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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>The Design and Implementation of the Tor Browser [DRAFT]</title><meta name="generator" content="DocBook XSL Stylesheets V1.76.1" /></head><body><div class="article" title="The Design and Implementation of the Tor Browser [DRAFT]"><div class="titlepage"><div><div><h2 class="title"><a id="design"></a>The Design and Implementation of the Tor Browser [DRAFT]</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Mike</span> <span class="surname">Perry</span></h3><div class="affiliation"><div class="address"><p><code class="email">&lt;<a class="email" href="mailto:mikeperry#torproject org">mikeperry#torproject org</a>&gt;</code></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Erinn</span> <span class="surname">Clark</span></h3><div class="affiliation"><div class="address"><p><code class="email">&lt;<a class="email" href="mailto:erinn#torproject org">erinn#torproject org</a>&gt;</code></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Steven</span> <span class="surname">Murdoch</span></h3><div class="affiliation"><div class="address"><p><code class="email">&lt;<a class="email" href="mailto:sjmurdoch#torproject org">sjmurdoch#torproject org</a>&gt;</code></p></div></div></div></div><div><p class="pubdate">March 11, 2013</p></div></div><hr /></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><dt><span class="sect1"><a href="#idp3154416">1. Introduction</a></span></dt><dd><dl><dt><span class="sect2"><a href="#components">1.1. Browser Component Overview</a></span></dt></dl></dd><dt><span class="sect1"><a href="#DesignRequirements">2. Design Requirements and Philosophy</a></span></dt><dd><dl><dt><span class="sect2"><a href="#security">2.1. Security Requirements</a></span></dt><dt><span class="sect2"><a href="#privacy">2.2. Privacy Requirements</a></span></dt><dt><span class="sect2"><a href="#philosophy">2.3. Philosophy</a></span></dt></dl></dd><dt><span class="sect1"><a href="#adversary">3. Adversary Model</a></span></dt><dd><dl><dt><span class="sect2"><a href="#adversary-goals">3.1. Adversary Goals</a></span></dt><dt><span class="sect2"><a href="#adversary-positioning">3.2. Adversary Capabilities - Positioning</a></span></dt><dt><span class="sect2"><a href="#attacks">3.3. Adversary Capabilities - Attacks</a></span></dt></dl></dd><dt><span class="sect1"><a href="#Implementation">4. Implementation</a></span></dt><dd><dl><dt><span class="sect2"><a href="#proxy-obedience">4.1. Proxy Obedience</a></span></dt><dt><span class="sect2"><a href="#state-separation">4.2. State Separation</a></span></dt><dt><span class="sect2"><a href="#disk-avoidance">4.3. Disk Avoidance</a></span></dt><dt><span class="sect2"><a href="#app-data-isolation">4.4. Application Data Isolation</a></span></dt><dt><span class="sect2"><a href="#identifier-linkability">4.5. Cross-Origin Identifier Unlinkability</a></span></dt><dt><span class="sect2"><a href="#fingerprinting-linkability">4.6. Cross-Origin Fingerprinting Unlinkability</a></span></dt><dt><span class="sect2"><a href="#new-identity">4.7. Long-Term Unlinkability via "New Identity" button</a></span></dt><dt><span class="sect2"><a href="#other-security">4.8. Other Security Measures</a></span></dt><dt><span class="sect2"><a href="#firefox-patches">4.9. Description of Firefox Patches</a></span></dt></dl></dd><dt><span class="appendix"><a href="#Transparency">A. Towards Transparency in Navigation Tracking</a></span></dt><dd><dl><dt><span class="sect1"><a href="#deprecate">A.1. Deprecation Wishlist</a></span></dt><dt><span class="sect1"><a href="#idp5843792">A.2. Promising Standards</a></span></dt></dl></dd></dl></div><div class="sect1" title="1. Introduction"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="idp3154416"></a>1. Introduction</h2></div></div></div><p>

This document describes the <a class="link" href="#adversary" title="3. Adversary Model">adversary model</a>,
<a class="link" href="#DesignRequirements" title="2. Design Requirements and Philosophy">design requirements</a>, and <a class="link" href="#Implementation" title="4. Implementation">implementation</a>  of the Tor Browser. It is current as of Tor Browser
2.3.25-5 and Torbutton 1.5.1.

</p><p>

This document is also meant to serve as a set of design requirements and to
describe a reference implementation of a Private Browsing Mode that defends
against active network adversaries, in addition to the passive forensic local
adversary currently addressed by the major browsers.

</p><div class="sect2" title="1.1. Browser Component Overview"><div class="titlepage"><div><div><h3 class="title"><a id="components"></a>1.1. Browser Component Overview</h3></div></div></div><p>

The Tor Browser is based on <a class="ulink" href="https://www.mozilla.org/en-US/firefox/organizations/" target="_top">Mozilla's Extended
Support Release (ESR) Firefox branch</a>. We have a <a class="link" href="#firefox-patches" title="4.9. Description of Firefox Patches">series of patches</a> against this browser to
enhance privacy and security. Browser behavior is additionally augmented
through the <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/tree/master" target="_top">Torbutton
extension</a>, though we are in the process of moving this
functionality into direct Firefox patches. We also <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/HEAD:/build-scripts/config/pound_tor.js" target="_top">change
a number of Firefox preferences</a> from their defaults.

