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From David White <dw11...@onemail.at>
Subject Re: PATCH: fix for asset service (branch-3-0 2005-03-16)
Date Mon, 21 Mar 2005 14:56:34 GMT
On Mon, 2005-03-21 at 09:11 -0500, Howard Lewis Ship wrote:
> >  All a CHF can do is confirm that the content that has
> been hashed is almost certainly identical with another piece of content
> that produced the same digest.
> 

> Exactly.  And that's all that's necessary.  That's a near perfect
> credential because it is non-trivial (i.e., virtually impossible) to
> generate the digest without the file. 

An UNIX box may be considered well and truly compromised if an attacker
gets access to the hashed passwords in /etc/shadow or /etc/passwd. The
same applies for the SAM in Windows. Why, if the CHF is truly one-way?
Because the bad guy can e.g. use a dictionary and end-user stupidity to
dramatically reduce the search space.

You ignored my argument about using the asset service to scan for
vulnerable Java classes or other resources. For an attacker, finding out
that a resource in www.someserver.com/TapestryAppContext is identical to
a resource that has been acquired by other means is useful information.
This attack is isomorphic to a dictionary attack on a password hash.

> The digest is long enough that
> you aren't going to guess it (not in the lifespan of the universe). As
> far as I understand it, this is exactly the kind of thing CHF are for.
> 

No, a CHF is most commonly used to ensure integrity of a message, thus
the name "message digest". The use of a CHF in an authentication routine
is deprecated when better mechanisms (e.g. Kerebos, smart cards,
challenge-response with asymmetric crypto, et. al.) are available. These
mechanisms may well use CHFs, but a CHF alone is *not* such a mechanism.

> You can overide the asset service in both 3.0 and 3.1 to fit your
> needs ... but all our energies are better spent elsewhere.
> 

The patch I sent serves the needs of the project I am working on. I
would be willing to contribute a further patch to handle clustered
environments. I also see value in supporting the use of a message digest
to verify content assets provided by untrusted sources.

Cordially,

David WHITE


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