/xlv3/openssl/0.9.7e-sgipl1/work/0.9.7e-sgipl1/openssl-
0.9.7e/doc/crypto
Page 1 (printed 10/20/05)
hmac(3) 18/Jul/2002 (0.9.7e) hmac(3)
NAME
HMAC, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_cleanup -
HMAC message authentication code
SYNOPSIS
#include <openssl/hmac.h>
unsigned char *HMAC(const EVP_MD *evp_md, const void *key,
int key_len, const unsigned char *d, int n,
unsigned char *md, unsigned int *md_len);
void HMAC_CTX_init(HMAC_CTX *ctx);
void HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md);
void HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md);
void HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
void HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
void HMAC_CTX_cleanup(HMAC_CTX *ctx);
void HMAC_cleanup(HMAC_CTX *ctx);
DESCRIPTION
HMAC is a MAC (message authentication code), i.e. a keyed
hash function used for message authentication, which is
based on a hash function.
HMAC() computes the message authentication code of the n
bytes at d using the hash function evp_md and the key key
which is key_len bytes long.
It places the result in md (which must have space for the
output of the hash function, which is no more than
EVP_MAX_MD_SIZE bytes). If md is NULL, the digest is placed
in a static array. The size of the output is placed in
md_len, unless it is NULL.
evp_md can be EVP_sha1(), EVP_ripemd160() etc. key and
evp_md may be NULL if a key and hash function have been set
in a previous call to HMAC_Init() for that HMAC_CTX.
HMAC_CTX_init() initialises a HMAC_CTX before first use. It
must be called.
HMAC_CTX_cleanup() erases the key and other data from the
HMAC_CTX and releases any associated resources. It must be
called when an HMAC_CTX is no longer required.
HMAC_cleanup() is an alias for HMAC_CTX_cleanup() included
for back compatibility with 0.9.6b, it is deprecated.
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hmac(3) 18/Jul/2002 (0.9.7e) hmac(3)
The following functions may be used if the message is not
completely stored in memory:
HMAC_Init() initializes a HMAC_CTX structure to use the hash
function evp_md and the key key which is key_len bytes long.
It is deprecated and only included for backward
compatibility with OpenSSL 0.9.6b.
HMAC_Init_ex() initializes or reuses a HMAC_CTX structure to
use the function evp_md and key key. Either can be NULL, in
which case the existing one will be reused. HMAC_CTX_init()
must have been called before the first use of an HMAC_CTX in
this function. N.B. HMAC_Init() had this undocumented
behaviour in previous versions of OpenSSL - failure to
switch to HMAC_Init_ex() in programs that expect it will
cause them to stop working.
HMAC_Update() can be called repeatedly with chunks of the
message to be authenticated (len bytes at data).
HMAC_Final() places the message authentication code in md,
which must have space for the hash function output.
RETURN VALUES
HMAC() returns a pointer to the message authentication code.
HMAC_CTX_init(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final()
and HMAC_CTX_cleanup() do not return values.
CONFORMING TO
RFC 2104
SEE ALSO
sha(3), evp(3)
HISTORY
HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and
HMAC_cleanup() are available since SSLeay 0.9.0.
HMAC_CTX_init(), HMAC_Init_ex() and HMAC_CTX_cleanup() are
available since OpenSSL 0.9.7.
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