Added backend check to twofactor primitives

2 files changed, 12 insertions, 0 deletions

diff --git a/docs/exceptions.rst b/docs/exceptions.rst
index 48c4bca8..e5010ebe 100644
--- a/docs/exceptions.rst
+++ b/docs/exceptions.rst
@@ -56,3 +56,9 @@ Exceptions
     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 3912d483..124d0ef5 100644
--- a/docs/hazmat/primitives/twofactor.rst
+++ b/docs/hazmat/primitives/twofactor.rst
@@ -52,6 +52,9 @@ codes (HMAC).
         :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 @@ similar to the following code.
         :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)