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Message-ID: <4F7D6D84.2060305@op5.se>
Date: Thu, 05 Apr 2012 12:01:40 +0200
From: Andreas Ericsson <ae@....se>
To: oss-security@...ts.openwall.com
CC: Marcus Meissner <meissner@...e.de>
Subject: Re: expat hash collision fix too predictable?
On 04/05/2012 11:30 AM, Marcus Meissner wrote:
> Hi,
>
> while reviewing a expat regression (likely caused by the hash collision denial of service fix, but unclear)
> i stumbled about the randomness it uses.
>
> static unsigned long
> generate_hash_secret_salt(void)
> {
> unsigned int seed = time(NULL) % UINT_MAX;
> srand(seed);
> return rand();
> }
>
> and it is seeded once at parser object creation.
>
> This is better than not seeding, but I am not sure if it is sufficient.
>
A pretty simple fix that makes it far better is to do
struct timeval tv;
unsigned int seed;
gettimeofday(&tv, NULL);
seed = (tv.tv_usec * 65531) % UINT_MAX;
srand(seed);
return rand();
The other option is ofcourse to not involve timestamps at all and
instead rely on a source with higher entropy, but this is usually
sufficient to make attacking it very unappealing. Especially when
considering that many xml docs contain a timestamp of when they were
generated, making the issue that much worse.
--
Andreas Ericsson andreas.ericsson@....se
OP5 AB www.op5.se
Tel: +46 8-230225 Fax: +46 8-230231
Considering the successes of the wars on alcohol, poverty, drugs and
terror, I think we should give some serious thought to declaring war
on peace.
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