On Jul 26, 2009, at 10:31 PM, Peter Gutmann wrote:

Jon Callas <j...@callas.org> writes:

You are of course correct, Peter, but are you saying that we shouldn't do
anything?

Well, I think it's necessary to consider the tradeoffs, if you don't know the other side's capabilities then it's a bit risky to assume that they're the
same as yours.

Let's look at it the other way, and suppose that I said that despite increases in processor power and distributed password crackers, we would leave the iteration count where it was in 1997, because if you increase it, it might go to small machines where that would be an issue. Consider the tradeoffs. You'd call me daft for refusing to protect the majority of people.


You are wrong with this.

*Messages* don't have this property, so long as they were encrypted to a
public key. It is unlocking the *key* that has this problem.

The data was encrypted using pre-shared secrets (i.e. packet type 3) which
does have this property.

(Don't ask me, I didn't create the requirements, I just got called in to help diagnose the problem, which was that at some point S2K's coming from PGP
Desktop were killing their embedded units.  Maybe they were even using
externally-generated private keys or who knows what rather than pre- shared secrets for messages, but whatever it was it was the S2K step that was causing
it).

Okay, password-protected files would get it, too. I won't ask why you're sending password protected files to an agent. I know you didn't design this.


That problem *only* exists when you import a key from a fast client into a slow client. That problem can be fixed either through some smart software (look at the iteration count and if it's higher than you like, change it the
next time you use the key), or the user can do it manually.

This doesn't work in a heterogeneous environment where the requirements will be something like messages having to comply with certain parts of the OpenPGP spec, and no more. Adding riders telling users how to manually configure individual applications doesn't work because end-users will never read the
technical spec, or even know that it exists.

I guess we could argue this point endlessly, but I really just brought it up to mention the unintended consequences of a particular design decision, and more generally the dangers of allowing unbounded integer and general data ranges in specs. Some implementations will enforce sensible limits, many won't (and will fail against fairly trivial attacks because of this), and without any guidance in the spec the ones that do take care to bound values are deemed non-compliant while the vulnerable ones that don't do any checking are deemed compliant. This is completely backwards for a security spec.

Sure, but. I think that "unintended consequences" is not quite the right way to put it. We don't intend to cause slow computers problems, but it was an intentional change with well-known upsides and downsides. Despite that, the upside seems to outweigh the downside.

This change shipped in September 2006. It's nearly three years old, and this would make only the second issue we had with it.

When it shipped, BlackBerries used signed math in computing the iteration count, and got it wrong. We made a BlackBerry export tool that reset the iteration count. That got fixed in 4.1 of the BB software, as I remember it.

So with millions helped and one field problem, it's not bad.

By the way, do you think it's safe to phase out MD5? That will break all the PGP 2 users.

        Jon




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