[tor-relays] Snowflake PT

[Mirimir]

On 08/22/2018 08:03 PM, teor wrote:
> 
>> On 23 Aug 2018, at 11:22, Mirimir <mirimir at riseup.net> wrote:
>>
>>> On 08/22/2018 05:41 PM, teor wrote:
>>>
>>>> On 23 Aug 2018, at 10:16, Mirimir <mirimir at riseup.net> wrote:
>>>>
>>>> On 08/22/2018 04:17 PM, teor wrote:
>>>>>
>>>>> I don’t know about the current deployment plan for Snowflake, but I
>>>>> can point you to the relevant parts of the git repository:
>>>>>
>>>>>> On 22 Aug 2018, at 07:58, Nathaniel Suchy <me at lunorian.is> wrote:
>>>>>>
>>>>>> Tor Browser 8 Alpha includes the Snowflake PT as it comes near a final release, the adoption and usage of the Snowflake PT will continue to rise. I now have the following questions...
>>>>>>
>>>>>> 1) Will a command line tool like an obfs4proxy come out so those of us with infrastructure can run high capacity snowflake bridges.
>>>>>
>>>>> Like Meek, Snowflake is a 3-component transport:
>>>>>
>>>>> User -> Proxy -> Bridge
> 
> Ok, here's a list of all the connections that happen in Snowflake:
> 
> 1. Proxy -> Website to download the Proxy JS
> 2. Proxy -> Broker to register with the Broker
> 3. Client -> Broker to get a list of Proxies
> 3. Client -> Proxy -> Bridge to transmit data

Thanks. So #3 is the WebRTC connection. Obviously not through Tor, or
any other proxy.

>>>> I've read some of the Snowflake documentation. But I've found it
>>>> confusing.
>>>
>>> The FAQ explains the different components:
>>> https://github.com/keroserene/snowflake#faq
>>
>> Thanks. This in particular was helpful:
>>
>> | 1. Volunteers visit websites which host the "snowflake" proxy.
>> | (just like flashproxy)
>>
>> I don't recall seeing such a clear statement in other docs.
>>
>>>> I vaguely recall that Snowflake came up in a recent Tor
>>>> browser install.
>>>
>>> Yes, the *Snowflake client* is in the new Tor Browser alpha.
>>>
>>>> And I vaguely recall that there was an option to act as
>>>> a Snowflake proxy, via WebRTC. Is that true?
>>>
>>> Yes, volunteers on non-censored connections can run the *Snowflake proxy*.
>>
>> Wait. From that quote, it's websites that are hosting the snowflake
>> proxy. So are "volunteers" running a snowflake script, which is hosted
>> on the proxy website?
> 
> Yes
> 
>>> (Running a proxy in Tor Browser is not possible, because Tor Browser
>>> disables WebRTC.)
>>
>> OK. Which is a good thing. Because it's an external IP leak.
>>
>>>> And if so, what IP address
>>>> would be exposed? Would it be the IP address of the device running Tor
>>>> browser? That would be rather iffy. Almost like inviting users to run
>>>> relays, no? But perhaps I'm just confused.
>>>
>>> The Snowflake client connects to the Snowflake proxy.
>>>
>>> Snowflake uses the STUN WebRTC method, so clients and proxies discover
>>> each others’ external IP addresses.
>>
>> The use of "clients and proxies" is confusing here. The proxy is hosted
>> on some website, so having its external IP address exposed isn't at all
>> problematic. But what I suspect is that it's clients and _volunteers_
>> that discover each others’ external IP addresses. So basically, the
>> snowflake script is circumventing the WebRTC block in Tor browser.
> 
> The Snowflake Client is a separate executable with its own WebRTC
> stack. It performs a restricted set of network operations. It is
> launched by Tor, which is launched by Tor Browser.
> 
> Tor Browser blocks WebRTC so that hostile websites can't run
> JavaScript that makes arbitrary connections outside Tor,
> revealing the user's IP address and other identifying data to an
> adversary.
> 
> They're very different scenarios.
> 
>>> If Snowflake used the TURN method, then the TURN server would discover
>>> both addresses:
>>> https://gitweb.torproject.org/pluggable-transports/snowflake.git/tree/client/rendezvous.go#n141
>>
>> Sure. But the problem here, if I understand this correctly, is that
>> volunteers are sharing their external IP addresses with snowflake
>> clients. Is that correct?
> 
> I'm not sure which IP addresses you think should be secret:
> * the IP addresses of the Tor users running the snowflake client
>   (the threat to these tor users is the same as any other tor user,
>    see below)
> * the IP addresses of the non-tor browser users running the snowflake
>   proxy (the threat to these non-tor users is similar to browsing other
>   WebRTC websites)
> 
>> And yes, I get that you say "volunteers on non-censored connections".
>> But "non-censored" doesn't mean non-monitored.
>>
>> There really needs to be a prominent warning about this.
> 
> What should the warning say, and who is it for?

The warning should say that, by volunteering as a snowflake proxy,
you're exposing your public IP address to adversaries posing as
snowflake clients.

>> Many people use
>> Tor for privacy and ~anonymity, not just for circumventing censorship.
> 
> Tor keeps user IP addresses separate from the sites they're accessing. In
> particular, the exit and the remote site never learn user IP addresses.
> 
> But Tor does reveal user IP addresses to the entry points into the network,
> which can be guards, bridges, or the entry point(s) for pluggable transports.
> 
> Anyone can run a guard or a bridge or a snowflake proxy.
> 
> How does the snowflake proxy create a different threat model?

Sure, anyone can run guard and bridges, and log IPs of Tor users. But
they're both basically just relays. And, except for non-published
bridges, I gather that there are mechanisms for detecting malicious
behavior. Such as large blocks of related IPs.

But what about snowflake clients? What's to stop adversaries from using
swarms of snowflake clients to enumerate users offering WebTRC links as
snowflake proxies? Will the Broker block suspicious snowflake clients?

I do think that this is a clever way to circumvent censorship, by making
it easy for volunteers to run snowflake proxies. However, it's arguably
a major change in risk model for mainstream Tor users. Who might not
think through the implications of volunteering as snowflake proxies.

It's far less risky than running relays, of course, in that the Tor
Project won't publish their IP addresses, and only snowflake clients and
bridges will see their IP addresses. So they won't get abuse complaints,
and their IPs won't likely get blacklisted.

Still, their IPs might more readily be enumerated. And repressive
governments might go after them.