xref: /openssl/doc/man3/HMAC.pod (revision 7ed6de99) 
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
27The following functions have been deprecated since OpenSSL 3.0, and can be
28hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
29see L<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
47The following function has been deprecated since OpenSSL 1.1.0, and can be
48hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable version value,
49see L<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
74HMAC() uses the default B<OSSL_LIB_CTX>.
75Use L<EVP_Q_mac(3)> instead if a library context is required.
76
77All of the functions described below are deprecated.
78Applications should instead use L<EVP_MAC_CTX_new(3)>, L<EVP_MAC_CTX_free(3)>,
79L<EVP_MAC_init(3)>, L<EVP_MAC_update(3)> and L<EVP_MAC_final(3)>
80or the 'quick' single-shot MAC function L<EVP_Q_mac(3)>.
81
82HMAC_CTX_new() creates a new HMAC_CTX in heap memory.
83
84HMAC_CTX_reset() clears an existing B<HMAC_CTX> and associated
85resources, making it suitable for new computations as if it was newly
86created with HMAC_CTX_new().
87
88HMAC_CTX_free() erases the key and other data from the B<HMAC_CTX>,
89releases any associated resources and finally frees the B<HMAC_CTX>
90itself. If the argument is NULL, nothing is done.
91
92The following functions may be used if the message is not completely
93stored in memory:
94
95HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use the hash
96function I<evp_md> and key I<key>. If both are NULL, or if I<key> is NULL
97and I<evp_md> is the same as the previous call, then the
98existing key is
99reused. I<ctx> must have been created with HMAC_CTX_new() before the first use
100of an B<HMAC_CTX> in this function.
101
102If HMAC_Init_ex() is called with I<key> NULL and I<evp_md> is not the
103same as the previous digest used by I<ctx> then an error is returned
104because reuse of an existing key with a different digest is not supported.
105
106HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash
107function I<evp_md> and the key I<key> which is I<key_len> bytes
108long.
109
110HMAC_Update() can be called repeatedly with chunks of the message to
111be authenticated (I<len> bytes at I<data>).
112
113HMAC_Final() places the message authentication code in I<md>, which
114must have space for the hash function output.
115
116HMAC_CTX_copy() copies all of the internal state from I<sctx> into I<dctx>.
117
118HMAC_CTX_set_flags() applies the specified flags to the internal EVP_MD_CTXs.
119These flags have the same meaning as for L<EVP_MD_CTX_set_flags(3)>.
120
121HMAC_CTX_get_md() returns the EVP_MD that has previously been set for the
122supplied HMAC_CTX.
123
124HMAC_size() returns the length in bytes of the underlying hash function output.
125
126=head1 RETURN VALUES
127
128HMAC() returns a pointer to the message authentication code or NULL if
129an error occurred.
130
131HMAC_CTX_new() returns a pointer to a new B<HMAC_CTX> on success or
132NULL if an error occurred.
133
134HMAC_CTX_reset(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() and
135HMAC_CTX_copy() return 1 for success or 0 if an error occurred.
136
137HMAC_CTX_get_md() return the EVP_MD previously set for the supplied HMAC_CTX or
138NULL if no EVP_MD has been set.
139
140HMAC_size() returns the length in bytes of the underlying hash function output
141or zero on error.
142
143=head1 CONFORMING TO
144
145RFC 2104
146
147=head1 SEE ALSO
148
149L<SHA1(3)>, EVP_Q_mac(3), L<evp(7)>
150
151=head1 HISTORY
152
153All functions except for HMAC() were deprecated in OpenSSL 3.0.
154
155HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL 1.1.0.
156
157HMAC_CTX_cleanup() existed in OpenSSL before version 1.1.0.
158
159HMAC_CTX_new(), HMAC_CTX_free() and HMAC_CTX_get_md() are new in OpenSSL 1.1.0.
160
161HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
162OpenSSL before version 1.0.0.
163
164=head1 COPYRIGHT
165
166Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.
167
168Licensed under the Apache License 2.0 (the "License").  You may not use
169this file except in compliance with the License.  You can obtain a copy
170in the file LICENSE in the source distribution or at
171L<https://www.openssl.org/source/license.html>.
172
173=cut
174