[tor-dev] Proposal for PoW DoS defenses during introduction (was Re: Proposal 305: ESTABLISH_INTRO Cell DoS Defense Extension)

Chelsea Holland Komlo me at chelseakomlo.com
Sun Jun 16 16:24:28 UTC 2019


Given the significant environmental impact of POW in other distributed
systems (blockchain), we should not implement schemes that solve a
problem for Tor but create problems for people elsewhere (potentially
irreversible environmental damage). 


Other less-destructive schemes exist to prevent DoS attacks. POW is a
method, not a goal in itself. Taking a step back and examining the full
spectrum of available tools would be better. 


On 2019-06-13 06:21, George Kadianakis wrote:
> juanjo <juanjo at avanix.es> writes:
>> Hello, this is my view of things, please be gentle as this is my first
>> proposal draft :)
> Hello,
> thanks for working on this. IMO any proof-of-work introduction proposal
> can be seen as orthogonal to David's prop305 which is a rate-limiting
> proposal (even tho it's not named as such) and hence deserves its own
> thread.
>> Client sends the INTRODUCE1 as normal.
>> Introduction Point checks the Current Requests Rate and checks the DoS
>> settings.
>> -DoS check is OK: send INTRODUCE2 to Hidden Service etc...
> So far so good (even tho this is not our usual proposal format).
>> -DoS settings/rate limit reached: then
>>      1.Introduction Point generates a random 8 bytes key (IPKey) and
>> associates it with the client circuit. Then send INTRODUCE_POW to the
>> Client with the IPKey.
> Is this a new cell? What's the format? Are these really keys or are they
> just nonces?
> IMO we should not do this through a new cell because that increases the
> round-trip by one. Instead we should just embed the PoW parameters in
> the onion service descriptor and clients find them there. 
>>      2.Client computes POW.
>>      Do{
>> Generates random 8 bytes key (ClientKey).
>> Generates hash(sha512/256 or sha3??) of
>> hash(IPKey + ClientKey)
>> } while (hash does not start with "abcde")
> That looks like a naive PoW scheme. It would perhaps be preferable to
> try to find a GPU/ASIC-resistant or memory-hard PoW scheme here, to
> minimize the advantage of adversaries with GPUs etc.?  Are there any
> good such schemes?
> Also services should definitely be able to configure the difficulty of
> the PoW, and IMO this should again happen through the descriptor.
>>      3. Client sends INTRODUCE_POWR to the I.P. with the generated POW
>> and the ClientKey.
> IMO this should happen as part of the INTRODUCE1 cell.
>>      4. I.P. checks the POW:
>>          -POW is correct: send INTRODUCE2 to HS.
>>          -POW is not correct: send INTRODUCE_POW_ERROR to client with
>> new IPKey.
>> *I say 8 bytes for the Keys just for example.
>> PROS AND CONS, who needs to update Tor version?:
>> --------------
>> Only rate limit: Introduction Point and Hidden Service. No breakage.
>> POW: Client, Introduction Point and Hidden Service. POW will break
>> compatibility with other v3 Hidden Services clients, if we implement a
>> way to bypass POW for old clients then this feature won't work as intended.
>> A forgotten guy here: Authenticated Rends cell: where we make sure the
>> Client established a connection to the Rend Point before requesting the
> Yep, that's yet another proposal (ticket #25066).
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