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# Integrity checking

The PSKC format allows for message authentication and integrity checking for some of the values stored within the PSKC file.

Integrity checking is done transparently when accessing attributes that are encrypted and contain a ValueMAC.

Once the PSKC encryption key has been set up, key values can be explicitly checked using the `check()` method:

    >>> pskc = PSKC('somefile.pskcxml')
    >>> pskc.encryption.derive_key('qwerty')
    >>> pskc.mac.algorithm
    'http://www.w3.org/2000/09/xmldsig#hmac-sha1'
    >>> all(key.check() for key in pskc.keys)
    True

## The MAC class

class `pskc.mac.``MAC`
  `algorithm`

    The name of the MAC algorithm to use (currently `HMAC-MD5`, `HMAC-SHA1`, `HMAC-SHA224`, `HMAC-SHA256`, `HMAC-SHA384` and `HMAC-SHA512` are supported).

  `key`

    For HMAC checking, this contains the binary value of the MAC key. The MAC key is generated specifically for each PSKC file and encrypted with the PSKC encryption key, so the PSKC file should be decrypted first (see PSKC encryption).

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