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Date: Wed, 09 May 2012 02:13:11 +0200
From: magnum <>
Subject: Re: New RAR OpenCL kernel

OK, then I might have got equivalent code now. I made 326 test rar files
and used a threshold of 3. All but 9 was cracked. Raised threshold to 4
and got 4 more. Raised to 5 and got all but one. That last one need a
threshold of 10. All these were pretty small files (*.[ch] from john's
src dir, the worst file was md5_eq.c rar'ed with -ptest)


On 05/08/2012 09:26 AM, Milen Rangelov wrote:
> I've never seen more than 3 of them in a row for a correct stream, yet
> having 2 or 3 in a row is quite common. I am doing all my checks in
> unpack29 though.
> On Mon, May 7, 2012 at 10:00 PM, magnum <> wrote:
>> On 05/07/2012 09:27 AM, Milen Rangelov wrote:
>>> 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 totally missed that one. Even if decode->DecodeNum[0] is valid several
>> times, I would *guess* this code is safe:
>> @@ -387,7 +387,7 @@ int rar_decode_number(unpack_data_t *unpack_data,
>> struct Decode *decode)
>>        rar_addbits(unpack_data, bits);
>> n=decode->DecodePos[bits]+((bit_field-decode->DecodeLen[bits-1])>>(16-bits));
>>        if (n >= decode->MaxNum) {
>> -               n=0;
>> +               return -1;
>>        }
>>        rar_dbgmsg("rar_decode_number return(%d)\n", decode->DecodeNum[n]);
>> ...and add code in read_tables() and rar_unpack29() so this -1
>> propagates to an immediate bailout. Seems to work fine but this alone
>> doesn't help performance as much as the threshold for repeated numbers.
>> BTW forget what I wrote seeing 26 of them, I did not reset the counter
>> properly. Besides, it depends on how/where you count them. I had the
>> check in read_tables() but it should be inside rar_decode_number()
>> itself (again returning -1 for bailout) because there are more callers.
>> magnum
>>> 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 <> 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 <>
>> 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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