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.. 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).

This is raised when the verify method of a one time password function's computed token does not match the expected token.

.. currentmodule:: cryptography.hazmat.primitives.twofactor.hotp

.. versionadded:: 0.3

HOTP objects take a key, length and algorithm parameter. The key should be :doc:`randomly generated bytes </random-numbers>` and is recommended to be 160 :term:`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.

>>> 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(20)
>>> 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 :term:`bits`. It is recommended that the key be 160 bits.

param int length:

Length of generated one time password as int.

param cryptography.hazmat.primitives.hashes.HashAlgorithm algorithm:
 

A :class:`~cryptography.hazmat.primitives.hashes` instance.

param backend:

A :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` instance.

param enforce_key_length:

A boolean flag defaulting to True that toggles whether a minimum key length of 128 :term:`bits` is enforced. This exists to work around the fact that as documented in Issue #2915, the Google Authenticator PAM module by default generates 80 bit keys. If this flag is set to False, the application develop should implement additional checks of the key length before passing it into :class:`~cryptography.hazmat.primitives.twofactor.hotp.HOTP`.

.. versionadded:: 1.5

raises ValueError:

This is raised if the provided key is shorter than 128 :term:`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.

raises cryptography.exceptions.UnsupportedAlgorithm:
 

This is raised if the provided backend does not implement :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`

.. 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.hazmat.primitives.twofactor.InvalidToken: This
         is raised when the supplied HOTP does not match the expected HOTP.

.. method:: get_provisioning_uri(account_name, counter, issuer)

    .. versionadded:: 1.0

    :param account_name: The display name of account, such as
        ``'Alice Smith'`` or ``'alice@example.com'``.
    :type account_name: :term:`text`
    :param issuer: The optional display name of issuer. This is typically
        the provider or service the user wants to access using the OTP
        token.
    :type issuer: :term:`text` or `None`
    :param int counter: The current value of counter.
    :return: A URI string.

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.

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

TOTP objects take a key, length, algorithm and time_step parameter. The key should be :doc:`randomly generated bytes </random-numbers>` 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.

>>> 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(20)
>>> 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 :term:`bits`. It is recommended that the key be 160 bits.

param int length:

Length of generated one time password as int.

param cryptography.hazmat.primitives.hashes.HashAlgorithm algorithm:
 

A :class:`~cryptography.hazmat.primitives.hashes` instance.

param int time_step:

The time step size. The recommended size is 30.

param backend:

A :class:`~cryptography.hazmat.backends.interfaces.HMACBackend` instance.

param enforce_key_length:

A boolean flag defaulting to True that toggles whether a minimum key length of 128 :term:`bits` is enforced. This exists to work around the fact that as documented in Issue #2915, the Google Authenticator PAM module by default generates 80 bit keys. If this flag is set to False, the application develop should implement additional checks of the key length before passing it into :class:`~cryptography.hazmat.primitives.twofactor.totp.TOTP`.

.. versionadded:: 1.5
raises ValueError:

This is raised if the provided key is shorter than 128 :term:`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.

raises cryptography.exceptions.UnsupportedAlgorithm:
 

This is raised if the provided backend does not implement :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`

.. 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.hazmat.primitives.twofactor.InvalidToken: This
         is raised when the supplied TOTP does not match the expected TOTP.

.. method:: get_provisioning_uri(account_name, issuer)

    .. versionadded:: 1.0

    :param account_name: The display name of account, such as
        ``'Alice Smith'`` or ``'alice@example.com'``.
    :type account_name: :term:`text`
    :param issuer: The optional display name of issuer. This is typically
        the provider or service the user wants to access using the OTP
        token.
    :type issuer: :term:`text` or `None`
    :return: A URI string.

Provisioning URI

The provisioning URI of HOTP and TOTP is a feature of Google Authenticator and not actually part of the HOTP or TOTP RFCs. However, it is widely supported by web sites and mobile applications which are using Two-Factor authentication.

For generating a provisioning URI you can use the get_provisioning_uri method of HOTP/TOTP instances.

counter = 5
account_name = 'alice@example.com'
issuer_name = 'Example Inc'

hotp_uri = hotp.get_provisioning_uri(account_name, counter, issuer_name)
totp_uri = totp.get_provisioning_uri(account_name, issuer_name)

A common usage is encoding the provisioning URI into QR code and guiding users to scan it with Two-Factor authentication applications in their mobile devices.