Date: Mon, 07 May 2012 01:59:25 +0200 From: magnum <john.magnum@...hmail.com> To: john-dev@...ts.openwall.com Subject: Re: New RAR OpenCL kernel 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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