[tor-dev] Looking up bridges in Globe et al. by fingerprint

[Karsten Loesing]

On 08/02/14 20:39, Roger Dingledine wrote:

Thanks for your reply.  This is very helpful!  Replying to your response 
inline and summarizing the design once again at the end of this email.

> On Mon, Feb 03, 2014 at 11:17:19AM +0100, Karsten Loesing wrote:
>>   1. Globe runs full fingerprints through SHA-1 on the server.
>>     - The Globe server learns a lot of bridge fingerprints, which
>> makes it a juicy target, because bridge fingerprints can (in the
>> future?) be used to fetch bridge descriptors containing bridge
>> addresses.
>>     + This variant is really easy to implement.
>
> This one sounds good to me. You could even check both the fingerprint
> you get and its SHA1, so people who get the Javascript won't be sharing
> their bridge fingerprint with you, and then this issue is limited to
> the folks who don't want to run javascript.

Good idea, I guess that's doable.

> (I had no idea that Globe was doing this Javascript trick on the client
> side, so I figured it was a straightforward security decision on my part,
> "is it worthwhile to look up this bridge fingerprint, even though it
> tells Globe about the fingerprint?" And I think that's a decision that
> most people are able to make.)

Agreed.

> (Also, you say 'in the future?' above, but I think it can happen right
> now -- if you ask Tonga about a bridge fingerprint, it will give you
> the corresponding descriptor. Unless we disabled that on the Tonga side
> when I wasn't looking.)

I said "(in the future?)" because I wasn't sure about this.  But I just 
looked a bit closer at the code and found that Tonga only sends out 
bridge descriptors over _encrypted_ connections, that is, requests 
coming in via Tor circuits.  That's why I never observed Tonga giving 
out a descriptor when asking it using curl or wget.  Makes sense now!

>>   2. Globe does not run full fingerprints through SHA-1 on the server
>> and instead forwards everything to Onionoo.
>>     - Not only the Globe server, but also the Onionoo server learns
>> bridge fingerprints.
>
> Yeah, this doesn't seem like a good option.

OK.

>>     + We teach users that they shouldn't copy their original bridge
>> fingerprint into the Globe search window.  For example, we could
>> tell them the single line of Python they need to run their
>> fingerprint through SHA-1.  While they might have "spoiled" their
>> first bridge, they'll know for their other bridges.
>>     - Nobody will understand the above.
>
> Well, I guess technically we could offer this too -- that way smart
> users have the possibility to pre-SHA1 their fingerprint before doing
> the search. And if Globe checks both, then it should just work either
> way.

We can combine the idea of teaching users that they shouldn't tell us 
their original fingerprint with the idea above of hashing their 
fingerprint for them: whenever the Globe server sees a fingerprint, it 
hashes it and asks Onionoo for matches.  And if Onionoo returns a single 
bridge for exactly that hash, Globe knows that the fingerprint given by 
the user was not hashed and includes a warning.

> (Except, what if the users pre-SHA1 their fingerprint, but then
> they run the javascript that SHA1's it again? Does that mean Globe should
> check two hashes deep? Hrm.)

Globe doesn't have to check two hashes deep, but Onionoo already does 
that.  A common use case, AIUI, is that people search for their original 
fingerprint, see the bridge details page showing the fingerprint hash, 
copy that somewhere, and next time ask for the fingerprint hash.  And 
that has to return the same bridge already.

>>   3. Tor prints out the SHA-1 fingerprint to its logs or to a
>> hashed-fingerprint file.
>>     + Bridge users who know to read the fingerprint file can as well
>> read the hashed-fingerprint file or the logs.  We could even add a
>> link "How do I search for my bridge" to the Globe page and explain
>> to look out for that file or read the logs.
>>     - Only very few people will find that file or click the link.
>> We'll have to do either 1 or 2 in addition to 3.
>
> I guess I don't object to this option, but I think you're right that
> it isn't a big win.

Okay, let's consider it in the not-too-far-away future.

>>   4. Sanitized bridge descriptors contain original fingerprints.
>>     + It will be a lot easier to search for bridges if it works the
>> same way as searching for relays.
>>     + The sanitizing process will be easier.
>>     - We lose the option to enable bridge clients to update their
>> bridge descriptors by sending the bridge fingerprint to the bridge
>> authority.
>>     - This is going to keep me busy for a while for updating the
>> various metrics code that expects SHA-1 fingerprints rather than
>> original fingerprints.  I can do that though.
>
> That doesn't sound like a good tradeoff either.

Agreed.  This suggestion seemed like a better idea when I thought that 
it was just planned that Tonga returns bridge descriptors when asked for 
fingerprints.  It doesn't make as much sense anymore.


Summarizing the design:

1. Globe browser

- If JavaScript is available, the browser-part of Globe computes the 
SHA-1 of any 40 hex chars given as input and sends that to the Globe 
server.  It uses a different parameter than the non-JavaScript version 
of Globe though, say, "search2=<str>", so that Globe server can distinguish.

- If JavaScript is not available, Globe browser simply sends the search 
paramter as "search=<str>" to the Globe server.

- There should be a link "how do I search for my bridge", or similar, 
that explains how to find their bridge's fingerprint and run it through 
SHA-1 before searching for it in Globe.

2. Globe server

- If asked for "search2=<str>", the Globe server simply forwards the 
request to the Onionoo server.  Nothing to worry about here, Onionoo 
will handle the case of <hash> being a SHA-1 fingerprint (if the user 
put in their original fingerprint) or the SHA-1 of the SHA-1 fingerprint 
(if the user put in a fingerprint hash).

- If asked for "search=<str>", the Globe server performs the same check 
as the JavaScript Globe would have done.  If it detects 40 hex chars, it 
computes the SHA-1 and sends it to Onionooo.  Otherwise it simply 
forwards to Onionoo.  When Onionoo replies, Globe server checks if the 
response contains a single bridge with the hashed fingerprint being the 
SHA-1 that Globe server computed.  In this case the <str> part that it 
was given was a non-hashed fingerprint.  If so, Globe server includes a 
warning that users shouldn't put in their original fingerprint and 
suggests to use the fingerprint hash instead in the future.

1.b Globe browser

- In fact, the JavaScript version of Globe browser could perform the 
same check as Globe server when receiving a response.  It could teach 
users not to put in their original fingerprint, because who knows, maybe 
next time they're searching using a non-JavaScript browser.  Also, they 
might wonder why the displayed fingerprint is different from what they 
typed in.

3. Tor

- Once this whole fingerprint hash business is more common, because 
people are actually getting used to using their fingerprint hash in 
Globe, we discuss writing a hashed-fingerprint file and printing the 
hashed fingerprint to the logs on bridges.

Christian, does this make sense to you?  Shall we move these ideas to 
Trac tickets?


All in all, we're reducing the problem that Globe learns lots of bridge 
fingerprints to Globe users that a) don't use JavaScript, b) don't read 
the "how to search for my bridge" link, *and* c) disregard the warning 
that they are not supposed to put in non-hashed fingerprints for their 
second on further bridges.  Sounds like a good compromise.

Again, thanks for your input here, Roger!

All the best,
Karsten