1=pod
2
3=head1 NAME
4
5EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt,
6EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info,
7EVP_PKEY_CTX_set_hkdf_mode -
8HMAC-based Extract-and-Expand key derivation algorithm
9
10=head1 SYNOPSIS
11
12 #include <openssl/kdf.h>
13
14 int EVP_PKEY_CTX_set_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);
15
16 int EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);
17
18 int EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
19                                 int saltlen);
20
21 int EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
22                                int keylen);
23
24 int EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
25                                 int infolen);
26
27=head1 DESCRIPTION
28
29The EVP_PKEY_HKDF algorithm implements the HKDF key derivation function.
30HKDF follows the "extract-then-expand" paradigm, where the KDF logically
31consists of two modules. The first stage takes the input keying material
32and "extracts" from it a fixed-length pseudorandom key K. The second stage
33"expands" the key K into several additional pseudorandom keys (the output
34of the KDF).
35
36EVP_PKEY_CTX_set_hkdf_mode() sets the mode for the HKDF operation. There
37are three modes that are currently defined:
38
39=over 4
40
41=item EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND
42
43This is the default mode. Calling L<EVP_PKEY_derive(3)> on an EVP_PKEY_CTX set
44up for HKDF will perform an extract followed by an expand operation in one go.
45The derived key returned will be the result after the expand operation. The
46intermediate fixed-length pseudorandom key K is not returned.
47
48In this mode the digest, key, salt and info values must be set before a key is
49derived or an error occurs.
50
51=item EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY
52
53In this mode calling L<EVP_PKEY_derive(3)> will just perform the extract
54operation. The value returned will be the intermediate fixed-length pseudorandom
55key K.
56
57The digest, key and salt values must be set before a key is derived or an
58error occurs.
59
60=item EVP_PKEY_HKDEF_MODE_EXPAND_ONLY
61
62In this mode calling L<EVP_PKEY_derive(3)> will just perform the expand
63operation. The input key should be set to the intermediate fixed-length
64pseudorandom key K returned from a previous extract operation.
65
66The digest, key and info values must be set before a key is derived or an
67error occurs.
68
69=back
70
71EVP_PKEY_CTX_set_hkdf_md() sets the message digest associated with the HKDF.
72
73EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to B<saltlen> bytes of the
74buffer B<salt>. Any existing value is replaced.
75
76EVP_PKEY_CTX_set1_hkdf_key() sets the key to B<keylen> bytes of the buffer
77B<key>. Any existing value is replaced.
78
79EVP_PKEY_CTX_add1_hkdf_info() sets the info value to B<infolen> bytes of the
80buffer B<info>. If a value is already set, it is appended to the existing
81value.
82
83=head1 STRING CTRLS
84
85HKDF also supports string based control operations via
86L<EVP_PKEY_CTX_ctrl_str(3)>.
87The B<type> parameter "md" uses the supplied B<value> as the name of the digest
88algorithm to use.
89The B<type> parameter "mode" uses the values "EXTRACT_AND_EXPAND",
90"EXTRACT_ONLY" and "EXPAND_ONLY" to determine the mode to use.
91The B<type> parameters "salt", "key" and "info" use the supplied B<value>
92parameter as a B<seed>, B<key> or B<info> value.
93The names "hexsalt", "hexkey" and "hexinfo" are similar except they take a hex
94string which is converted to binary.
95
96=head1 NOTES
97
98A context for HKDF can be obtained by calling:
99
100 EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
101
102The total length of the info buffer cannot exceed 1024 bytes in length: this
103should be more than enough for any normal use of HKDF.
104
105The output length of an HKDF expand operation is specified via the length
106parameter to the L<EVP_PKEY_derive(3)> function.
107Since the HKDF output length is variable, passing a B<NULL> buffer as a means
108to obtain the requisite length is not meaningful with HKDF in any mode that
109performs an expand operation. Instead, the caller must allocate a buffer of the
110desired length, and pass that buffer to L<EVP_PKEY_derive(3)> along with (a
111pointer initialized to) the desired length. Passing a B<NULL> buffer to obtain
112the length is allowed when using EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.
113
114Optimised versions of HKDF can be implemented in an ENGINE.
115
116=head1 RETURN VALUES
117
118All these functions return 1 for success and 0 or a negative value for failure.
119In particular a return value of -2 indicates the operation is not supported by
120the public key algorithm.
121
122=head1 EXAMPLES
123
124This example derives 10 bytes using SHA-256 with the secret key "secret",
125salt value "salt" and info value "label":
126
127 EVP_PKEY_CTX *pctx;
128 unsigned char out[10];
129 size_t outlen = sizeof(out);
130 pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
131
132 if (EVP_PKEY_derive_init(pctx) <= 0)
133     /* Error */
134 if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
135     /* Error */
136 if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
137     /* Error */
138 if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
139     /* Error */
140 if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
141     /* Error */
142 if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
143     /* Error */
144
145=head1 CONFORMING TO
146
147RFC 5869
148
149=head1 SEE ALSO
150
151L<EVP_PKEY_CTX_new(3)>,
152L<EVP_PKEY_CTX_ctrl_str(3)>,
153L<EVP_PKEY_derive(3)>
154
155=head1 HISTORY
156
157All of the functions described here were converted from macros to functions in
158OpenSSL 3.0.
159
160=head1 COPYRIGHT
161
162Copyright 2016-2021 The OpenSSL Project Authors. All Rights Reserved.
163
164Licensed under the Apache License 2.0 (the "License").  You may not use
165this file except in compliance with the License.  You can obtain a copy
166in the file LICENSE in the source distribution or at
167L<https://www.openssl.org/source/license.html>.
168
169=cut
170