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Date: Mon, 7 May 2012 10:27:51 +0300
From: Milen Rangelov <gat3way@...il.com>
To: john-dev@...ts.openwall.com
Subject: Re: New RAR OpenCL kernel

I think we can narrow the case further (that's what I did, still not sure
it's completely safe).

To understand why a bad stream returns long series of same values, you need
to have a look at the end of rar_decode_number() function. You can do one
more stricter check: if rar_decode_number() returns repeatedly
decode->DecodeNum[0], then it's "likely" a bad stream. Still I think it
might happen for some valid stream too. But I guess there must be some
threshold that can be considered relatively safe.

I think with ZIP we have somewhat similar situation - it is possible
(though very unlikely) that for a small file the verifier matches, file is
inflated correctly and checksum matches, yet password candidate is wrong.



On Mon, May 7, 2012 at 2:59 AM, magnum <john.magnum@...hmail.com> wrote:

> On 05/05/2012 02:09 AM, Milen Rangelov wrote:
> > Well, there are two cases here, a PPM block or a LZ block. The PPM case
> > would very quickly give you an error if stream is invallid and unpack29
> > would switch back to the pure LZSS case.
>
> Do you mean you modified something in unpack29 for this case? When I
> debugged it I got the impression it already rejects properly in this case.
>
> > Then for Lempel-Ziv  it's highly
> > unlikely that you'd get the same result from rar_decode_number()  several
> > times at a row unless the stream is bad. The key point here being
> "several"
> > and that's where  my concerns are.
>
> Nice. I tested a little and I've seen the same result at most 26 times
> in a row for a valid stream while bad streams can have several hundreds.
> The question is what threshold would be safe...
>
> I think I'll have a look at Jim's pkzip code again, maybe something in
> there is usable for the LZ case.
>
> magnum
>
>
>
> > On Sat, May 5, 2012 at 2:45 AM, magnum <john.magnum@...hmail.com> wrote:
> >
> >> On 05/04/2012 08:19 PM, Milen Rangelov wrote:
> >>> OK, my tests are all successful until now. Speed is relatively the
> same,
> >>> doesn't matter whether the compressed file is a 4KB text file, or a
> 50MB
> >>> avi movie :) Unfortunately there is still overhead as compared to the
> HE
> >>> case, speed being somewhat 20% slower. I am still not 100% whether this
> >>> would hold true in all cases and if I am wrong, it might happen that
> for
> >>> some archives we can get false negatives.
> >>>
> >>> I am not yet 100% convinced I am correct yet though. The approach is
> >> based
> >>> on 2 assumptions, first one can easily be proved, the second one is
> >> related
> >>> to the LZ algorithm and I cannot yet prove what I check is safe and
> can't
> >>> occur in a valid LZ stream.
> >>
> >> This is good news Milen, just what I hoped for. I am SURE there are such
> >> short-cuts because cRARk runs full speed no matter what you throw at it.
> >> Please do share your findings, you know I would.
> >>
> >> magnum
> >>
> >>
> >
>
>
>

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