Merge branch 'master' into urandom-engine

* master: (66 commits)
  Chanloge + versionadded
  Added an example usage
  Typo fix
  Added to toctree
  Rename and document
  Linkify the things we have that others don't
  add HKDF to changelog
  Strings have quote marks at both ends.
  HKDF example.
  Properly mark all test cases as dependant on HMAC.
  Remove language about the separate stages of HKDF until we expose multiple stages of HKDF.
  Don't forget InvalidKey.
  Fix typo
  Import exception classes instead of the exceptions module.
  Lose the bit about passwords.
  https a bunch of links.
  Pseudorandom is a word.
  Backtick the entire equation.
  Clarify salt language and link to the paper in addition to the RFC.
  Don't expose extract and expand on this class yet because we don't know how best to expose verify functionality, continue testing the stages using the private methods.
  ...

Conflicts:
	docs/hazmat/backends/openssl.rst

6 files changed, 156 insertions, 11 deletions

diff --git a/docs/hazmat/backends/commoncrypto.rst b/docs/hazmat/backends/commoncrypto.rst
index af2032b6..16a61337 100644
--- a/docs/hazmat/backends/commoncrypto.rst
+++ b/docs/hazmat/backends/commoncrypto.rst
@@ -11,7 +11,16 @@ The `CommonCrypto`_ C library provided by Apple on OS X and iOS.
 
 .. data:: cryptography.hazmat.backends.commoncrypto.backend
 
-    This is the exposed API for the CommonCrypto backend. It has one public attribute.
+    This is the exposed API for the CommonCrypto backend.
+
+    It implements the following interfaces:
+
+    * :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`
+
+    It has one additional public attribute.
 
     .. attribute:: name
 
diff --git a/docs/hazmat/backends/index.rst b/docs/hazmat/backends/index.rst
index dbc0724e..983a44e9 100644
--- a/docs/hazmat/backends/index.rst
+++ b/docs/hazmat/backends/index.rst
@@ -32,4 +32,5 @@ Individual Backends
 
     openssl
     commoncrypto
+    multibackend
     interfaces
diff --git a/docs/hazmat/backends/multibackend.rst b/docs/hazmat/backends/multibackend.rst
new file mode 100644
index 00000000..63177bef
--- /dev/null
+++ b/docs/hazmat/backends/multibackend.rst
@@ -0,0 +1,28 @@
+.. hazmat::
+
+MultiBackend
+============
+
+.. currentmodule:: cryptography.hazmat.backends.multibackend
+
+.. class:: MultiBackend(backends)
+
+    .. versionadded:: 0.2
+
+    This class allows you to combine multiple backends into a single backend
+    which offers the combined features of all of its constituents.
+
+    .. code-block:: pycon
+
+        >>> from cryptography.hazmat.backends.multibackend import MultiBackend
+        >>> from cryptography.hazmat.primitives import hashes
+        >>> backend1.hash_supported(hashes.SHA256())
+        False
+        >>> backend2.hash_supported(hashes.SHA1())
+        True
+        >>> multi_backend = MultiBackend([backend1, backend2])
+        >>> multi_backend.hash_supported(hashes.SHA1())
+        True
+
+    :param backends: A ``list`` of backend objects. Backends are checked for
+                     feature support in the order they appear in this list.
diff --git a/docs/hazmat/backends/openssl.rst b/docs/hazmat/backends/openssl.rst
index 1d40b93c..f7d6b710 100644
--- a/docs/hazmat/backends/openssl.rst
+++ b/docs/hazmat/backends/openssl.rst
@@ -9,6 +9,15 @@ The `OpenSSL`_ C library.
 
     This is the exposed API for the OpenSSL backend.
 
+    It implements the following interfaces:
+
+    * :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
+    * :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`
+
+    It also exposes the following:
+
     .. attribute:: name
 
         The string name of this backend: ``"openssl"``
diff --git a/docs/hazmat/primitives/interfaces.rst b/docs/hazmat/primitives/interfaces.rst
index 09a5a4ce..cbca5ed6 100644
--- a/docs/hazmat/primitives/interfaces.rst
+++ b/docs/hazmat/primitives/interfaces.rst
@@ -130,7 +130,13 @@ Asymmetric Interfaces
 
         The public exponent.
 
-    .. attribute:: key_length
+    .. attribute:: private_exponent
+
+        :type: int
+
+        The private exponent.
+
+    .. attribute:: key_size
 
         :type: int
 
@@ -152,7 +158,7 @@ Asymmetric Interfaces
 
         :type: int
 
-        The private exponent.
+        The private exponent. Alias for :attr:`private_exponent`.
 
     .. attribute:: n
 
@@ -179,7 +185,7 @@ Asymmetric Interfaces
 
         The public modulus.
 
-    .. attribute:: key_length
+    .. attribute:: key_size
 
         :type: int
 
diff --git a/docs/hazmat/primitives/key-derivation-functions.rst b/docs/hazmat/primitives/key-derivation-functions.rst
index 551dbd6d..1937c2ec 100644
--- a/docs/hazmat/primitives/key-derivation-functions.rst
+++ b/docs/hazmat/primitives/key-derivation-functions.rst
@@ -13,7 +13,8 @@ Different KDFs are suitable for different tasks such as:
 
     Deriving a key suitable for use as input to an encryption algorithm.
     Typically this means taking a password and running it through an algorithm
-    such as :class:`~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC` or HKDF.
+    such as :class:`~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC` or
+    :class:`~cryptography.hazmat.primitives.kdf.hkdf.HKDF`.
     This process is typically known as `key stretching`_.
 