</p><p>

To help protect against potential Tor Exit Node eavesdroppers, we include
<a class="ulink" href="https://www.eff.org/https-everywhere" target="_top">HTTPS-Everywhere</a>. To
provide users with optional defense-in-depth against Javascript and other
potential exploit vectors, we also include <a class="ulink" href="http://noscript.net/" target="_top">NoScript</a>. To protect against
PDF-based Tor proxy bypass and to improve usability, we include the <a class="ulink" href="https://addons.mozilla.org/en-us/firefox/addon/pdfjs/" target="_top">PDF.JS</a>
extension. We also modify <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/HEAD:/build-scripts/config/extension-overrides.js" target="_top">several
extension preferences</a> from their defaults.

</p></div></div><div class="sect1" title="2. Design Requirements and Philosophy"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="DesignRequirements"></a>2. Design Requirements and Philosophy</h2></div></div></div><p>

The Tor Browser Design Requirements are meant to describe the properties of a
Private Browsing Mode that defends against both network and local forensic
adversaries.

</p><p>

There are two main categories of requirements: <a class="link" href="#security" title="2.1. Security Requirements">Security Requirements</a>, and <a class="link" href="#privacy" title="2.2. Privacy Requirements">Privacy Requirements</a>. Security Requirements are the
minimum properties in order for a browser to be able to support Tor and
similar privacy proxies safely. Privacy requirements are the set of properties
that cause us to prefer one browser over another.

</p><p>

While we will endorse the use of browsers that meet the security requirements,
it is primarily the privacy requirements that cause us to maintain our own
browser distribution.

</p><p>

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
      "OPTIONAL" in this document are to be interpreted as described in
      <a class="ulink" href="https://www.ietf.org/rfc/rfc2119.txt" target="_top">RFC 2119</a>.

</p><div class="sect2" title="2.1. Security Requirements"><div class="titlepage"><div><div><h3 class="title"><a id="security"></a>2.1. Security Requirements</h3></div></div></div><p>

The security requirements are primarily concerned with ensuring the safe use
of Tor. Violations in these properties typically result in serious risk for
the user in terms of immediate deanonymization and/or observability. With
respect to browser support, security requirements are the minimum properties
in order for Tor to support the use of a particular browser.

</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><a class="link" href="#proxy-obedience" title="4.1. Proxy Obedience"><span class="command"><strong>Proxy
Obedience</strong></span></a><p>The browser
MUST NOT bypass Tor proxy settings for any content.</p></li><li class="listitem"><a class="link" href="#state-separation" title="4.2. State Separation"><span class="command"><strong>State
Separation</strong></span></a><p>

The browser MUST NOT provide the content window with any state from any other
browsers or any non-Tor browsing modes. This includes shared state from
independent plugins, and shared state from Operating System implementations of
TLS and other support libraries.

</p></li><li class="listitem"><a class="link" href="#disk-avoidance" title="4.3. Disk Avoidance"><span class="command"><strong>Disk
Avoidance</strong></span></a><p>

The browser MUST NOT write any information that is derived from or that
reveals browsing activity to the disk, or store it in memory beyond the
duration of one browsing session, unless the user has explicitly opted to
store their browsing history information to disk.

</p></li><li class="listitem"><a class="link" href="#app-data-isolation" title="4.4. Application Data Isolation"><span class="command"><strong>Application Data
Isolation</strong></span></a><p>

The components involved in providing private browsing MUST be self-contained,
or MUST provide a mechanism for rapid, complete removal of all evidence of the
use of the mode. In other words, the browser MUST NOT write or cause the
operating system to write <span class="emphasis"><em>any information</em></span> about the use
of private browsing to disk outside of the application's control. The user
must be able to ensure that secure deletion of the software is sufficient to
remove evidence of the use of the software. All exceptions and shortcomings
due to operating system behavior MUST be wiped by an uninstaller. However, due
to permissions issues with access to swap, implementations MAY choose to leave
it out of scope, and/or leave it to the Operating System/platform to implement
ephemeral-keyed encrypted swap.

</p></li></ol></div></div><div class="sect2" title="2.2. Privacy Requirements"><div class="titlepage"><div><div><h3 class="title"><a id="privacy"></a>2.2. Privacy Requirements</h3></div></div></div><p>

The privacy requirements are primarily concerned with reducing linkability:
the ability for a user's activity on one site to be linked with their activity
on another site without their knowledge or explicit consent. With respect to
browser support, privacy requirements are the set of properties that cause us
to prefer one browser over another.

</p><p>

For the purposes of the unlinkability requirements of this section as well as
the descriptions in the <a class="link" href="#Implementation" title="4. Implementation">implementation
section</a>, a <span class="command"><strong>url bar origin</strong></span> means at least the
second-level DNS name.  For example, for mail.google.com, the origin would be
google.com. Implementations MAY, at their option, restrict the url bar origin
to be the entire fully qualified domain name.

</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><a class="link" href="#identifier-linkability" title="4.5. Cross-Origin Identifier Unlinkability"><span class="command"><strong>Cross-Origin
Identifier Unlinkability</strong></span></a><p>

User activity on one url bar origin MUST NOT be linkable to their activity in
any other url bar origin by any third party automatically or without user
interaction or approval. This requirement specifically applies to linkability
from stored browser identifiers, authentication tokens, and shared state. The
requirement does not apply to linkable information the user manually submits
to sites, or due to information submitted during manual link traversal. This
functionality SHOULD NOT interfere with interactive, click-driven federated
login in a substantial way.

