Re: [tcpm] tcp-auth-opt issue: support for NATs

At Thu, 07 Aug 2008 11:51:34 -0700,
Joe Touch wrote:
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> Eric Rescorla wrote:
> ...
> |> Supporting NATs can be done in two different ways:
> |
> | I think this is at least potentially an issue even aside from NAT. When
> | I install keys on my host, you're proposing that I need to configure
> | both the peer IP address and the key-id. This isn't necessarily
> | bad, but it does mean that if (for instance) the other side decides
> | to renumber, nothing will work absent manual reconfiguration. Is
> | that something people want?
> 
> <indiv hat on>
> If we ignore source IP addresses, we probably end up needing to go
> through a set of possible keys when a SYN arrives.

Well, not if you use unique key-ids.

> This seems like it
> amplifies the impact of a SYN attack - the more keys we have, the more
> each falsly-encrypted SYN costs.

Well, that's certainly true at some level, but if you use any
randomness at all in key-id assignment, you would expect to have
about N/256 key-ids where N is the number of keys. Unless N is
very large, the number of duplicate key-ids is likely to be high
enough that this doesn't seem like a very impressive attack,
especially since the attacker can force you to compute a MAC with
*every* packet he sends, not just SYNs.

> Alternately, a good KMS could figure out when these IP addresses
> changed, and update the TSAD accordingly. I would prefer this latter
> solution...

Yeah, but what does that say about manual key management, which is what
we're going to have for the foreseeable future.

-Ekr
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