On Fri, Sep 06, 2013 at 03:08:54PM +0200, Tomas Mraz wrote:
On Fri, 2013-09-06 at 09:04 -0400, Matthew Miller wrote:
> The cracklib dicts in Fedora is 8.3M. (I'm sure some of this is my fault, as
> I've added to it over the years.) The cracklib pam module supports a
> compressed dictionary, but apparently it has a serious performance impact
> (
https://bugzilla.redhat.com/show_bug.cgi?id=1004896).
>
> Meanwhile, in many systems today, local passwords are entirely unused.
> Authentication is done via keys or by kerberos.
>
> At the same time, we have an increased need for smaller systems. That 8MB
> starts to be a meaningful fraction of a container or an ultra-small cloud
> image.
>
> I do recognize the value of protecting against dictionary-based attacks when
> passwords are used. Maybe we could have a policy which requires _longer_
> passwords but uses a much smaller dictionary?
The other option would be to fix the gzip support in cracklib to cache
the unpacked data somehow. However that would require to keep the
unpacked dictionary in RAM when cracklib is loaded, which is suboptimal
as well. Or we could make the cracklib-dicts optional somehow so it is
possible to install an ultra small cloud image without the dictionary at
all - I expect ultra small cloud image not needing password quality
checking at all.
As an unscientific test, I looked at how long it takes to decompress
the file pw_dict.pwd file with gzip
# gzip -c -9 /usr/share/cracklib/pw_dict.pwd > pw_dict.pwd.gz
# echo 2 > /proc/sys/vm/drop_caches
# time gunzip -c pw_dict.pwd.gz > pw_dict.pwd
real 0m0.112s
user 0m0.104s
sys 0m0.007s
IOW, AFAICT, decompression time of pw_dict.pwd should not even be
noticable, unless it is decompressing it multiple times during a
single password change operation ? Even then it would have to be
decompressing it at least 10 times to become really noticable.
Daniel
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