Add X963KDF from ANSI X9.63:2001

The implemention allows the use a Hash function to implement a KDF very
similar to ConcatKDFHash, just different enough to require a separate
derivation function.

Closes #2203

Signed-off-by: Simo Sorce <simo@redhat.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/src/cryptography/hazmat/primitives/kdf/x963kdf.py b/src/cryptography/hazmat/primitives/kdf/x963kdf.py
new file mode 100644
index 00000000..83789b31
--- /dev/null
+++ b/src/cryptography/hazmat/primitives/kdf/x963kdf.py
@@ -0,0 +1,70 @@
+# This file is dual licensed under the terms of the Apache License, Version
+# 2.0, and the BSD License. See the LICENSE file in the root of this repository
+# for complete details.
+
+from __future__ import absolute_import, division, print_function
+
+import struct
+
+from cryptography import utils
+from cryptography.exceptions import (
+    AlreadyFinalized, InvalidKey, UnsupportedAlgorithm, _Reasons
+)
+from cryptography.hazmat.backends.interfaces import HashBackend
+from cryptography.hazmat.primitives import constant_time, hashes
+from cryptography.hazmat.primitives.kdf import KeyDerivationFunction
+
+
+def _int_to_u32be(n):
+    return struct.pack('>I', n)
+
+
+@utils.register_interface(KeyDerivationFunction)
+class X963KDF(object):
+    def __init__(self, algorithm, length, sharedinfo, backend):
+
+        max_len = algorithm.digest_size * (2 ** 32 - 1)
+        if length > max_len:
+            raise ValueError(
+                "Can not derive keys larger than {0} bits.".format(max_len))
+        if not (sharedinfo is None or isinstance(sharedinfo, bytes)):
+            raise TypeError("sharedinfo must be bytes.")
+        self._algorithm = algorithm
+        self._length = length
+        self._sharedinfo = sharedinfo
+
+        if not isinstance(backend, HashBackend):
+            raise UnsupportedAlgorithm(
+                "Backend object does not implement HashBackend.",
+                _Reasons.BACKEND_MISSING_INTERFACE
+            )
+        self._backend = backend
+        self._used = False
+
+    def derive(self, key_material):
+        if self._used:
+            raise AlreadyFinalized
+        self._used = True
+
+        if not isinstance(key_material, bytes):
+            raise TypeError("key_material must be bytes.")
+
+        output = [b""]
+        outlen = 0
+        counter = 1
+
+        while self._length > outlen:
+            h = hashes.Hash(self._algorithm, self._backend)
+            h.update(key_material)
+            h.update(_int_to_u32be(counter))
+            if self._sharedinfo is not None:
+                h.update(self._sharedinfo)
+            output.append(h.finalize())
+            outlen += len(output[-1])
+            counter += 1
+
+        return b"".join(output)[:self._length]
+
+    def verify(self, key_material, expected_key):
+        if not constant_time.bytes_eq(self.derive(key_material), expected_key):
+            raise InvalidKey