parameter must equal the size
78       of the intermediate fixed-length pseudorandom key otherwise an error
79       will occur.  For that mode, the fixed output size can be looked up by
80       calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest on
81       the EVP_KDF_CTX.
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EXAMPLES

84       This example derives 10 bytes using SHA-256 with the secret key
85       "secret", salt value "salt" and info value "label":
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87        EVP_KDF *kdf;
88        EVP_KDF_CTX *kctx;
89        unsigned char out[10];
90        OSSL_PARAM params[5], *p = params;
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92        kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
93        kctx = EVP_KDF_CTX_new(kdf);
94        EVP_KDF_free(kdf);
95
96        *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
97                                                SN_sha256, strlen(SN_sha256));
98        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
99                                                 "secret", (size_t)6);
100        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
101                                                 "label", (size_t)5);
102        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
103                                                 "salt", (size_t)4);
104        *p = OSSL_PARAM_construct_end();
105        if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
106            error("EVP_KDF_derive");
107        }
108
109        EVP_KDF_CTX_free(kctx);
110

CONFORMING TO

112       RFC 5869
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SEE ALSO

115       EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3),
116       EVP_KDF_CTX_get_kdf_size(3), EVP_KDF_CTX_set_params(3),
117       EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3), EVP_KDF-TLS13_KDF(7)
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120       Copyright 2016-2021 The OpenSSL Project Authors. All Rights Reserved.
121
122       Licensed under the Apache License 2.0 (the "License").  You may not use
123       this file except in compliance with the License.  You can obtain a copy
124       in the file LICENSE in the source distribution or at
125       <https://www.openssl.org/source/license.html>.
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1293.0.5                             2022-11-01               EVP_KDF-HKDF(7ossl)
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