(gcrypt.info.gz) Hashing and MACing Subsystem Architecture

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 13.3 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.
 
 `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.
 

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(gcrypt.info.gz) Multi-Precision-Integer Subsystem Architecture