</p></li><li class="listitem"><a class="link" href="#fingerprinting-linkability" title="4.6. Cross-Origin Fingerprinting Unlinkability"><span class="command"><strong>Cross-Origin
Fingerprinting Unlinkability</strong></span></a><p>

User activity on one url bar origin MUST NOT be linkable to their activity in
any other url bar origin by any third party. This property specifically applies to
linkability from fingerprinting browser behavior.

</p></li><li class="listitem"><a class="link" href="#new-identity" title="4.7. Long-Term Unlinkability via &quot;New Identity&quot; button"><span class="command"><strong>Long-Term
Unlinkability</strong></span></a><p>

The browser MUST provide an obvious, easy way for the user to remove all of
its authentication tokens and browser state and obtain a fresh identity.
Additionally, the browser SHOULD clear linkable state by default automatically
upon browser restart, except at user option.

</p></li></ol></div></div><div class="sect2" title="2.3. Philosophy"><div class="titlepage"><div><div><h3 class="title"><a id="philosophy"></a>2.3. Philosophy</h3></div></div></div><p>

In addition to the above design requirements, the technology decisions about
Tor Browser are also guided by some philosophical positions about technology.

</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Preserve existing user model</strong></span><p>

The existing way that the user expects to use a browser must be preserved. If
the user has to maintain a different mental model of how the sites they are
using behave depending on tab, browser state, or anything else that would not
normally be what they experience in their default browser, the user will
inevitably be confused. They will make mistakes and reduce their privacy as a
result. Worse, they may just stop using the browser, assuming it is broken.

</p><p>

User model breakage was one of the <a class="ulink" href="https://blog.torproject.org/blog/toggle-or-not-toggle-end-torbutton" target="_top">failures
of Torbutton</a>: Even if users managed to install everything properly,
the toggle model was too hard for the average user to understand, especially
in the face of accumulating tabs from multiple states crossed with the current
Tor-state of the browser.

</p></li><li class="listitem"><span class="command"><strong>Favor the implementation mechanism least likely to
break sites</strong></span><p>

In general, we try to find solutions to privacy issues that will not induce
site breakage, though this is not always possible.

</p></li><li class="listitem"><span class="command"><strong>Plugins must be restricted</strong></span><p>

Even if plugins always properly used the browser proxy settings (which none of
them do) and could not be induced to bypass them (which all of them can), the
activities of closed-source plugins are very difficult to audit and control.
They can obtain and transmit all manner of system information to websites,
often have their own identifier storage for tracking users, and also
contribute to fingerprinting.

</p><p>

Therefore, if plugins are to be enabled in private browsing modes, they must
be restricted from running automatically on every page (via click-to-play
placeholders), and/or be sandboxed to restrict the types of system calls they
can execute. If the user agent allows the user to craft an exemption to allow
a plugin to be used automatically, it must only apply to the top level url bar
domain, and not to all sites, to reduce cross-origin fingerprinting
linkability.

</p></li><li class="listitem"><span class="command"><strong>Minimize Global Privacy Options</strong></span><p>

<a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3100" target="_top">Another
failure of Torbutton</a> was the options panel. Each option
that detectably alters browser behavior can be used as a fingerprinting tool.
Similarly, all extensions <a class="ulink" href="http://blog.chromium.org/2010/06/extensions-in-incognito.html" target="_top">should be
disabled in the mode</a> except as an opt-in basis. We should not load
system-wide and/or Operating System provided addons or plugins.

</p><p>
Instead of global browser privacy options, privacy decisions should be made
<a class="ulink" href="https://wiki.mozilla.org/Privacy/Features/Site-based_data_management_UI" target="_top">per
url bar origin</a> to eliminate the possibility of linkability
between domains. For example, when a plugin object (or a Javascript access of
window.plugins) is present in a page, the user should be given the choice of
allowing that plugin object for that url bar origin only. The same
goes for exemptions to third party cookie policy, geo-location, and any other
privacy permissions.
     </p><p>
If the user has indicated they wish to record local history storage, these
permissions can be written to disk. Otherwise, they should remain memory-only. 
     </p></li><li class="listitem"><span class="command"><strong>No filters</strong></span><p>

Site-specific or filter-based addons such as <a class="ulink" href="https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/" target="_top">AdBlock
Plus</a>, <a class="ulink" href="http://requestpolicy.com/" target="_top">Request Policy</a>,
<a class="ulink" href="http://www.ghostery.com/about" target="_top">Ghostery</a>, <a class="ulink" href="http://priv3.icsi.berkeley.edu/" target="_top">Priv3</a>, and <a class="ulink" href="http://sharemenot.cs.washington.edu/" target="_top">Sharemenot</a> are to be
avoided. We believe that these addons do not add any real privacy to a proper
<a class="link" href="#Implementation" title="4. Implementation">implementation</a> of the above <a class="link" href="#privacy" title="2.2. Privacy Requirements">privacy requirements</a>, and that development efforts
should be focused on general solutions that prevent tracking by all
third parties, rather than a list of specific URLs or hosts.
     </p><p>
Filter-based addons can also introduce strange breakage and cause usability
nightmares, and will also fail to do their job if an adversary simply
registers a new domain or creates a new url path. Worse still, the unique
filter sets that each user creates or installs will provide a wealth of
fingerprinting targets.
      </p><p>

As a general matter, we are also generally opposed to shipping an always-on Ad
blocker with Tor Browser. We feel that this would damage our credibility in
terms of demonstrating that we are providing privacy through a sound design
alone, as well as damage the acceptance of Tor users by sites that support
themselves through advertising revenue.

