[tor-dev] Tor with collective signatures

bancfc at openmailbox.org bancfc at openmailbox.org
Fri Jul 22 13:04:43 UTC 2016

On 2016-07-21 17:05, isis agora lovecruft wrote:
> Nicolas Gailly transcribed 59K bytes:
>> Hi,
>> Here's a new version of the proposal with some minor fixes discussed
>> with teor last time.
>> 0.4:
>>     - changed *included* to *appended*
>>     - 3.2: end of paragraph, a valid consensus document contains a 
>> majority
>>            of CoSi signatures.
>>     - Acknowledgments include teor and Tom Ritter.
>> As always, critics / feedbacks / thoughts are more than welcome :)
>> Thanks !
>> Nicolas
>> Ps: Our team and I are going to be at PETS this year, so if you don't
>> have time now to
>> read the whole thing, but you are still willing to know about CoSi and
>> how it could improve
>> Tor security, I/we will be happy to talk with some of you there also.
> Hello all,
> At PETS this afternoon, Nicolas Gailly, Philipp Jovanovic, Ismail 
> Khoffi,
> Georg Koppen, Nick Mathewson, and I met to discuss the collective 
> signing
> proposal.  I'm just going to breifly summarise the discussion here.
> One of the major concerns voiced was that, if we made it mandatory that 
> a
> collective signature on a consensus be verifiable (for some N number of
> signers, where N might be all of them but it's not important) for a 
> client to
> accept and use a consensus, then attacks upon the witnesses (or any 
> disruption
> to the witness signing system) will cause clients to no longer be able 
> to
> bootstrap.  Conversely, if we made it so that it only emitted some 
> warning
> when the collective signature could not be verified, then (likely) no 
> users
> would see this warning (or even if they did, they'd treat it in the 
> same
> manner as a TLS certificate warning and simply click through it).
> There is also concern that, with enforcing collective signatures, that 
> the Tor
> network has a larger attack surface w.r.t. (D)DoSing: an adversary 
> could DoS 5
> of the 9 DirAuths *or* they could DoS whatever necessary percentage of 
> the
> witness servers.  Additionally, an adversary who controls some portion 
> of the
> witness servers may DoS other witnesses in order to amplify the 
> relative
> proportion of the collective signature which they control.
> There was some discussion over whether to integrate this into core tor, 
> or
> rather to just use Nicolas' CoSi Golang tool in a separate process.  
> Everyone
> agreed that rewriting something from Go to C is suboptimal.
> One idea was if we used CoSi, but rather than "don't trust/use a 
> consensus if
> it doesn't have a good CoSi" we could use it as a mechanism for clients 
> to
> report (to some system somewhere? perhaps part of the prop#267 
> consensus
> transparency logs?) when CoSis don't verify successfully.

+1 this write-up. Besides sending stats, I still think its useful for 
the CoSi Go client to locally log its results so advanced users (who 
don't treat warnings like browser SSL errors) can take the necessary 
steps if they see they are attacked.

> Another idea was to use CoSi to sign the metadata file which Firefox's 
> updater
> uses to learn where to fetch updates so that a client would know that 
> the same
> Tor Browser updates were being served to other different vantage 
> points.
> Todo list:
>  1. It's not super necessary, but more analysis of the bandwidth 
> overhead for
>     running this protocol would be nice, i.e. network-wide overhead, 
> not just
>     the overhead for a single witness.
>  2. It would be nice to have some RFC-like description so that 
> alternate
>     implementations could be created, e.g. include encodings, state 
> machines,
>     message formats.  (We strive to maintain our specifications with 
> the
>     delusion that there are secretly hundreds of other tor 
> implementations in
>     every existing language, and that any of them should be compatible 
> if they
>     follow the specification.)
>  3. Update the proposal to mention that each DirAuth would have their 
> own
>     tree, thus the consensus document in the end would have somewhere 
> between
>     5 and 9 CoSi signatures.
>  4. There's a typo in ยง5.2: s/witnesse's/witnesses'/
> Thanks, everyone, for the great discussion!
> Best regards,
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