I wouldn't expect outright successful attacks like forged replacements
for arbitrary files.
I would expect someone to have on hand a small number of blobs that are
different but have different hashes and, eventually, to drop said files
into a blob-based infrastructure to wreak havoc.
So: a way to locally mark a given checksum as "controversial" seems
prudent, to me (hence, support for such in my blob-db code/spec).
-t
On Fri, 29 Apr 2005, Tom Lord wrote:
I would expect someone to have on hand a small number of blobs that are
different but have different hashes and, eventually, to drop said files
into a blob-based infrastructure to wreak havoc.
This is just ridiculous. The number of known collisions in SHA1 is
*exactly zero* at this point in time --- not guaranteed to stay that way,
of course, but generating collisions is likely to remain relatively
expensive for some time. The collisions are highly structured; they are
not just arbitrary blobs. If, after doing your 2^69 work or so to
generate a real honest-to-goodness SHA-1 collision, you think an
attacker would "DROP THEM IN A REPOSITORY TO CREATE HAVOC"? You'd have to
break into the repository, etc, and then you'd find that *NOTHING
REFERENCED THEM* and so *ABSOLUTELY NOTHING WOULD HAPPEN*.
It's far more likely that SHA1 collisions will be used to generate forged
X509 certificates, for a number of highly technical reasons.
Git's highly constrained and derided 'brittle' file formats also serve
to protect against the collision attacks against SHA-1 which are beginning
to look possible.
So: a way to locally mark a given checksum as "controversial" seems
prudent, to me (hence, support for such in my blob-db code/spec).
Arguably that's what *upgrades* to the spec might be for -- git has a
solid philosophy of not creating 'features' unless it is sure that they
are needed/will be used, and I think this is always the wise route in
software development. Of much specification comes no code.
And, if you actually create a 'flexible' blob-db spec with 'room for
expansion' -- congratulations, you've just made yourself more vulnerable
to collision attacks.
--scott
terrorist MI5 SKILLET hack AMLASH security KMPLEBE KUFIRE SCRANTON
D5 SLBM LINCOLN KUDESK SMOTH Kojarena Moscow HTAUTOMAT WSBURNT Chechnya
( http://cscott.net/ )