### 17.3 Hashing and MACing Subsystem Architecture ¶

The interface to work with message digests and CRC algorithms is made up of functions from the `gcry_md_` name space. The implementation follows the open-use-close paradigm and uses registered algorithm modules for the actual work. Although CRC algorithms are not considered cryptographic hash algorithms, they share enough properties so that it makes sense to handle them in the same way. It is possible to use several algorithms at once with one context and thus compute them all on the same data.

The most important functions are:

`gcry_md_open`

  Create a new message digest instance and optionally enable one algorithm. A flag may be used to turn the message digest algorithm into a HMAC algorithm.

`gcry_md_enable`

  Enable an additional algorithm for the instance.

`gcry_md_setkey`

  Set the key for the MAC.

`gcry_md_write`

  Pass more data for computing the message digest to an instance.

  There is no strict alignment requirements for data, but the best performance can be archived if data is aligned to cacheline boundary.

`gcry_md_putc`

  Buffered version of `gcry_md_write` implemented as a macro.

`gcry_md_read`

  Finalize the computation of the message digest or HMAC and return the result.

`gcry_md_close`

  Release an instance

`gcry_md_hash_buffer`

  Convenience function to directly compute a message digest over a memory buffer without the need to create an instance first.

There are also functions to query properties of algorithms or the instance, like enabled algorithms, digest length, map algorithm names. It is also possible to reset an instance or to copy the current state of an instance at any time. Debug functions to write the hashed data to files are available as well.