Date: Tue, 05 Oct 2004 08:29:19 +0200 From: Alex de Kruijff <freebsd@akruijff.dds.nl> To: Giorgos Keramidas <keramida@linux.gr> Cc: Colin Percival <cperciva@wadham.ox.ac.uk> Subject: Re: compare-by-hash (was Re: sharing /etc/passwd) Message-ID: <20041005062919.GE917@alex.lan> In-Reply-To: <20040928090551.GA1800@orion.daedalusnetworks.priv> References: <Pine.LNX.4.33.0111071900280.24824-100000@moroni.pp.asu.edu> <20011107211316.A7830@nomad.lets.net> <20040925140242.GB78219@gothmog.gr> <41575DFC.9020206@wadham.ox.ac.uk> <20040927091710.GC914@orion.daedalusnetworks.priv> <41582024.2080205@wadham.ox.ac.uk> <20040928090551.GA1800@orion.daedalusnetworks.priv>
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On Tue, Sep 28, 2004 at 12:05:51PM +0300, Giorgos Keramidas wrote: > On 2004-09-27 07:13, Colin Percival <cperciva@wadham.ox.ac.uk> wrote: > > Giorgos Keramidas wrote: > > >Increasing the number of bits the hash key uses will decrease the > > >possibility of a collision but never eliminate it entirely, AFAICT. > > > > How small does a chance of error need to be before you're willing to > > ignore it? > > That's a good question. I'm not sure I have a definitive answer, but > the possibility of a collision is indeed scary. Especially since I > haven't seen a study of the real probability of a collition is, given > the fact that passwords aren't (normally) random binary data but a > much smaller subset of the universe being hashed. I could be wrong but arn't hash values more random dan anything a user can in put. > > If an appropriately strong hash is used (eg, SHA1), then the probability > > of obtaining an incorrect /etc/*pwd.db with a correct hash is much > > smaller than the probability of a random incorrect password being > > accepted. Remember, passwords are stored by their MD5 hashes, so a > > random password has a 2^(-128) chance of working. > > I was probably being unreasonably paranoid about 'modified' passwords > that don't get detected as modified, but what you describe is also > true. You could simply scp these few files afther the rsync. There's files aren't that large. -- Alex Articles based on solutions that I use: http://www.kruijff.org/alex/FreeBSD/
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