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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):
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