bugtraq - Re: [HACKERS] Postgres: pg_hba.conf, md5, pg

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Date: Sat, 23 Apr 2005 15:53:25 +0100
From: Antoine Martin <antoine@...afix.co.uk>
To: Stephen Frost <sfrost@...wman.net>
Cc: Bruno Wolff III <bruno@...ff.to>, Tom Lane <tgl@....pgh.pa.us>,
	"Jim C. Nasby" <decibel@...ibel.org>, pgsql-hackers@...tgresql.org,
	bugtraq@...urityfocus.com
Subject: Re: [HACKERS] Postgres: pg_hba.conf, md5, pg_shadow, encrypted


On Sat, 2005-04-23 at 09:02 -0400, Stephen Frost wrote:
> * Antoine Martin (antoine@...afix.co.uk) wrote:
> > Basically, multiple input data that have the same output hash, which is
> > of no use when what you are trying to find is the input.
> > Finding collisions quicker for a known input is one thing, but that is
> > not going to reduce the search space, not even your storage space (it is
> > unlikely that the colliding results would all be valid input).
> 
> Erm, you aren't necessairly trying to find the input...  It may be the
> case that you're trying to find what you need to authenticate to this
> server, or any other PostgreSQL server where the same userid & input are
> used.  In that case you just need something that hashes to the same
> thing.
Agreed, what I said was that it is highly unlikely you will find
colliding inputs that are valid, so the "SHA weakness" does not really
help you here as it does not reduce the search space:
You are much better off pre-calculating hashes for possible usernames &
passwords than working backwards and generating all possible hashes
hoping that one would happen to be matching a real entry...
Usernames are not exactly random, passwords are less predictable, the
chance of a useful collision on the username+password is remote at best.
> Using a random salt would mean that it's different per server so
> breaking it on one doesn't help you against another server unless you
> happened to find the actual original input.
Absolutely.
> 
> > Is adding the non-guessable salt that hard anyway?
> 
> It is if you want to continue to support the 'md5' method in pg_hba.conf
> because the wireline protocol will probably need to change.  A less
> intrusive alternative would be to add an 'with encrypted password 'xyz'
> with random salt' or some such which would only be supported with the
> 'password' method in pg_hba.conf.
> 
> One problem with this is that you then can't just switch from password
> to md5 or back again.  Perhaps that's ok though?  Comments?
Just add another authentication method - call it 'md5-salt' (sharing
most of the 'md5' code), you get backwards compatibility and you advise
users to migrate to the new salt hash. Shouldn't be too hard...
Might as well do a 'sha512-salt' too.

Antoine