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Bitzi Developer Discussion: crc32 support
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crc32 support
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| one thing i'd like to see in bitzi/bitcollider is support for crc32 hashes. while they are rather short (32bits), they are widely used in irc/ftp filesharing (sfv files), and also often included in filenames. the actual hash algorithm IS already included in bitcllider (the SmallHash in ftuuhash), so all that'd be needed would be some glue there. i'd be willing to help with that, actually i've already hacked on it a bit, but couldn't get it to work yet (i'd say lack of a little docs on the inner workings of bitcollider... it crashes at 452: for(attr = attrList; attr->key; attr++) with (Attribute *) being invalid, but i never saw that before struct before...) so... would there be interest in this? (i.e. at least adding database side support?)
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-- somebody, December 30, 2002 02:15 am
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Re: crc32 support
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| As you note, we could add it very easily. I am a bit reluctant to promote its use, however, because CRC32 is not a "secure" checksum. It is trivial for anyone to twiddle any file to result in any desired CRC32 value, so it is only useful as a check against accidental corruption, not intentional. For example, if there is a 40MB file, "Snow White Theatrical Rerelease -- Free Preview Trailer.mpg", with hex CRC32 #F73E0831, any competent mischiefmaker could easily take a 40MB porn MPG, clip it to exactly the length of the Snow White trailer, and twiddle the bits until it also has the exact same CRC32 #F73E0831. The great thing about secure hashes (like SHA1 and TigerTree in the Bitzi Bitprint) is that such mischief isn't possible. Any 1-bit change in a file changes the hash completely, and you could twiddle it for a million years and never restore the secure hash to its unaltered value (except by changing the file contents back). Can you speak some more about the benefits of tracking CRC32s, even given this weakness?
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-- gojomo, January 16, 2003 02:25 pm
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Re: crc32 support
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| (lost the old password...)
well, i understand that a crc32 is a rather short/weak hash, as i said.
(as are, btw, ftuuhashes (which bitzi already supports), and those don't even protect against 'accidential' corruption, there are lots of ftuuhashes matching more than one bitprint (corrupted copies) in the database... dont have a example to post, but i could look up some)
the advantage in storing crc32s is, as i said, is that they are widely used, in ftp/irc filesharing, to protect against corruption.
often crc32s for file releases are available, so if bitzi would store crc32s, those could be matched against bitzi entries to sort out corrupted files.
think of cases like:
http://bitzi.com/search/targeted_by?targeted_by=2231678
if the entries had crc32 tags, and one could get a crc32 for the original file, it would be possible to match them.
ofcourse it would be better if everybody had used secure hashes from the start, and people should be encouraged to use them, but this would (at relatively low cost) add a some backward compatibilty to bitzi, and maybe get more users who used crc32s before to use the more secure hashes available.
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-- somebody1, January 16, 2003 08:50 pm
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Re: crc32 support
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| OK, we're all for backward-compatibility. How are the CRC values usually shared/displayed? As 8-digit hex values? Are they usually labelled/punctuated in any particular way?
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-- gojomo, January 16, 2003 09:22 pm
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Re: crc32 support
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| cool!
shared either in .sfv files (lists of filename<whitespace>crc32),
or just in filenames, maybe enclosed in [] or thelike,
and displayed always as 8 uppercase hex digits.
i don't know of any special labeling
btw, md5sums are also usually encoded in (lowercase) hex, and bitzi's base32 md5sums display is rather useless, as there isn't even a conversion utility around...
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-- somebody1, January 16, 2003 09:50 pm
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Re: crc32 support
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| CRC32 rulez! lol.. But I'm submitting them along the secure ones ofcourse. The more values, the better. Not including them is just saving some space and just a little of processing power. It's not less secure than the first 20 bytes or filelength and provides good backward-compatibility. Ofcourse it should be user-verified anyway. And it's ok if you trust the source. Sharereactor or your favorite anime group. I think everyone also should get some P2P certificate one day anyway. Hmm... I'll make an online conversion utility then... http://darkfader.net/toolbox/convert/
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-- darkfader, July 30, 2003 07:51 pm
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Re: crc32 support
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| Yes, although this thread is quite old, but the support for CRC32 cheksums via a checkbox for example would be much appreciated. Mostly all anime fansub groups now include CRC32 checksums nowadays, so looking up if an existing one is already there bitzi can replace the common winsfv and other tools if its just a single file and not a whole b atch which is probably faster checked with an .sfv file.
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-- coprophiliac, January 05, 2005 01:46 pm
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