# 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

    A URI of the MAC algorithm used for message authentication. See the section Supported MAC algorithms below for a list of algorithm URIs.

    Assigned values to this attribute will be converted to the canonical URI for the algorithm if it is known. For instance, the value `HMAC-SHA-256` will automatically be converted into `http://www.w3.org/2001/04/xmldsig-more#hmac-sha256`.

  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).

  setup(...)

    Configure an encrypted MAC key for creating a new PSKC file.

    Parameters

      - algorithm (str) – encryption algorithm

      - key (bytes) – the encryption key to use

    None of the arguments are required. By default HMAC-SHA1 will be used as a MAC algorithm. If no key is configured a random key will be generated with the length of the output of the configured hash.

    This function will automatically be called when the configured encryption algorithm requires a message authentication code.

## Supported MAC algorithms

The module should support all HMAC algorithms that can be constructed from hash algorithms that are available in the standard Python `hashlib` module. At the least the following algorithms should be supported:

URI | Description
--- | ---
`http://www.w3.org/2001/04/xmldsig-more#hmac-md5` | MD5-based HMAC
`http://www.w3.org/2000/09/xmldsig#hmac-sha1` | SHA-1 based HMAC
`http://www.w3.org/2001/04/xmldsig-more#hmac-sha224` `http://www.w3.org/2001/04/xmldsig-more#hmac-sha256` `http://www.w3.org/2001/04/xmldsig-more#hmac-sha384` `http://www.w3.org/2001/04/xmldsig-more#hmac-sha512` | SHA-2 family based HMACs
`http://www.w3.org/2001/04/xmldsig-more#hmac-ripemd160` | RIPEMD-160 based HMAC