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## 8.2 Working with MAC algorithms

To use most of these function it is necessary to create a context; this is done using:

- Function:
*gcry_error_t***gcry_mac_open***(gcry_mac_hd_t **`hd`, int`algo`, unsigned int`flags`, gcry_ctx_t`ctx`) -
Create a MAC object for algorithm

`algo`.`flags`may be given as an bitwise OR of constants described below.`hd`is guaranteed to either receive a valid handle or NULL.`ctx`is context object to associate MAC object with.`ctx`maybe set to NULL.For a list of supported algorithms, see See section Available MAC algorithms.

The flags allowed for

`mode`are:`GCRY_MAC_FLAG_SECURE`

Allocate all buffers and the resulting MAC in "secure memory". Use this if the MAC data is highly confidential.

In order to use a handle for performing MAC algorithm operations, a ‘key’ has to be set first:

- Function:
*gcry_error_t***gcry_mac_setkey***(gcry_mac_hd_t*`h`, const void *`key`, size_t`keylen`) -
Set the MAC key to the value of

`key`of length`keylen`bytes. With HMAC algorithms, there is no restriction on the length of the key. With CMAC algorithms, the length of the key is restricted to those supported by the underlying block cipher.

GMAC algorithms need initialization vector to be set, which can be performed with function:

- Function:
*gcry_error_t***gcry_mac_setiv***(gcry_mac_hd_t*`h`, const void *`iv`, size_t`ivlen`) -
Set the IV to the value of

`iv`of length`ivlen`bytes.

After you are done with the MAC calculation, you should release the resources by using:

- Function:
*void***gcry_mac_close***(gcry_mac_hd_t*`h`) -
Release all resources of MAC context

`h`.`h`should not be used after a call to this function. A`NULL`

passed as`h`is ignored. The function also clears all sensitive information associated with this handle.

Often you have to do several MAC operations using the same algorithm. To avoid the overhead of creating and releasing context, a reset function is provided:

- Function:
*gcry_error_t***gcry_mac_reset***(gcry_mac_hd_t*`h`) -
Reset the current context to its initial state. This is effectively identical to a close followed by an open and setting same key.

Note that gcry_mac_reset is implemented as a macro.

Now that we have prepared everything to calculate MAC, it is time to see how it is actually done.

- Function:
*gcry_error_t***gcry_mac_write***(gcry_mac_hd_t*`h`, const void *`buffer`, size_t`length`) -
Pass

`length`bytes of the data in`buffer`to the MAC object with handle`h`to update the MAC values.

The way to read out the calculated MAC is by using the function:

- Function:
*gcry_error_t***gcry_mac_read***(gcry_mac_hd_t*`h`, void *`buffer`, size_t *`length`) -
`gcry_mac_read`

returns the MAC after finalizing the calculation. Function copies the resulting MAC value to`buffer`of the length`length`. If`length`is larger than length of resulting MAC value, then length of MAC is returned through`length`.

To compare existing MAC value with recalculated MAC, one is to use the function:

- Function:
*gcry_error_t***gcry_mac_verify***(gcry_mac_hd_t*`h`, void *`buffer`, size_t`length`) -
`gcry_mac_verify`

finalizes MAC calculation and compares result with`length`bytes of data in`buffer`. Error code`GPG_ERR_CHECKSUM`

is returned if the MAC value in the buffer`buffer`does not match the MAC calculated in object`h`.

MAC algorithms are identified by internal algorithm numbers (see
`gcry_mac_open`

for a list). However, in most applications they are
used by names, so two functions are available to map between string
representations and MAC algorithm identifiers.

- Function:
*const char ****gcry_mac_algo_name***(int*`algo`) -
Map the MAC algorithm id

`algo`to a string representation of the algorithm name. For unknown algorithms this function returns the string`"?"`

. This function should not be used to test for the availability of an algorithm.

- Function:
*int***gcry_mac_map_name***(const char **`name`) -
Map the algorithm with

`name`to a MAC algorithm identifier. Returns 0 if the algorithm name is not known. This function should not be used to test for the availability of an algorithm.

To test whether an algorithm is actually available for use, the following macro should be used:

- Function:
*gcry_error_t***gcry_mac_test_algo***(int*`algo`) -
The macro returns 0 if the MAC algorithm

`algo`is available for use.

If the length of a message digest is not known, it can be retrieved using the following function:

- Function:
*unsigned int***gcry_mac_get_algo_maclen***(int*`algo`) -
Retrieve the length in bytes of the MAC yielded by algorithm

`algo`. This is often used prior to`gcry_mac_read`

to allocate sufficient memory for the MAC value. On error`0`

is returned.

- Function:
*unsigned int***gcry_mac_get_algo_keylen***(*`algo`) -
This function returns length of the key for MAC algorithm

`algo`. If the algorithm supports multiple key lengths, the default supported key length is returned. On error`0`

is returned. The key length is returned as number of octets.

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