Hash-Based Message Authentication Code (HMAC)
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HMAC generates a MAC given a 16 byte (128-bit) symmetric key and a message.

**Functions**:

.. function:: HMACEval(key []byte, msg []byte) ([]byte, error)

    Compute a HMAC-SHA512 of the message.

    :param key: 16-byte symmetric MAC key
    :type key: []byte
    :param msg: Arbitrary-length message to HMAC
    :type msg: []byte

    :returns: 64-byte HMAC
    :rtype: []byte, error

.. function:: HMACEqual(a []byte, b []byte) (bool)

    Compare whether two MACs are the same in a constant-time manner.

    :param a: First MAC
    :type key: []byte
    :param b: Second MAC
    :type msg: []byte

    :rtype: bool