Store MD5 + permissions for all files

[Ryan Dean Bair <ryandbair@gmail.com>]

I was brainstorming recently and thought it'd be useful if we stored
md5sums of all installed files and their default permissions. 

This information could be used in higher level tools such as apt to find
corrupt or tainted files and replace them with clean versions. This
would make recovering from an accidental rm -rf or chmod -R command or
damaged hard drives or filesystems much, much easier and could aid in
the removal of rootkits as well (provided the sums weren't tampered with
as well).  

I know there are existing tools that can do this, but they would require
being run after every new package installation or update, which can take
a long time. By integrating with dpkg, we could avoid this by providing
pre-computed sums or calculating sums on the fly for only new or updated
files. Integration would also give us the benefit of a more automated
file/permissions repair because system would know which packages would
need reinstalled to repair the damaged files.

So what are the thoughts on this? Something possible? I haven't touched
dpkg source code yet, but I do have a tad bit of experience in C(++).
Just wanted to get some initial opinions first.

Thanks