Re: [babel] Some open HMAC issues

[Juliusz Chroboczek <[e-mail]>]

>> But why is it needed? It's just one more thing to configure, which makes
>> configuration more verbose and prone to errors...

> Without a KeyID, the receiving complexity is proportional to the number of
> configured keys. That means that nodes configured with n keys not
> only spend O(n) times more CPU per received packet,

Where n is just the number of keys configured, and not something an
attacker can control (see Sections 1.1 and 4.3 of draft-do-babel-hmac).
In practice, I expect n to be usually just 1, and at most 2 during key
rotation.

Without an explicit KeyID, the set of keys is just a set -- it can be
implemented for example as a directory of keys, where adding a new key is
just an scp to the directory, or it can be implemented as a set of TLVs
flooded by HNCP, where adding a key consists in flooding a new TLV.

If you have an explicit KeyID, then you need to make sure that the KeyID
is consistent among all of the routers in a given security domain.  You
can no longer just scp the key, you need to pick a KeyID that is currently
unused.  If you're using an automated distribution protocol, then the
protocol must be able to choose a KeyID that is currently unused not only
by nodes that participate in said protocol, but also by nodes that don't.

Worse is better, David.  It is better to have a protocol that has a slight
inefficiency but is easy to deploy and to manage, than a protocol that is
slightly more efficient but that nobody will want to deploy because of the
very significant complications due to the KeyID.

-- Juliusz