</p><p>
Users are free to install these addons if they wish, but doing
so is not recommended, as it will alter the browser request fingerprint.
      </p></li><li class="listitem"><span class="command"><strong>Stay Current</strong></span><p>
We believe that if we do not stay current with the support of new web
technologies, we cannot hope to substantially influence or be involved in
their proper deployment or privacy realization. However, we will likely disable
high-risk features pending analysis, audit, and mitigation.
      </p></li></ol></div></div></div><div class="sect1" title="3. Adversary Model"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="adversary"></a>3. Adversary Model</h2></div></div></div><p>

A Tor web browser adversary has a number of goals, capabilities, and attack
types that can be used to illustrate the design requirements for the
Tor Browser. Let's start with the goals.

</p><div class="sect2" title="3.1. Adversary Goals"><div class="titlepage"><div><div><h3 class="title"><a id="adversary-goals"></a>3.1. Adversary Goals</h3></div></div></div><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Bypassing proxy settings</strong></span><p>The adversary's primary goal is direct compromise and bypass of 
Tor, causing the user to directly connect to an IP of the adversary's
choosing.</p></li><li class="listitem"><span class="command"><strong>Correlation of Tor vs Non-Tor Activity</strong></span><p>If direct proxy bypass is not possible, the adversary will likely
happily settle for the ability to correlate something a user did via Tor with
their non-Tor activity. This can be done with cookies, cache identifiers,
javascript events, and even CSS. Sometimes the fact that a user uses Tor may
be enough for some authorities.</p></li><li class="listitem"><span class="command"><strong>History disclosure</strong></span><p>
The adversary may also be interested in history disclosure: the ability to
query a user's history to see if they have issued certain censored search
queries, or visited censored sites.
     </p></li><li class="listitem"><span class="command"><strong>Correlate activity across multiple sites</strong></span><p>

The primary goal of the advertising networks is to know that the user who
visited siteX.com is the same user that visited siteY.com to serve them
targeted ads. The advertising networks become our adversary insofar as they
attempt to perform this correlation without the user's explicit consent.

</p></li><li class="listitem"><span class="command"><strong>Fingerprinting/anonymity set reduction</strong></span><p>

Fingerprinting (more generally: "anonymity set reduction") is used to attempt
to gather identifying information on a particular individual without the use
of tracking identifiers. If the dissident or whistleblower's timezone is
available, and they are using a rare build of Firefox for an obscure operating
system, and they have a specific display resolution only used on one type of
laptop, this can be very useful information for tracking them down, or at
least <a class="link" href="#fingerprinting">tracking their activities</a>.

</p></li><li class="listitem"><span class="command"><strong>History records and other on-disk
information</strong></span><p>
In some cases, the adversary may opt for a heavy-handed approach, such as
seizing the computers of all Tor users in an area (especially after narrowing
the field by the above two pieces of information). History records and cache
data are the primary goals here.
     </p></li></ol></div></div><div class="sect2" title="3.2. Adversary Capabilities - Positioning"><div class="titlepage"><div><div><h3 class="title"><a id="adversary-positioning"></a>3.2. Adversary Capabilities - Positioning</h3></div></div></div><p>
The adversary can position themselves at a number of different locations in
order to execute their attacks.
    </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Exit Node or Upstream Router</strong></span><p>
The adversary can run exit nodes, or alternatively, they may control routers
upstream of exit nodes. Both of these scenarios have been observed in the
wild.
     </p></li><li class="listitem"><span class="command"><strong>Ad servers and/or Malicious Websites</strong></span><p>
The adversary can also run websites, or more likely, they can contract out
ad space from a number of different ad servers and inject content that way. For
some users, the adversary may be the ad servers themselves. It is not
inconceivable that ad servers may try to subvert or reduce a user's anonymity 
through Tor for marketing purposes.
     </p></li><li class="listitem"><span class="command"><strong>Local Network/ISP/Upstream Router</strong></span><p>
The adversary can also inject malicious content at the user's upstream router
when they have Tor disabled, in an attempt to correlate their Tor and Non-Tor
activity.
     </p><p>

Additionally, at this position the adversary can block Tor, or attempt to
recognize the traffic patterns of specific web pages at the entrance to the Tor
network.

