.. hazmat::

OATH
====

.. currentmodule:: cryptography.hazmat.oath

This module contains algorithms under the umbrella of the
Initiative for Open Authentication (OATH).

Currently, it contains an algorithm for generating and verifying
one time password values based on Hash-based message authentication
codes (HMAC).

.. currentmodule:: cryptography.hazmat.oath.hotp

.. class:: HOTP(key, length, backend)

    HOTP objects take a ``key`` and ``length`` parameter. The ``key``
    should be randomly generated bytes and is recommended to be 160 bits in
    length. The ``length`` parameter controls the length of the generated
    one time password and must be >= 6 and <= 8.

    This is an implementation of :rfc:`4226`.

    .. doctest::

        >>> import os
        >>> from cryptography.hazmat.backends import default_backend
        >>> from cryptography.hazmat.oath.hotp import HOTP

        >>> key = b"12345678901234567890"
        >>> hotp = HOTP(key, 6, backend=default_backend())
        >>> hotp.generate(0)
        '755224'
        >>> hotp.verify(b"755224", 0)

    :param bytes key: Secret key as ``bytes``. This value must be generated in a
                      cryptographically secure fashion and be at least 128 bits.
                      It is recommended that the key be 160 bits.
    :param int length: Length of generated one time password as ``int``.
    :param backend: A
        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
        provider.
    :raises ValueError: This is raised if the provided ``key`` is shorter 128 bits
                        or if the ``length`` parameter is not between 6 to 8.


    .. method:: generate(counter)

        :param int counter: The counter value used to generate the one time password.
        :return bytes: A one time password value.

    .. method:: verify(hotp, counter)

        :param bytes hotp: The one time password value to validate.
        :param bytes counter: The counter value to validate against.
        :raises cryptography.exceptions.InvalidToken: This is raised when the supplied HOTP
                                                      does not match the expected HOTP.