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Message-ID: <CABh=JRE+g+SWGR-7NpJAtYkyPiydxu+-07=FmdJWOiXV1Hsn+g@mail.gmail.com>
Date: Wed, 16 May 2012 01:23:39 +0300
From: Milen Rangelov <gat3way@...il.com>
To: john-dev@...ts.openwall.com
Subject: Re: RAR early reject
P.S by "no rar archiver" I mean two different winrar versions I use to
generate rar files - one rather old on a XP machine, and one recent one.
Both have the same limits on max order and memory to use for models. Still
I am not at all convinced this is always the case.
On Wed, May 16, 2012 at 1:21 AM, Milen Rangelov <gat3way@...il.com> wrote:
> True.
>
> If I understand correctly, it should never go beyond 63 as no rar archiver
> I've seen allows it. I think a similar check can be done for maxmb (winrar
> imposes a limit of 128MB). This helps. But we definitely need some early
> reject in the LZ case.
>
> On Wed, May 16, 2012 at 12:59 AM, magnum <john.magnum@...hmail.com> wrote:
>
>> On 05/15/2012 09:05 AM, Milen Rangelov wrote:
>> > I am reaching the same conclusion. I was able to do another check for
>> the
>> > PPM part, order should never be more than 63 (as this is the archiver
>> > limit) and this is safe I think.
>>
>> Do you mean max_order in ppm_decode_init() right after reading it from
>> rar_get_char()? Or did you test somewhere else?
>>
>> I also found from original unpack.cpp that filter_pos in add_vm_code()
>> should never be > 1024 but I haven't ever seen it happen. Maybe I'll
>> drop that test again even though it's there in the original code (for
>> some reason it was not in clamav's code). BTW I only recently realized
>> how easy it is to compare the original .cpp code with clamav's code.
>> They really just ported it. The original code has more comments.
>>
>> We are currently rejecting 93% of the data on average, compared to
>> reading all of the data each time. That may sound good, but for a 500 MB
>> file we still need to decrypt and read 3.6 MB on average.
>>
>> magnum
>>
>>
>
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