</p></li><li class="listitem"><span class="command"><strong>Physical Access</strong></span><p>
Some users face adversaries with intermittent or constant physical access.
Users in Internet cafes, for example, face such a threat. In addition, in
countries where simply using tools like Tor is illegal, users may face
confiscation of their computer equipment for excessive Tor usage or just
general suspicion.
     </p></li></ol></div></div><div class="sect2" title="3.3. Adversary Capabilities - Attacks"><div class="titlepage"><div><div><h3 class="title"><a id="attacks"></a>3.3. Adversary Capabilities - Attacks</h3></div></div></div><p>

The adversary can perform the following attacks from a number of different 
positions to accomplish various aspects of their goals. It should be noted
that many of these attacks (especially those involving IP address leakage) are
often performed by accident by websites that simply have Javascript, dynamic 
CSS elements, and plugins. Others are performed by ad servers seeking to
correlate users' activity across different IP addresses, and still others are
performed by malicious agents on the Tor network and at national firewalls.

</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Read and insert identifiers</strong></span><p>

The browser contains multiple facilities for storing identifiers that the
adversary creates for the purposes of tracking users. These identifiers are
most obviously cookies, but also include HTTP auth, DOM storage, cached
scripts and other elements with embedded identifiers, client certificates, and
even TLS Session IDs.

</p><p>

An adversary in a position to perform MITM content alteration can inject
document content elements to both read and inject cookies for arbitrary
domains. In fact, even many "SSL secured" websites are vulnerable to this sort of
<a class="ulink" href="http://seclists.org/bugtraq/2007/Aug/0070.html" target="_top">active
sidejacking</a>. In addition, the ad networks of course perform tracking
with cookies as well.

</p><p>

These types of attacks are attempts at subverting our <a class="link" href="#identifier-linkability" title="4.5. Cross-Origin Identifier Unlinkability">Cross-Origin Identifier Unlinkability</a> and <a class="link" href="#new-identity" title="4.7. Long-Term Unlinkability via &quot;New Identity&quot; button">Long-Term Unlikability</a> design requirements.

</p></li><li class="listitem"><a id="fingerprinting"></a><span class="command"><strong>Fingerprint users based on browser
attributes</strong></span><p>

There is an absurd amount of information available to websites via attributes
of the browser. This information can be used to reduce anonymity set, or even
uniquely fingerprint individual users. Attacks of this nature are typically
aimed at tracking users across sites without their consent, in an attempt to
subvert our <a class="link" href="#fingerprinting-linkability" title="4.6. Cross-Origin Fingerprinting Unlinkability">Cross-Origin
Fingerprinting Unlinkability</a> and <a class="link" href="#new-identity" title="4.7. Long-Term Unlinkability via &quot;New Identity&quot; button">Long-Term Unlikability</a> design requirements.

</p><p>

Fingerprinting is an intimidating
problem to attempt to tackle, especially without a metric to determine or at
least intuitively understand and estimate which features will most contribute
to linkability between visits.

</p><p>

The <a class="ulink" href="https://panopticlick.eff.org/about.php" target="_top">Panopticlick study
done</a> by the EFF uses the <a class="ulink" href="https://en.wikipedia.org/wiki/Entropy_%28information_theory%29" target="_top">Shannon
entropy</a> - the number of identifying bits of information encoded in
browser properties - as this metric. Their <a class="ulink" href="https://wiki.mozilla.org/Fingerprinting#Data" target="_top">result data</a> is
definitely useful, and the metric is probably the appropriate one for
determining how identifying a particular browser property is. However, some
quirks of their study means that they do not extract as much information as
they could from display information: they only use desktop resolution and do
not attempt to infer the size of toolbars. In the other direction, they may be
over-counting in some areas, as they did not compute joint entropy over
multiple attributes that may exhibit a high degree of correlation. Also, new
browser features are added regularly, so the data should not be taken as
final.

</p><p>

Despite the uncertainty, all fingerprinting attacks leverage the following
attack vectors:

</p><div class="orderedlist"><ol class="orderedlist" type="a"><li class="listitem"><span class="command"><strong>Observing Request Behavior</strong></span><p>

Properties of the user's request behavior comprise the bulk of low-hanging
fingerprinting targets. These include: User agent, Accept-* headers, pipeline
usage, and request ordering. Additionally, the use of custom filters such as
AdBlock and other privacy filters can be used to fingerprint request patterns
(as an extreme example).

</p></li><li class="listitem"><span class="command"><strong>Inserting Javascript</strong></span><p>

Javascript can reveal a lot of fingerprinting information. It provides DOM
objects such as window.screen and window.navigator to extract information
about the useragent.

Also, Javascript can be used to query the user's timezone via the
<code class="function">Date()</code> object, <a class="ulink" href="https://www.khronos.org/registry/webgl/specs/1.0/#5.13" target="_top">WebGL</a> can
reveal information about the video card in use, and high precision timing
information can be used to <a class="ulink" href="http://w2spconf.com/2011/papers/jspriv.pdf" target="_top">fingerprint the CPU and
interpreter speed</a>. In the future, new JavaScript features such as
<a class="ulink" href="http://w3c-test.org/webperf/specs/ResourceTiming/" target="_top">Resource
Timing</a> may leak an unknown amount of network timing related
information.

</p></li><li class="listitem"><span class="command"><strong>Inserting Plugins</strong></span><p>

The Panopticlick project found that the mere list of installed plugins (in
navigator.plugins) was sufficient to provide a large degree of
fingerprintability. Additionally, plugins are capable of extracting font lists,
interface addresses, and other machine information that is beyond what the
browser would normally provide to content. In addition, plugins can be used to
store unique identifiers that are more difficult to clear than standard
cookies.  <a class="ulink" href="http://epic.org/privacy/cookies/flash.html" target="_top">Flash-based
cookies</a> fall into this category, but there are likely numerous other
examples. Beyond fingerprinting, plugins are also abysmal at obeying the proxy
settings of the browser.

