hmac.pod revision 306195 
1=pod
2
3=head1 NAME
4
5HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup,
6HMAC_cleanup - HMAC message authentication code
7
8=head1 SYNOPSIS
9
10 #include <openssl/hmac.h>
11
12 unsigned char *HMAC(const EVP_MD *evp_md, const void *key,
13               int key_len, const unsigned char *d, int n,
14               unsigned char *md, unsigned int *md_len);
15
16 void HMAC_CTX_init(HMAC_CTX *ctx);
17
18 int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
19               const EVP_MD *md);
20 int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
21               	   const EVP_MD *md, ENGINE *impl);
22 int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
23 int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
24
25 void HMAC_CTX_cleanup(HMAC_CTX *ctx);
26 void HMAC_cleanup(HMAC_CTX *ctx);
27
28=head1 DESCRIPTION
29
30HMAC is a MAC (message authentication code), i.e. a keyed hash
31function used for message authentication, which is based on a hash
32function.
33
34HMAC() computes the message authentication code of the B<n> bytes at
35B<d> using the hash function B<evp_md> and the key B<key> which is
36B<key_len> bytes long.
37
38It places the result in B<md> (which must have space for the output of
39the hash function, which is no more than B<EVP_MAX_MD_SIZE> bytes).
40If B<md> is NULL, the digest is placed in a static array.  The size of
41the output is placed in B<md_len>, unless it is B<NULL>.
42
43B<evp_md> can be EVP_sha1(), EVP_ripemd160() etc.
44
45HMAC_CTX_init() initialises a B<HMAC_CTX> before first use. It must be
46called.
47
48HMAC_CTX_cleanup() erases the key and other data from the B<HMAC_CTX>
49and releases any associated resources. It must be called when an
50B<HMAC_CTX> is no longer required.
51
52HMAC_cleanup() is an alias for HMAC_CTX_cleanup() included for back
53compatibility with 0.9.6b, it is deprecated.
54
55The following functions may be used if the message is not completely
56stored in memory:
57
58HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash
59function B<evp_md> and the key B<key> which is B<key_len> bytes
60long. It is deprecated and only included for backward compatibility
61with OpenSSL 0.9.6b.
62
63HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use the hash
64function B<evp_md> and key B<key>. If both are NULL (or B<evp_md> is the same
65as the previous digest used by B<ctx> and B<key> is NULL) the existing key is
66reused. B<ctx> must have been created with HMAC_CTX_new() before the first use
67of an B<HMAC_CTX> in this function. B<N.B. HMAC_Init() had this undocumented
68behaviour in previous versions of OpenSSL - failure to switch to HMAC_Init_ex()
69in programs that expect it will cause them to stop working>.
70
71B<NB: if HMAC_Init_ex() is called with B<key> NULL and B<evp_md> is not the
72same as the previous digest used by B<ctx> then an error is returned
73because reuse of an existing key with a different digest is not supported.>
74
75HMAC_Update() can be called repeatedly with chunks of the message to
76be authenticated (B<len> bytes at B<data>).
77
78HMAC_Final() places the message authentication code in B<md>, which
79must have space for the hash function output.
80
81=head1 RETURN VALUES
82
83HMAC() returns a pointer to the message authentication code or NULL if
84an error occurred.
85
86HMAC_Init_ex(), HMAC_Update() and HMAC_Final() return 1 for success or 0 if
87an error occurred.
88
89HMAC_CTX_init() and HMAC_CTX_cleanup() do not return values.
90
91=head1 CONFORMING TO
92
93RFC 2104
94
95=head1 SEE ALSO
96
97L<sha(3)|sha(3)>, L<evp(3)|evp(3)>
98
99=head1 HISTORY
100
101HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup()
102are available since SSLeay 0.9.0.
103
104HMAC_CTX_init(), HMAC_Init_ex() and HMAC_CTX_cleanup() are available
105since OpenSSL 0.9.7.
106
107HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
108versions of OpenSSL before 1.0.0.
109
110=cut
111