1=pod
2
3=head1 NAME
4
5HMAC,
6HMAC_CTX_new,
7HMAC_CTX_reset,
8HMAC_CTX_free,
9HMAC_Init,
10HMAC_Init_ex,
11HMAC_Update,
12HMAC_Final,
13HMAC_CTX_copy,
14HMAC_CTX_set_flags,
15HMAC_CTX_get_md,
16HMAC_size
17- HMAC message authentication code
18
19=head1 SYNOPSIS
20
21 #include <openssl/hmac.h>
22
23 unsigned char *HMAC(const EVP_MD *evp_md, const void *key, int key_len,
24                     const unsigned char *data, size_t data_len,
25                     unsigned char *md, unsigned int *md_len);
26
27Deprecated since OpenSSL 3.0, can be hidden entirely by defining
28B<OPENSSL_API_COMPAT> with a suitable version value, see
29L<openssl_user_macros(7)>:
30
31 HMAC_CTX *HMAC_CTX_new(void);
32 int HMAC_CTX_reset(HMAC_CTX *ctx);
33
34 int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
35                  const EVP_MD *md, ENGINE *impl);
36 int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, size_t len);
37 int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
38
39 void HMAC_CTX_free(HMAC_CTX *ctx);
40
41 int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx);
42 void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags);
43 const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);
44
45 size_t HMAC_size(const HMAC_CTX *e);
46
47Deprecated since OpenSSL 1.1.0, can be hidden entirely by defining
48B<OPENSSL_API_COMPAT> with a suitable version value, see
49L<openssl_user_macros(7)>:
50
51 int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
52               const EVP_MD *md);
53
54=head1 DESCRIPTION
55
56HMAC is a MAC (message authentication code), i.e. a keyed hash
57function used for message authentication, which is based on a hash
58function.
59
60HMAC() computes the message authentication code of the I<data_len> bytes at
61I<data> using the hash function I<evp_md> and the key I<key> which is
62I<key_len> bytes long. The I<key> may also be NULL with I<key_len> being 0.
63
64It places the result in I<md> (which must have space for the output of
65the hash function, which is no more than B<EVP_MAX_MD_SIZE> bytes).
66If I<md> is NULL, the digest is placed in a static array.  The size of
67the output is placed in I<md_len>, unless it is NULL. Note: passing a NULL
68value for I<md> to use the static array is not thread safe.
69
70I<evp_md> is a message digest such as EVP_sha1(), EVP_ripemd160() etc.
71HMAC does not support variable output length digests such as EVP_shake128() and
72EVP_shake256().
73
74All of the functions described below are deprecated.
75Applications should instead use L<EVP_MAC_CTX_new(3)>, L<EVP_MAC_CTX_free(3)>,
76L<EVP_MAC_init(3)>, L<EVP_MAC_update(3)> and L<EVP_MAC_final(3)>
77or the 'quick' single-shot MAC function L<EVP_Q_mac(3)>.
78
79HMAC_CTX_new() creates a new HMAC_CTX in heap memory.
80
81HMAC_CTX_reset() clears an existing B<HMAC_CTX> and associated
82resources, making it suitable for new computations as if it was newly
83created with HMAC_CTX_new().
84
85HMAC_CTX_free() erases the key and other data from the B<HMAC_CTX>,
86releases any associated resources and finally frees the B<HMAC_CTX>
87itself.
88
89The following functions may be used if the message is not completely
90stored in memory:
91
92HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use the hash
93function I<evp_md> and key I<key>. If both are NULL, or if I<key> is NULL
94and I<evp_md> is the same as the previous call, then the
95existing key is
96reused. I<ctx> must have been created with HMAC_CTX_new() before the first use
97of an B<HMAC_CTX> in this function.
98
99If HMAC_Init_ex() is called with I<key> NULL and I<evp_md> is not the
100same as the previous digest used by I<ctx> then an error is returned
101because reuse of an existing key with a different digest is not supported.
102
103HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash
104function I<evp_md> and the key I<key> which is I<key_len> bytes
105long.
106
107HMAC_Update() can be called repeatedly with chunks of the message to
108be authenticated (I<len> bytes at I<data>).
109
110HMAC_Final() places the message authentication code in I<md>, which
111must have space for the hash function output.
112
113HMAC_CTX_copy() copies all of the internal state from I<sctx> into I<dctx>.
114
115HMAC_CTX_set_flags() applies the specified flags to the internal EVP_MD_CTXs.
116These flags have the same meaning as for L<EVP_MD_CTX_set_flags(3)>.
117
118HMAC_CTX_get_md() returns the EVP_MD that has previously been set for the
119supplied HMAC_CTX.
120
121HMAC_size() returns the length in bytes of the underlying hash function output.
122
123=head1 RETURN VALUES
124
125HMAC() returns a pointer to the message authentication code or NULL if
126an error occurred.
127
128HMAC_CTX_new() returns a pointer to a new B<HMAC_CTX> on success or
129NULL if an error occurred.
130
131HMAC_CTX_reset(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() and
132HMAC_CTX_copy() return 1 for success or 0 if an error occurred.
133
134HMAC_CTX_get_md() return the EVP_MD previously set for the supplied HMAC_CTX or
135NULL if no EVP_MD has been set.
136
137HMAC_size() returns the length in bytes of the underlying hash function output
138or zero on error.
139
140=head1 CONFORMING TO
141
142RFC 2104
143
144=head1 SEE ALSO
145
146L<SHA1(3)>, EVP_Q_mac(3), L<evp(7)>
147
148=head1 HISTORY
149
150All functions except for HMAC() were deprecated in OpenSSL 3.0.
151
152HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL 1.1.0.
153
154HMAC_CTX_cleanup() existed in OpenSSL before version 1.1.0.
155
156HMAC_CTX_new(), HMAC_CTX_free() and HMAC_CTX_get_md() are new in OpenSSL 1.1.0.
157
158HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
159OpenSSL before version 1.0.0.
160
161=head1 COPYRIGHT
162
163Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.
164
165Licensed under the Apache License 2.0 (the "License").  You may not use
166this file except in compliance with the License.  You can obtain a copy
167in the file LICENSE in the source distribution or at
168L<https://www.openssl.org/source/license.html>.
169
170=cut
171