Begin designing the KDF interfaces. Fixes #511
diff --git a/cryptography/exceptions.py b/cryptography/exceptions.py
index 2654b45..e2542a1 100644
--- a/cryptography/exceptions.py
+++ b/cryptography/exceptions.py
@@ -38,3 +38,7 @@
class InternalError(Exception):
pass
+
+
+class InvalidKey(Exception):
+ pass
diff --git a/cryptography/hazmat/primitives/interfaces.py b/cryptography/hazmat/primitives/interfaces.py
index 293fcd7..1a27644 100644
--- a/cryptography/hazmat/primitives/interfaces.py
+++ b/cryptography/hazmat/primitives/interfaces.py
@@ -257,3 +257,19 @@
"""
The public exponent of the RSA key. Alias for public_exponent.
"""
+
+
+class KeyDerivationFunction(six.with_metaclass(abc.ABCMeta)):
+ @abc.abstractmethod
+ def derive(self, key_material):
+ """
+ Deterministically generates and returns a new key based on the existing
+ key material.
+ """
+
+ @abc.abstractmethod
+ def verify(self, key_material, expected_key):
+ """
+ Checks whether the key generated by the key material matches the
+ expected derived key. Raises an exception if they do not match.
+ """
diff --git a/docs/exceptions.rst b/docs/exceptions.rst
index 1fbd326..f9e29f3 100644
--- a/docs/exceptions.rst
+++ b/docs/exceptions.rst
@@ -30,3 +30,9 @@
This is raised when a backend doesn't support the requested algorithm (or
combination of algorithms).
+
+
+.. class:: InvalidKey
+
+ This is raised when the verify method of a key derivation function does not
+ compare equal.
diff --git a/docs/hazmat/primitives/interfaces.rst b/docs/hazmat/primitives/interfaces.rst
index bf78e36..ac48dd2 100644
--- a/docs/hazmat/primitives/interfaces.rst
+++ b/docs/hazmat/primitives/interfaces.rst
@@ -204,4 +204,34 @@
The public exponent. Alias for :attr:`public_exponent`.
+Key Derivation Functions
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. class:: KeyDerivationFunction
+
+ .. method:: derive(key_material)
+
+ :param key_material bytes: The raw key material. Depending on what key
+ derivation function you are using this could
+ be either random material, or a user
+ supplied password.
+ :return: The resulting key.
+
+ The generates and returns a new key from the supplied key material.
+
+ .. method:: verify(key_material, expected_key)
+
+ :param key_material bytes: The raw key material. This is the same as
+ ``key_material`` in :meth:`derive`.
+ :param expected_key bytes: What the expected result of deriving a new
+ key is.
+ :raises cryptography.exceptions.InvalidKey: This is raised when the
+ derived key does not match
+ the expected key.
+
+ This checks whether deriving a key from the supplied ``key_material``
+ generates the same key as the ``expected_key``, and raises an exception
+ if they do not match. This can be used for something like checking
+ whether a user's password attempt matches the stored derived key.
+
.. _`RSA`: http://en.wikipedia.org/wiki/RSA_(cryptosystem)