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diff --git a/docs/hazmat/primitives/twofactor.rst b/docs/hazmat/primitives/twofactor.rst new file mode 100644 index 00000000..3912d483 --- /dev/null +++ b/docs/hazmat/primitives/twofactor.rst @@ -0,0 +1,162 @@ +.. hazmat:: + +Two-factor Authentication +========================= + +.. currentmodule:: cryptography.hazmat.primitives.twofactor + +This module contains algorithms related to two-factor authentication. + +Currently, it contains an algorithm for generating and verifying +one time password values based on Hash-based message authentication +codes (HMAC). + +.. currentmodule:: cryptography.hazmat.primitives.twofactor.hotp + +.. class:: HOTP(key, length, algorithm, backend) + + .. versionadded:: 0.3 + + HOTP objects take a ``key``, ``length`` and ``algorithm`` 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.primitives.twofactor.hotp import HOTP + >>> from cryptography.hazmat.primitives.hashes import SHA1 + >>> key = os.urandom(16) + >>> hotp = HOTP(key, 6, SHA1(), backend=default_backend()) + >>> hotp_value = hotp.generate(0) + >>> hotp.verify(hotp_value, 0) + + :param bytes key: Per-user secret key. This value must be kept secret + 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 HashAlgorithm algorithm: A + :class:`~cryptography.hazmat.primitives.hashes` + provider. + :param backend: A + :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` + provider. + :raises ValueError: This is raised if the provided ``key`` is shorter than + 128 bits or if the ``length`` parameter is not 6, 7 or 8. + :raises TypeError: This is raised if the provided ``algorithm`` is not + :class:`~cryptography.hazmat.primitives.hashes.SHA1()`, + :class:`~cryptography.hazmat.primitives.hashes.SHA256()` or + :class:`~cryptography.hazmat.primitives.hashes.SHA512()` or if the + ``length`` parameter is not an integer. + + .. 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 int counter: The counter value to validate against. + :raises cryptography.exceptions.InvalidToken: This is raised when the + supplied HOTP does not match the expected HOTP. + +Throttling +~~~~~~~~~~ + +Due to the fact that the HOTP algorithm generates rather short tokens that are +6 - 8 digits long, brute force attacks are possible. It is highly recommended +that the server that validates the token implement a throttling scheme that +locks out the account for a period of time after a number of failed attempts. +The number of allowed attempts should be as low as possible while still +ensuring that usability is not significantly impacted. + +Re-synchronization of the Counter +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The server's counter value should only be incremented on a successful HOTP +authentication. However, the counter on the client is incremented every time a +new HOTP value is requested. This can lead to the counter value being out of +synchronization between the client and server. + +Due to this, it is highly recommended that the server sets a look-ahead window +that allows the server to calculate the next ``x`` HOTP values and check them +against the supplied HOTP value. This can be accomplished with something +similar to the following code. + +.. code-block:: python + + def verify(hotp, counter, look_ahead): + assert look_ahead >= 0 + correct_counter = None + + otp = HOTP(key, 6, default_backend()) + for count in range(counter, counter + look_ahead): + try: + otp.verify(hotp, count) + correct_counter = count + except InvalidToken: + pass + + return correct_counter + +.. currentmodule:: cryptography.hazmat.primitives.twofactor.totp + +.. class:: TOTP(key, length, algorithm, time_step, backend) + + TOTP objects take a ``key``, ``length``, ``algorithm`` and ``time_step`` + parameter. The ``key`` should be randomly generated bytes and is recommended + to be as long as your hash function's output (e.g 256-bit for SHA256). + The ``length`` parameter controls the length of the generated one time + password and must be >= 6 and <= 8. + + This is an implementation of :rfc:`6238`. + + .. doctest:: + + >>> import os + >>> import time + >>> from cryptography.hazmat.backends import default_backend + >>> from cryptography.hazmat.primitives.twofactor.totp import TOTP + >>> from cryptography.hazmat.primitives.hashes import SHA1 + >>> key = os.urandom(16) + >>> totp = TOTP(key, 8, SHA1(), 30, backend=default_backend()) + >>> time_value = time.time() + >>> totp_value = totp.generate(time_value) + >>> totp.verify(totp_value, time_value) + + :param bytes key: Per-user secret key. This value must be kept secret + 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 HashAlgorithm algorithm: A + :class:`~cryptography.hazmat.primitives.hashes` + provider. + :param int time_step: The time step size. The recommended size is 30. + :param backend: A + :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` + provider. + :raises ValueError: This is raised if the provided ``key`` is shorter than + 128 bits or if the ``length`` parameter is not 6, 7 or 8. + :raises TypeError: This is raised if the provided ``algorithm`` is not + :class:`~cryptography.hazmat.primitives.hashes.SHA1()`, + :class:`~cryptography.hazmat.primitives.hashes.SHA256()` or + :class:`~cryptography.hazmat.primitives.hashes.SHA512()` or if the + ``length`` parameter is not an integer. + + .. method:: generate(time) + + :param int time: The time value used to generate the one time password. + :return bytes: A one time password value. + + .. method:: verify(totp, time) + + :param bytes totp: The one time password value to validate. + :param int time: The time value to validate against. + :raises cryptography.exceptions.InvalidToken: This is raised when the + supplied TOTP does not match the expected TOTP. |