[tor-dev] Tor with collective signatures

[Nicolas Gailly]

Hi,

First, thanks a lot both of you for your in-depth comments, I know it takes time,
but they've been very helpful!

Then I mean to ask you about what you said regarding the fallback
directory keys. So keys are not embedded but only hashes, OK. As you
pointed out, CoSi would need to download full descriptors in order to
safely get the private keys of all the witnesses.
This solution would need to clearly be defined as you said in order to
be correct. However, there exists another solution and I'd like to know 
if it looks reasonable in the context of Tor .

=> Considering the two roles of Fallback directory mirrors are different
(serving the consensus document to Tor /  collectively signing on it),
we thought it would make sense to create new keys for this role and
embed those into the Tor source code. These keys would have to be
ed25519 keys (same size as the ID), so the size overhead
is minimal (32 bytes * 100 = 3.2KB vs 1.5MB). The only reason I can
think of why Tor includes hashes instead of public keys is because of
local storage: How does Tor consider an increase of storage need by 3.2KB ? Is
it acceptable or not ?

That would :

* Bring down the need for exchanging the descriptors between the
fallbacks / no more bandwidth /

* No complex scenario such as "fallback key download mechanism"

* Simplify the verification on the client side also (no need to get the
keys if you already have them).

* Add an additional steps for the relays operators where they would need
to create that key and inform the Tor people about their public key.

 Maybe you thought of this solution and there's a reason why you did not
mention it, or maybe you did not, then what do you think of it ?

Cheers,

Nicolas




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