™..

hmac - Online Linux Manual Page

Section : 3
Updated : 2013-02-11
Source : 1.0.1e
Note : OpenSSL

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);
​
​ int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
​               const EVP_MD *md);
​ int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
​                   const EVP_MD *md, ENGINE *impl);
​ int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
​ int 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

bytes at

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

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.

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.

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

, which must have space for the hash function output.

RETURN VALUES

HMAC

(

)

returns a pointer to the message authentication code or NULL if an error occurred.

HMAC_Init_ex(

)

HMAC_Update(

)

and

HMAC_Final(

)

return 1 for success or 0 if an error occurred.

HMAC_CTX_init(

)

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.

HMAC_Init_ex(

)

HMAC_Update(

)

and

HMAC_Final(

)

did not return values in versions of OpenSSL before 1.0.0.

Johanes Gumabo

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