</p></li><li class="listitem"><span class="command"><strong>Inserting CSS</strong></span><p>

<a class="ulink" href="https://developer.mozilla.org/En/CSS/Media_queries" target="_top">CSS media
queries</a> can be inserted to gather information about the desktop size,
widget size, display type, DPI, user agent type, and other information that
was formerly available only to Javascript.

</p></li></ol></div></li><li class="listitem"><a id="website-traffic-fingerprinting"></a><span class="command"><strong>Website traffic fingerprinting</strong></span><p>

Website traffic fingerprinting is an attempt by the adversary to recognize the
encrypted traffic patterns of specific websites. In the case of Tor, this
attack would take place between the user and the Guard node, or at the Guard
node itself.
     </p><p> The most comprehensive study of the statistical properties of this
attack against Tor was done by <a class="ulink" href="http://lorre.uni.lu/~andriy/papers/acmccs-wpes11-fingerprinting.pdf" target="_top">Panchenko
et al</a>. Unfortunately, the publication bias in academia has encouraged
the production of a number of follow-on attack papers claiming "improved"
success rates, in some cases even claiming to completely invalidate any
attempt at defense. These "improvements" are actually enabled primarily by
taking a number of shortcuts (such as classifying only very small numbers of
web pages, neglecting to publish ROC curves or at least false positive rates,
and/or omitting the effects of dataset size on their results). Despite these
subsequent "improvements", we are skeptical of the efficacy of this attack in
a real world scenario, <span class="emphasis"><em>especially</em></span> in the face of any
defenses.

</p><p>

In general, with machine learning, as you increase the <a class="ulink" href="https://en.wikipedia.org/wiki/VC_dimension" target="_top">number and/or complexity of
categories to classify</a> while maintaining a limit on reliable feature
information you can extract, you eventually run out of descriptive feature
information, and either true positive accuracy goes down or the false positive
rate goes up. This error is called the <a class="ulink" href="http://www.cs.washington.edu/education/courses/csep573/98sp/lectures/lecture8/sld050.htm" target="_top">bias
in your hypothesis space</a>. In fact, even for unbiased hypothesis
spaces, the number of training examples required to achieve a reasonable error
bound is <a class="ulink" href="https://en.wikipedia.org/wiki/Probably_approximately_correct_learning#Equivalence" target="_top">a
function of the complexity of the categories</a> you need to classify.

</p><p>

In the case of this attack, the key factors that increase the classification
complexity (and thus hinder a real world adversary who attempts this attack)
are large numbers of dynamically generated pages, partially cached content,
and also the non-web activity of entire Tor network. This yields an effective
number of "web pages" many orders of magnitude larger than even <a class="ulink" href="http://lorre.uni.lu/~andriy/papers/acmccs-wpes11-fingerprinting.pdf" target="_top">Panchenko's
"Open World" scenario</a>, which suffered continous near-constant decline
in the true positive rate as the "Open World" size grew (see figure 4). This
large level of classification complexity is further confounded by a noisy and
low resolution featureset - one which is also realtively easy for the defender
to manipulate at low cost.

</p><p>

To make matters worse for a real-world adversary, the ocean of Tor Internet
activity (at least, when compared to a lab setting) makes it a certainty that
an adversary attempting examine large amounts of Tor traffic will ultimately
be overwhelmed by false positives (even after making heavy tradeoffs on the
ROC curve to minimize false positives to below 0.01%). This problem is known
in the IDS literature as the <a class="ulink" href="http://www.raid-symposium.org/raid99/PAPERS/Axelsson.pdf" target="_top">Base Rate
Fallacy</a>, and it is the primary reason that anomaly and activity
classification-based IDS and antivirus systems have failed to materialize in
the marketplace (despite early success in academic literature).

</p><p>

Still, we do not believe that these issues are enough to dismiss the attack
outright. But we do believe these factors make it both worthwhile and
effective to <a class="link" href="#traffic-fingerprinting-defenses">deploy
light-weight defenses</a> that reduce the accuracy of this attack by
further contributing noise to hinder successful feature extraction.

</p></li><li class="listitem"><span class="command"><strong>Remotely or locally exploit browser and/or
OS</strong></span><p>

Last, but definitely not least, the adversary can exploit either general
browser vulnerabilities, plugin vulnerabilities, or OS vulnerabilities to
install malware and surveillance software. An adversary with physical access
can perform similar actions. Regrettably, this last attack capability is
outside of the browser's ability to defend against, but it is worth mentioning
for completeness. In fact, <a class="ulink" href="http://tails.boum.org/contribute/design/" target="_top">The Tails system</a> can
provide some defense against this adversary, and it does include the Tor
Browser. We do however aim to defend against an adersary that has passive
forensic access the disk after browsing activity takes place, as part of our
<a class="link" href="#disk-avoidance" title="4.3. Disk Avoidance">Disk Avoidance</a> defenses.

