On Fri, Jan 19, 2018 at 07:25:43PM +0100, Carl Eugen Hoyos wrote: > 2018-01-19 18:51 GMT+01:00 Michael Niedermayer <mich...@niedermayer.cc>: > > On Thu, Jan 18, 2018 at 09:28:40PM +0100, Carl Eugen Hoyos wrote: > >> Hi! > >> > >> The probe score for swf files (with uncompressed headers) is currently > >> very high after testing a little more than 24bit, attached patch > >> reduces the score. > > > > hmm > > the first 24 bits are tested and all but 2 values are rejected > > > > thats 23 bits that must match > > Ok. > > > then 4 values are tested which can be from 1 to 31 bits each > > the tests are each either == 0 or != 0 so they are a bit weak > > but its at least adding 4 bits that must match and also len > > I was sure this is not equivalent to four bits on a specific > position (but for the patch I assumed a "best case scenario" > where this actually were the case). > > > itself has to be smallish so we could argue that this gets us to > > about 28 bits > > > and then buf[3] is checked to be < 20 > > at this point we need about 32bits to be matching, still not huge but > > id think this is stronger than what file extensions prove which are > > MAX/2 > > We return MAX/2 for many "initial 32 bits" tests and I believe > we made good experience (and for MAX/2 the extension is > ignored or do I misremember?) >
> > The test is weakened by using 0 / not 0 / ascii as values though > > > > Have you seen an actual probe failure ? > > No, I was looking at another issue. we have many files with wrong extensions, its not uncommon so if we have no example of this failing then it is clearly a stronger check than file extension checking which is at score 50 so the score should be higher [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB The worst form of inequality is to try to make unequal things equal. -- Aristotle
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