HOTP and TOTP should also have optional backends (#5402)

docs/hazmat/primitives/twofactor.rst

@@ -18,7 +18,7 @@ codes (HMAC).
 
 .. currentmodule:: cryptography.hazmat.primitives.twofactor.hotp
 
-.. class:: HOTP(key, length, algorithm, backend, enforce_key_length=True)
+.. class:: HOTP(key, length, algorithm, backend=None, enforce_key_length=True)
 
     .. versionadded:: 0.3
 
@@ -49,15 +49,15 @@ codes (HMAC).
     :param cryptography.hazmat.primitives.hashes.HashAlgorithm algorithm: A
         :class:`~cryptography.hazmat.primitives.hashes`
         instance.
-    :param backend: A
+    :param backend: An optional
         :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
+        If this flag is set to False, the application developer should
+        implement additional checks of the key length before passing it into
         :class:`~cryptography.hazmat.primitives.twofactor.hotp.HOTP`.
 
         .. versionadded:: 1.5
@@ -141,7 +141,7 @@ similar to the following code.
 
 .. currentmodule:: cryptography.hazmat.primitives.twofactor.totp
 
-.. class:: TOTP(key, length, algorithm, time_step, backend, enforce_key_length=True)
+.. class:: TOTP(key, length, algorithm, time_step, backend=None, enforce_key_length=True)
 
     TOTP objects take a ``key``, ``length``, ``algorithm`` and ``time_step``
     parameter. The ``key`` should be :doc:`randomly generated bytes
@@ -173,7 +173,7 @@ similar to the following code.
         :class:`~cryptography.hazmat.primitives.hashes`
         instance.
     :param int time_step: The time step size. The recommended size is 30.
-    :param backend: A
+    :param backend: An optional
         :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
         instance.
     :param enforce_key_length: A boolean flag defaulting to True that toggles

src/cryptography/hazmat/primitives/twofactor/hotp.py

@@ -9,6 +9,7 @@ import struct
 import six
 
 from cryptography.exceptions import UnsupportedAlgorithm, _Reasons
+from cryptography.hazmat.backends import _get_backend
 from cryptography.hazmat.backends.interfaces import HMACBackend
 from cryptography.hazmat.primitives import constant_time, hmac
 from cryptography.hazmat.primitives.hashes import SHA1, SHA256, SHA512
@@ -18,8 +19,9 @@ from cryptography.hazmat.primitives.twofactor.utils import _generate_uri
 
 class HOTP(object):
     def __init__(
-        self, key, length, algorithm, backend, enforce_key_length=True
+        self, key, length, algorithm, backend=None, enforce_key_length=True
     ):
+        backend = _get_backend(backend)
         if not isinstance(backend, HMACBackend):
             raise UnsupportedAlgorithm(
                 "Backend object does not implement HMACBackend.",

src/cryptography/hazmat/primitives/twofactor/totp.py

@@ -5,6 +5,7 @@
 from __future__ import absolute_import, division, print_function
 
 from cryptography.exceptions import UnsupportedAlgorithm, _Reasons
+from cryptography.hazmat.backends import _get_backend
 from cryptography.hazmat.backends.interfaces import HMACBackend
 from cryptography.hazmat.primitives import constant_time
 from cryptography.hazmat.primitives.twofactor import InvalidToken
@@ -19,9 +20,10 @@ class TOTP(object):
         length,
         algorithm,
         time_step,
-        backend,
+        backend=None,
         enforce_key_length=True,
     ):
+        backend = _get_backend(backend)
         if not isinstance(backend, HMACBackend):
             raise UnsupportedAlgorithm(
                 "Backend object does not implement HMACBackend.",