From d945a5213f2b2bbb189bbc2be407aa35e0dab204 Mon Sep 17 00:00:00 2001
From: Alex Gaynor <alex.gaynor@gmail.com>
Date: Sat, 5 Nov 2016 21:18:15 -0400
Subject: [PATCH] Fixes #3211 -- fixed hkdf's output with short length

Index: python-cryptography/cryptography/hazmat/primitives/kdf/hkdf.py
===================================================================
--- python-cryptography.orig/cryptography/hazmat/primitives/kdf/hkdf.py	2017-01-01 22:24:27.090828930 +0200
+++ python-cryptography/cryptography/hazmat/primitives/kdf/hkdf.py	2017-01-01 22:24:27.086828861 +0200
@@ -99,7 +99,7 @@
         output = [b""]
         counter = 1
 
-        while (self._algorithm.digest_size // 8) * len(output) < self._length:
+        while self._algorithm.digest_size * (len(output) - 1) < self._length:
             h = hmac.HMAC(key_material, self._algorithm, backend=self._backend)
             h.update(output[-1])
             h.update(self._info)
Index: python-cryptography/tests/hazmat/primitives/test_hkdf.py
===================================================================
--- python-cryptography.orig/tests/hazmat/primitives/test_hkdf.py	2017-01-01 22:24:27.090828930 +0200
+++ python-cryptography/tests/hazmat/primitives/test_hkdf.py	2017-01-01 22:24:27.086828861 +0200
@@ -152,6 +152,17 @@
 
             hkdf.verify(b"foo", six.u("bar"))
 
+    def test_derive_short_output(self, backend):
+        hkdf = HKDF(
+            hashes.SHA256(),
+            4,
+            salt=None,
+            info=None,
+            backend=backend
+        )
+
+        assert hkdf.derive(b"\x01" * 16) == b"gJ\xfb{"
+
 
 @pytest.mark.hmac
 class TestHKDFExpand(object):