Document and implement the public API for when the backend doesn't support the requested algorithm
diff --git a/docs/exceptions.rst b/docs/exceptions.rst
new file mode 100644
index 0000000..b391e62
--- /dev/null
+++ b/docs/exceptions.rst
@@ -0,0 +1,9 @@
+Exceptions
+==========
+
+.. currentmodule:: cryptography.exceptions
+
+.. class:: NoSuchAlgorithm
+
+ This is raised when a backend doesn't support the requested algorithm (or
+ combination of algorithms).
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index b8b1c83..48bad92 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -42,12 +42,21 @@
:class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
provider.
+ If the backend doesn't support the requested combination of ``cipher``
+ and ``mode`` a :class:`cryptography.exceptions.NoSuchAlgorithm` will
+ be raised.
+
.. method:: decryptor()
:return: A decrypting
:class:`~cryptography.hazmat.primitives.interfaces.CipherContext`
provider.
+ If the backend doesn't support the requested combination of ``cipher``
+ and ``mode`` a :class:`cryptography.exceptions.NoSuchAlgorithm` will
+ be raised.
+
+
.. currentmodule:: cryptography.hazmat.primitives.interfaces
.. class:: CipherContext
diff --git a/docs/index.rst b/docs/index.rst
index 4fd5d3b..eb30b5d 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -31,6 +31,7 @@
:maxdepth: 2
architecture
+ exceptions
contributing
security
community