xref: /freebsd/crypto/openssl/doc/man3/HMAC.pod (revision e17f5b1d) 
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,
24                     int key_len, const unsigned char *d, int n,
25                     unsigned char *md, unsigned int *md_len);
26
27 HMAC_CTX *HMAC_CTX_new(void);
28 int HMAC_CTX_reset(HMAC_CTX *ctx);
29
30 int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
31                  const EVP_MD *md, ENGINE *impl);
32 int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
33 int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
34
35 void HMAC_CTX_free(HMAC_CTX *ctx);
36
37 int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx);
38 void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags);
39 const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);
40
41 size_t HMAC_size(const HMAC_CTX *e);
42
43Deprecated:
44
45 #if OPENSSL_API_COMPAT < 0x10100000L
46 int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
47               const EVP_MD *md);
48 #endif
49
50=head1 DESCRIPTION
51
52HMAC is a MAC (message authentication code), i.e. a keyed hash
53function used for message authentication, which is based on a hash
54function.
55
56HMAC() computes the message authentication code of the B<n> bytes at
57B<d> using the hash function B<evp_md> and the key B<key> which is
58B<key_len> bytes long.
59
60It places the result in B<md> (which must have space for the output of
61the hash function, which is no more than B<EVP_MAX_MD_SIZE> bytes).
62If B<md> is NULL, the digest is placed in a static array.  The size of
63the output is placed in B<md_len>, unless it is B<NULL>. Note: passing a NULL
64value for B<md>  to use the static array is not thread safe.
65
66B<evp_md> is a message digest such as EVP_sha1(), EVP_ripemd160() etc. HMAC does
67not support variable output length digests such as EVP_shake128() and
68EVP_shake256().
69
70HMAC_CTX_new() creates a new HMAC_CTX in heap memory.
71
72HMAC_CTX_reset() zeroes an existing B<HMAC_CTX> and associated
73resources, making it suitable for new computations as if it was newly
74created with HMAC_CTX_new().
75
76HMAC_CTX_free() erases the key and other data from the B<HMAC_CTX>,
77releases any associated resources and finally frees the B<HMAC_CTX>
78itself.
79
80The following functions may be used if the message is not completely
81stored in memory:
82
83HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use the hash
84function B<evp_md> and key B<key>. If both are NULL, or if B<key> is NULL
85and B<evp_md> is the same as the previous call, then the
86existing key is
87reused. B<ctx> must have been created with HMAC_CTX_new() before the first use
88of an B<HMAC_CTX> in this function.
89
90If HMAC_Init_ex() is called with B<key> NULL and B<evp_md> is not the
91same as the previous digest used by B<ctx> then an error is returned
92because reuse of an existing key with a different digest is not supported.
93
94HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash
95function B<evp_md> and the key B<key> which is B<key_len> bytes
96long.
97
98HMAC_Update() can be called repeatedly with chunks of the message to
99be authenticated (B<len> bytes at B<data>).
100
101HMAC_Final() places the message authentication code in B<md>, which
102must have space for the hash function output.
103
104HMAC_CTX_copy() copies all of the internal state from B<sctx> into B<dctx>.
105
106HMAC_CTX_set_flags() applies the specified flags to the internal EVP_MD_CTXs.
107These flags have the same meaning as for L<EVP_MD_CTX_set_flags(3)>.
108
109HMAC_CTX_get_md() returns the EVP_MD that has previously been set for the
110supplied HMAC_CTX.
111
112HMAC_size() returns the length in bytes of the underlying hash function output.
113
114=head1 RETURN VALUES
115
116HMAC() returns a pointer to the message authentication code or NULL if
117an error occurred.
118
119HMAC_CTX_new() returns a pointer to a new B<HMAC_CTX> on success or
120B<NULL> if an error occurred.
121
122HMAC_CTX_reset(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() and
123HMAC_CTX_copy() return 1 for success or 0 if an error occurred.
124
125HMAC_CTX_get_md() return the EVP_MD previously set for the supplied HMAC_CTX or
126NULL if no EVP_MD has been set.
127
128HMAC_size() returns the length in bytes of the underlying hash function output
129or zero on error.
130
131=head1 CONFORMING TO
132
133RFC 2104
134
135=head1 SEE ALSO
136
137L<SHA1(3)>, L<evp(7)>
138
139=head1 HISTORY
140
141HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL 1.1.0.
142
143HMAC_CTX_cleanup() existed in OpenSSL before version 1.1.0.
144
145HMAC_CTX_new(), HMAC_CTX_free() and HMAC_CTX_get_md() are new in OpenSSL 1.1.0.
146
147HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
148OpenSSL before version 1.0.0.
149
150=head1 COPYRIGHT
151
152Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.
153
154Licensed under the OpenSSL license (the "License").  You may not use
155this file except in compliance with the License.  You can obtain a copy
156in the file LICENSE in the source distribution or at
157L<https://www.openssl.org/source/license.html>.
158
159=cut
160