</p></li></ol></div></div></div><div class="sect1" title="4. Implementation"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Implementation"></a>4. Implementation</h2></div></div></div><p>

The Implementation section is divided into subsections, each of which
corresponds to a <a class="link" href="#DesignRequirements" title="2. Design Requirements and Philosophy">Design Requirement</a>.
Each subsection is divided into specific web technologies or properties. The
implementation is then described for that property.

</p><p>

In some cases, the implementation meets the design requirements in a non-ideal
way (for example, by disabling features). In rare cases, there may be no
implementation at all. Both of these cases are denoted by differentiating
between the <span class="command"><strong>Design Goal</strong></span> and the <span class="command"><strong>Implementation
Status</strong></span> for each property. Corresponding bugs in the <a class="ulink" href="https://trac.torproject.org/projects/tor/report" target="_top">Tor bug tracker</a>
are typically linked for these cases.

</p><div class="sect2" title="4.1. Proxy Obedience"><div class="titlepage"><div><div><h3 class="title"><a id="proxy-obedience"></a>4.1. Proxy Obedience</h3></div></div></div><p>

Proxy obedience is assured through the following:
   </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">Firefox proxy settings, patches, and build flags
 <p>
Our <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/HEAD:/build-scripts/config/pound_tor.js" target="_top">Firefox
preferences file</a> sets the Firefox proxy settings to use Tor directly as a
SOCKS proxy. It sets <span class="command"><strong>network.proxy.socks_remote_dns</strong></span>,
<span class="command"><strong>network.proxy.socks_version</strong></span>,
<span class="command"><strong>network.proxy.socks_port</strong></span>, and
<span class="command"><strong>network.dns.disablePrefetch</strong></span>.
 </p><p>

We also patch Firefox in order to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0016-Prevent-WebSocket-DNS-leak.patch" target="_top">prevent
a DNS leak due to a WebSocket rate-limiting check</a>. As stated in the
patch, we believe the direct DNS resolution performed by this check is in
violation of the W3C standard, but <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=751465" target="_top">this DNS proxy leak
remains present in stock Firefox releases</a>.

</p><p>

During the transition to Firefox 17-ESR, a code audit was undertaken to verify
that there were no system calls or XPCOM activity in the source tree that did
not use the browser proxy settings. The only violation we found was that
WebRTC was capable of creating UDP sockets and was compiled in by default. We
subsequently disabled it using the Firefox build option
<span class="command"><strong>--disable-webrtc</strong></span>.

</p><p>

We have verified that these settings and patches properly proxy HTTPS, OCSP,
HTTP, FTP, gopher (now defunct), DNS, SafeBrowsing Queries, all javascript
activity, including HTML5 audio and video objects, addon updates, wifi
geolocation queries, searchbox queries, XPCOM addon HTTPS/HTTP activity,
WebSockets, and live bookmark updates. We have also verified that IPv6
connections are not attempted, through the proxy or otherwise (Tor does not
yet support IPv6). We have also verified that external protocol helpers, such
as smb urls and other custom protocol handlers are all blocked.

</p><p>

Numerous other third parties have also reviewed and tested the proxy settings
and have provided test cases based on their work. See in particular <a class="ulink" href="http://decloak.net/" target="_top">decloak.net</a>.

</p></li><li class="listitem">Disabling plugins

<p>Plugins have the ability to make arbitrary OS system calls and  <a class="ulink" href="http://decloak.net/" target="_top">bypass proxy settings</a>. This includes
the ability to make UDP sockets and send arbitrary data independent of the
browser proxy settings.
 </p><p>
Torbutton disables plugins by using the
<span class="command"><strong>@mozilla.org/plugin/host;1</strong></span> service to mark the plugin tags
as disabled. This block can be undone through both the Torbutton Security UI,
and the Firefox Plugin Preferences.
 </p><p>
If the user does enable plugins in this way, plugin-handled objects are still
restricted from automatic load through Firefox's click-to-play preference
<span class="command"><strong>plugins.click_to_play</strong></span>.
 </p><p>
In addition, to reduce any unproxied activity by arbitrary plugins at load
time, and to reduce the fingerprintability of the installed plugin list, we
also patch the Firefox source code to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0005-Block-all-plugins-except-flash.patch" target="_top">prevent the load of any plugins except
for Flash and Gnash</a>.

</p></li><li class="listitem">External App Blocking and Drag Event Filtering
  <p>

External apps can be induced to load files that perform network activity.
Unfortunately, there are cases where such apps can be launched automatically
with little to no user input. In order to prevent this, Torbutton installs a
component to <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/blob_plain/HEAD:/src/components/external-app-blocker.js" target="_top">
provide the user with a popup</a> whenever the browser attempts to launch
a helper app.

</p><p>

Additionally, modern desktops now pre-emptively fetch any URLs in Drag and
Drop events as soon as the drag is initiated. This download happens
independent of the browser's Tor settings, and can be triggered by something
as simple as holding the mouse button down for slightly too long while
clicking on an image link. We had to patch Firefox to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0018-Emit-observer-event-to-filter-the-Drag-Drop-url-list.patch" target="_top">emit
an observer event during dragging</a> to allow us to filter the drag
events from Torbutton before the OS downloads the URLs the events contained.

