New submission from Joachim Wagner:
(First time submitting a patch to this system.)
The hmac module uses a fixed blocksize of 64 bytes. This is fine for
many hash functions like md5, sha1 and sha256, but not for sha512 or
in the general case. The RFC referenced in the python documentation
specifies that the blocksize has to match the hash function. The
attached patch is the first of three proposed solutions:
1. use the undocumented block_size attribute of the hashing objects
provided in the hashlib modules and fallback to 64 bytes if the
attribute is missing (maybe a depreciated warning would be better); in
this case it would be a good idea to document to block_size attribute
(not included in the patch attached); performance could be improved by
making block_size a class attribute
2. document that the blocksize is 64 and that the RFC is only
correctly implemented if the hash function also has a blocksize of 64
bytes; optionally include the workaround to subclass hmac.HMAC and
overwrite the blocksize (this is documented in the source code, but
unfortunately not in the python docu)
3. make the blocksize a keyword argument to the constructor and
document that it has to match the hash function's blocksize for full
RFC compliance
Regards,
Joachim
----------
components: None
files: hmac_1.patch
messages: 57106
nosy: jowagner
severity: normal
status: open
title: hmac module violates RFC for some hash functions, e.g. sha512
type: behavior
versions: Python 3.0
Added file: http://bugs.python.org/file8689/hmac_1.patch
__________________________________
Tracker <[EMAIL PROTECTED]>
<http://bugs.python.org/issue1385>
__________________________________
--- hmac.orig 2007-11-04 17:44:46.000000000 +0000
+++ hmac.py 2007-11-04 18:31:39.000000000 +0000
@@ -48,7 +48,15 @@
self.inner = self.digest_cons()
self.digest_size = self.inner.digest_size
- blocksize = self.blocksize
+ try:
+ blocksize = self.digest_cons().block_size
+ if blocksize < 16:
+ # very low blocksize
+ # probably a legacy value like in Lib/sha.py
+ blocksize = self.blocksize
+ except AttributeError:
+ blocksize = self.blocksize
+
if len(key) > blocksize:
key = self.digest_cons(key).digest()
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