[issue31560] hashlib.blake2: error in example for signed cookies

[Dmitry Chestnykh]

New submission from Dmitry Chestnykh:

The example for creating and verifying signed cookies with hashlib.blake2 
doesn't work. Corrected in GitHub PR #3694.

--
assignee: docs@python
components: Documentation
messages: 302787
nosy: dchest, docs@python
priority: normal
pull_requests: 3692
severity: normal
status: open
title: hashlib.blake2: error in example for signed cookies
versions: Python 3.7

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[issue26798] add BLAKE2 to hashlib

[Dmitry Chestnykh]

Dmitry Chestnykh added the comment:

> I have replaced verify() with compare_digest().

+>>> compare_digesty(cookie, '0102030405060708090a0b0c0d0e0f00')

Typo here. Also, this doesn't look like it compares the digest. Maybe you can 
keep the verify() function, but make it use compare_digest() -- this looks more 
clear to me:

>>> def verify(cookie, sig):
... good_sig = sign(cookie)
... return compare_digest(goodsig, sig)


> Python requires a C89 compatible compiler and 32bit architecture. C89 doesn't 
> mandate 64bit integers. As far as I remember there is (or was) one buildbot 
> with a compiler, that doesn't have 64 ints on an old SPARC system. All major 
> platforms have 64bit ints. I might modify the implementation when the patch 
> has landed.

Oh, I see. Thanks!

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[issue26798] add BLAKE2 to hashlib

[Dmitry Chestnykh]

Dmitry Chestnykh added the comment:

Christian: yes, and I'm also happy that you kept the drawing of hash tree, as 
it helps a lot with understanding of terminology.

I had a quick look at the patch and it looks good to me.

Some comments, which you can ignore:

In keyed hashing example there's:

+>>> def verify(cookie, sig):
+... good_sig = sign(cookie)
+... if len(sig) != len(good_sig):
+...  return False
+... # Use constant-time comparison to avoid timing attacks.
+... result = 0
+... for x, y in zip(sig, good_sig):
+... result |= ord(x) ^ ord(y)
+... return result == 0

Perhaps, you can replace comparison with hmac.compare_digest(sig, goodsig) now 
that we have it in Python.

+*Salted hashing* (or just hashing) with BLAKE2 or any other general-purpose
+cryptographic hash function, such as SHA-256, is not suitable for hashing
+passwords.  See `BLAKE2 FAQ <https://blake2.net/#qa>`_ for more
+information.

Maybe also point readers to hashlib.html#key-derivation


+On platforms with support for 64bit integer types (some 32bit platforms,
+all 64bit platforms), blake2b and blake2s are supported.

Theoretically, blake2s shouldn't require 64bit int, reference code only uses it 
for sizes -- is this something worth fixing? Are there platforms that have 
uint32_t, but not uint64_t?


@@ -162,7 +192,7 @@ class HashLibTestCase(unittest.TestCase):
 for cons in self.hash_constructors:
 h = cons()
 self.assertIsInstance(h.name, str)
-self.assertIn(h.name, self.supported_hash_names)
+#self.assertIn(h.name, self.supported_hash_names)
 self.assertEqual(h.name, hashlib.new(h.name).name)


Was this commented-out on purpose? Also, in setup.py:

+#if os.uname().machine == "x86_64":
+# Every x86_64 machine has at least SSE2.
+#blake2_macros.append(('BLAKE2_USE_SSE', '1'))

--
nosy: +dchest

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