 * Password storage
@@ -85,7 +86,7 @@ Different KDFs are suitable for different tasks such as:
 
     .. method:: derive(key_material)
 
-        :param key_material bytes: The input key material. For PBKDF2 this
+        :param bytes key_material: The input key material. For PBKDF2 this
             should be a password.
         :return bytes: the derived key.
         :raises cryptography.exceptions.AlreadyFinalized: This is raised when
@@ -98,9 +99,9 @@ Different KDFs are suitable for different tasks such as:
 
     .. method:: verify(key_material, expected_key)
 
-        :param key_material bytes: The input key material. This is the same as
+        :param bytes key_material: The input key material. This is the same as
                                    ``key_material`` in :meth:`derive`.
-        :param expected_key bytes: The expected result of deriving a new key,
+        :param bytes expected_key: The expected result of deriving a new key,
                                    this is the same as the return value of
                                    :meth:`derive`.
         :raises cryptography.exceptions.InvalidKey: This is raised when the
@@ -118,8 +119,99 @@ Different KDFs are suitable for different tasks such as:
         checking whether the password a user provides matches the stored derived
         key.
 
+
+.. currentmodule:: cryptography.hazmat.primitives.kdf.hkdf
+
+.. class:: HKDF(algorithm, length, salt, info, backend)
+
+    .. versionadded:: 0.2
+
+    `HKDF`_ (HMAC-based Extract-and-Expand Key Derivation Function) is suitable
+    for deriving keys of a fixed size used for other cryptographic operations.
+
+    .. doctest::
+
+        >>> import os
+        >>> from cryptography.hazmat.primitives import hashes
+        >>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
+        >>> from cryptography.hazmat.backends import default_backend
+        >>> backend = default_backend()
+        >>> salt = os.urandom(16)
+        >>> info = b"hkdf-example"
+        >>> hkdf = HKDF(
+        ...     algorithm=hashes.SHA256(),
+        ...     length=32,
+        ...     salt=salt,
+        ...     info=info,
+        ...     backend=backend
+        ... )
+        >>> key = hkdf.derive(b"input key")
+        >>> hkdf = HKDF(
+        ...     algorithm=hashes.SHA256(),
+        ...     length=32,
+        ...     salt=salt,
+        ...     info=info,
+        ...     backend=backend
+        ... )
+        >>> hkdf.verify(b"input key", key)
+
+    :param algorithm: An instance of a
+        :class:`~cryptography.hazmat.primitives.interfaces.HashAlgorithm`
+        provider.
+
+    :param int length: The desired length of the derived key. Maximum is
+        ``255 * (algorithm.digest_size // 8)``.
+
+    :param bytes salt: A salt. Randomizes the KDF's output. Optional, but
+        highly recommended. Ideally as many bits of entropy as the security
+        level of the hash: often that means cryptographically random and as
+        long as the hash output. Worse (shorter, less entropy) salt values can
+        still meaningfully contribute to security. May be reused. Does not have
+        to be secret, but may cause stronger security guarantees if secret; see
+        `RFC 5869`_ and the `HKDF paper`_ for more details. If ``None`` is
+        explicitly passed a default salt of ``algorithm.digest_size // 8`` null
+        bytes will be used.
+
+    :param bytes info: Application specific context information.  If ``None``
+        is explicitly passed an empty byte string will be used.
+
+    :params backend: A
+        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
+        provider.
+
+    .. method:: derive(key_material)
+
+        :param bytes key_material: The input key material.
+        :retunr bytes: The derived key.
+
+        Derives a new key from the input key material by performing both the
+        extract and expand operations.
+
+    .. method:: verify(key_material, expected_key)
+
+        :param key_material bytes: The input key material. This is the same as
+                                   ``key_material`` in :meth:`derive`.
+        :param expected_key bytes: The expected result of deriving a new key,
+                                   this is the same as the return value of
+                                   :meth:`derive`.
+        :raises cryptography.exceptions.InvalidKey: This is raised when the
+                                                    derived key does not match
+                                                    the expected key.
+        :raises cryptography.exceptions.AlreadyFinalized: This is raised when
+                                                          :meth:`derive` or
+                                                          :meth:`verify` is
+                                                          called more than
+                                                          once.
+
+        This checks whether deriving a new key from the supplied
+        ``key_material`` generates the same key as the ``expected_key``, and
+        raises an exception if they do not match.
+
 .. _`NIST SP 800-132`: http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf
 .. _`Password Storage Cheat Sheet`: https://www.owasp.org/index.php/Password_Storage_Cheat_Sheet
-.. _`PBKDF2`: http://en.wikipedia.org/wiki/PBKDF2
-.. _`scrypt`: http://en.wikipedia.org/wiki/Scrypt
-.. _`key stretching`: http://en.wikipedia.org/wiki/Key_stretching
+.. _`PBKDF2`: https://en.wikipedia.org/wiki/PBKDF2
+.. _`scrypt`: https://en.wikipedia.org/wiki/Scrypt
+.. _`key stretching`: https://en.wikipedia.org/wiki/Key_stretching
+.. _`HKDF`:
+.. _`RFC 5869`: https://tools.ietf.org/html/rfc5869
+.. _`HKDF paper`: https://eprint.iacr.org/2010/264