Re: Val Henson's critique of hash-based content storage systems

["C. Scott Ananian" <cscott@cscott.net>]

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

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