[tor-commits] [community/master] Rewrote and reviewed cloudflare captcha monitoring project

[pili at torproject.org]

commit fdb8ba63730ccbf9c1debfbae46f78030d11b7f8
Author: Pili Guerra <pili at piliguerra.com>
Date:   Tue Feb 4 11:29:13 2020 +0100

    Rewrote and reviewed cloudflare captcha monitoring project
---
 .../gsoc/cloudflare-captcha-monitoring/contents.lr | 41 +++++++++++++++-------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/content/gsoc/cloudflare-captcha-monitoring/contents.lr b/content/gsoc/cloudflare-captcha-monitoring/contents.lr
index c5daeb4..332ca2a 100644
--- a/content/gsoc/cloudflare-captcha-monitoring/contents.lr
+++ b/content/gsoc/cloudflare-captcha-monitoring/contents.lr
@@ -25,26 +25,43 @@ title: Cloudflare Captcha Monitoring
 ---
 subtitle:
 
-We should track the rate that cloudflare gives captchas to Tor users over time.
+This project should implement a mechanism to track the rate that Cloudflare fronted webpages return captchas to Tor users over time.
 
 ---
 body:
 
-My suggested way of doing that tracking is to sign up a very simple static webpage to be fronted by cloudflare, and then fetch it via Tor over time, and record and graph the rates of getting a captcha vs getting the real page.
+# Problem
 
-The reason for the "simple static page" is to make it really easy to distinguish whether we're getting hit with a captcha. The "distinguishing one dynamic web page from another" challenge makes exitmap tricky in the general case, but we can remove that variable here.
+A large number of Tor users report getting hit by infinite captcha loops when visiting webpages fronted by Cloudflare. This makes them feel punished for using Tor to protect their privacy and prevents them from legitimately accessing websites.
 
-One catch is that Cloudflare currently gives alt-svc headers in response to fetches from Tor addresses. So that means we need a web client that can follow alt-srv headers -- maybe we need a full Selenium like client?
+# Proposal
 
-Once we get the infrastructure set up, we would be smart to run a second one which is just wget or curl or lynx or something, i.e. which doesn't behave like Tor Browser, in order to be able to track the difference between how Cloudflare responds to Tor Browser vs other browsers.
+For this project we would like to track in practice how often Cloudflare fronted webpages return captchas to Tor clients.
 
-I imagine that Cloudflare should be internally tracking how they're handling Tor requests, but having a public tracker (a) gives the data to everybody, and (b) helps Cloudflare have a second opinion in case their internal data diverges from the public version.
+Our proposed approach consists of:
 
-The Berkeley ICSI group did research that included this sort of check:
-​https://www.freehaven.net/anonbib/#differential-ndss2016
-​https://www.freehaven.net/anonbib/#exit-blocking2017
-but what I have in mind here is essentially a simpler subset of this research, skipping the complicated part of "how do you tell what kind of response you got" and with an emphasis on automation and consistency.
+1. Setting up a very simple static webpage to be fronted by Cloudflare
+2. Write an application which  small client to periodically fetch this static webpage via Tor and record how often a captcha is returned
+3. Record and graph captcha vs real page rates
+4. Using the pre-existing architecture, run a second client that does not fetch this webpage via Tor. This will allow us to contrast and compare how Cloudflare responds to Tor Browser vs other browsers.
+5. Track and publish these details publicly
 
-There are two interesting metrics to track over time: one is the fraction of exit relays that are getting hit with captchas, and the other is the chance that a Tor client, choosing an exit relay in the normal weighted faction, will get hit by a captcha.
+There are two interesting metrics to track over time: 
 
-Then there are other interesting patterns to look for, e.g. "are certain IP addresses punished consistently and others never punished, or is whether you get a captcha much more probabilistic and transient?" And does that pattern change over time?
+- the fraction of exit relays that are getting hit with captchas, and
+- the chance that a Tor client, choosing an exit relay in the normal weighted faction, will get hit by a captcha.
+
+Then there are other interesting patterns to look for:
+
+- Are certain IP addresses punished consistently and others never punished?
+- Is whether you get a captcha much more probabilistic and transient?
+- Does that pattern change over time?
+
+# Resources
+
+There is pre-existing research by the Berkeley ICSI group which includes these sorts of checks:
+
+- https://www.freehaven.net/anonbib/#differential-ndss2016
+- https://www.freehaven.net/anonbib/#exit-blocking2017
+
+For the original ticket and discussion, please see ticket [#33010](http://bugs.torproject.org/33010)
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