Added backend check to twofactor primitives
diff --git a/docs/exceptions.rst b/docs/exceptions.rst
index 48c4bca..e5010eb 100644
--- a/docs/exceptions.rst
+++ b/docs/exceptions.rst
@@ -56,3 +56,9 @@
This is raised when the verify method of a one time password function's
computed token does not match the expected token.
+.. class:: UnsupportedInterface
+
+ .. versionadded:: 0.3
+
+ This is raised when the provided backend does not support the required
+ interface.
diff --git a/docs/hazmat/primitives/twofactor.rst b/docs/hazmat/primitives/twofactor.rst
index 3912d48..124d0ef 100644
--- a/docs/hazmat/primitives/twofactor.rst
+++ b/docs/hazmat/primitives/twofactor.rst
@@ -52,6 +52,9 @@
: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.UnsupportedInterface: This is raised if the
+ provided ``backend`` does not implement
+ :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
.. method:: generate(counter)
@@ -148,6 +151,9 @@
: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.UnsupportedInterface: This is raised if the
+ provided ``backend`` does not implement
+ :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
.. method:: generate(time)