hmac.pod revision 325337 
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>. Note: passing a NULL
42value for B<md>  to use the static array is not thread safe.
43
44B<evp_md> can be EVP_sha1(), EVP_ripemd160() etc.
45
46HMAC_CTX_init() initialises a B<HMAC_CTX> before first use. It must be
47called.
48
49HMAC_CTX_cleanup() erases the key and other data from the B<HMAC_CTX>
50and releases any associated resources. It must be called when an
51B<HMAC_CTX> is no longer required.
52
53HMAC_cleanup() is an alias for HMAC_CTX_cleanup() included for back
54compatibility with 0.9.6b, it is deprecated.
55
56The following functions may be used if the message is not completely
57stored in memory:
58
59HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash
60function B<evp_md> and the key B<key> which is B<key_len> bytes
61long. It is deprecated and only included for backward compatibility
62with OpenSSL 0.9.6b.
63
64HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use the hash
65function B<evp_md> and key B<key>. If both are NULL (or B<evp_md> is the same
66as the previous digest used by B<ctx> and B<key> is NULL) the existing key is
67reused. B<ctx> must have been created with HMAC_CTX_new() before the first use
68of an B<HMAC_CTX> in this function. B<N.B. HMAC_Init() had this undocumented
69behaviour in previous versions of OpenSSL - failure to switch to HMAC_Init_ex()
70in programs that expect it will cause them to stop working>.
71
72B<NB: if HMAC_Init_ex() is called with B<key> NULL and B<evp_md> is not the
73same as the previous digest used by B<ctx> then an error is returned
74because reuse of an existing key with a different digest is not supported.>
75
76HMAC_Update() can be called repeatedly with chunks of the message to
77be authenticated (B<len> bytes at B<data>).
78
79HMAC_Final() places the message authentication code in B<md>, which
80must have space for the hash function output.
81
82=head1 RETURN VALUES
83
84HMAC() returns a pointer to the message authentication code or NULL if
85an error occurred.
86
87HMAC_Init_ex(), HMAC_Update() and HMAC_Final() return 1 for success or 0 if
88an error occurred.
89
90HMAC_CTX_init() and HMAC_CTX_cleanup() do not return values.
91
92=head1 CONFORMING TO
93
94RFC 2104
95
96=head1 SEE ALSO
97
98L<sha(3)|sha(3)>, L<evp(3)|evp(3)>
99
100=head1 HISTORY
101
102HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup()
103are available since SSLeay 0.9.0.
104
105HMAC_CTX_init(), HMAC_Init_ex() and HMAC_CTX_cleanup() are available
106since OpenSSL 0.9.7.
107
108HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
109versions of OpenSSL before 1.0.0.
110
111=cut
112