</p></li></ol></div></div><div class="sect2" title="4.2. State Separation"><div class="titlepage"><div><div><h3 class="title"><a id="state-separation"></a>4.2. State Separation</h3></div></div></div><p>
Tor Browser State is separated from existing browser state through use of a
custom Firefox profile. Furthermore, plugins are disabled, which prevents
Flash cookies from leaking from a pre-existing Flash directory.
   </p></div><div class="sect2" title="4.3. Disk Avoidance"><div class="titlepage"><div><div><h3 class="title"><a id="disk-avoidance"></a>4.3. Disk Avoidance</h3></div></div></div><div class="sect3" title="Design Goal:"><div class="titlepage"><div><div><h4 class="title"><a id="idp5587232"></a>Design Goal:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote">

The User Agent MUST (at user option) prevent all disk records of browser activity.
The user should be able to optionally enable URL history and other history
features if they so desire.

</blockquote></div></div><div class="sect3" title="Implementation Status:"><div class="titlepage"><div><div><h4 class="title"><a id="idp5588592"></a>Implementation Status:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote">

We achieve this goal through several mechanisms. First, we set the Firefox
Private Browsing preference
<span class="command"><strong>browser.privatebrowsing.autostart</strong></span>. In addition, four Firefox patches are needed to prevent disk writes, even if
Private Browsing Mode is enabled. We need to

<a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0002-Make-Permissions-Manager-memory-only.patch" target="_top">prevent
the permissions manager from recording HTTPS STS state</a>,
<a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0003-Make-Intermediate-Cert-Store-memory-only.patch" target="_top">prevent
intermediate SSL certificates from being recorded</a>,
<a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0013-Make-Download-manager-memory-only.patch" target="_top">prevent
download history from being recorded</a>, and
<a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0006-Make-content-pref-service-memory-only-clearable.patch" target="_top">prevent
the content preferences service from recording site zoom</a>.

For more details on these patches, <a class="link" href="#firefox-patches" title="4.9. Description of Firefox Patches">see the
Firefox Patches section</a>.

</blockquote></div><div class="blockquote"><blockquote class="blockquote">

As an additional defense-in-depth measure, we set the following preferences:
<span class="command"><strong></strong></span>,
<span class="command"><strong>browser.cache.disk.enable</strong></span>,
<span class="command"><strong>browser.cache.offline.enable</strong></span>,
<span class="command"><strong>dom.indexedDB.enabled</strong></span>,
<span class="command"><strong>network.cookie.lifetimePolicy</strong></span>,
<span class="command"><strong>signon.rememberSignons</strong></span>,
<span class="command"><strong>browser.formfill.enable</strong></span>,
<span class="command"><strong>browser.download.manager.retention</strong></span>,
<span class="command"><strong>browser.sessionstore.privacy_level</strong></span>,
and <span class="command"><strong>network.cookie.lifetimePolicy</strong></span>. Many of these
preferences are likely redundant with
<span class="command"><strong>browser.privatebrowsing.autostart</strong></span>, but we have not done the
auditing work to ensure that yet.

</blockquote></div><div class="blockquote"><blockquote class="blockquote">

Torbutton also <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/blob/HEAD:/src/components/tbSessionStore.js" target="_top">contains
code</a> to prevent the Firefox session store from writing to disk.
    </blockquote></div><div class="blockquote"><blockquote class="blockquote">

For more details on disk leak bugs and enhancements, see the <a class="ulink" href="https://trac.torproject.org/projects/tor/query?keywords=~tbb-disk-leak&amp;status=!closed" target="_top">tbb-disk-leak tag in our bugtracker</a></blockquote></div></div></div><div class="sect2" title="4.4. Application Data Isolation"><div class="titlepage"><div><div><h3 class="title"><a id="app-data-isolation"></a>4.4. Application Data Isolation</h3></div></div></div><p>

Tor Browser Bundle MUST NOT cause any information to be written outside of the
bundle directory. This is to ensure that the user is able to completely and
safely remove the bundle without leaving other traces of Tor usage on their
computer.

</p><p>

To ensure TBB directory isolation, we set
<span class="command"><strong>browser.download.useDownloadDir</strong></span>,
<span class="command"><strong>browser.shell.checkDefaultBrowser</strong></span>, and
<span class="command"><strong>browser.download.manager.addToRecentDocs</strong></span>. We also set the
$HOME environment variable to be the TBB extraction directory.
   </p></div><div class="sect2" title="4.5. Cross-Origin Identifier Unlinkability"><div class="titlepage"><div><div><h3 class="title"><a id="identifier-linkability"></a>4.5. Cross-Origin Identifier Unlinkability</h3></div></div></div><p>

The Tor Browser MUST prevent a user's activity on one site from being linked
to their activity on another site. When this goal cannot yet be met with an
existing web technology, that technology or functionality is disabled. Our
<a class="link" href="#privacy" title="2.2. Privacy Requirements">design goal</a> is to ultimately eliminate the need to disable arbitrary
technologies, and instead simply alter them in ways that allows them to
function in a backwards-compatible way while avoiding linkability. Users
should be able to use federated login of various kinds to explicitly inform
sites who they are, but that information should not transparently allow a
third party to record their activity from site to site without their prior
consent.

</p><p>

The benefit of this approach comes not only in the form of reduced
linkability, but also in terms of simplified privacy UI. If all stored browser
state and permissions become associated with the url bar origin, the six or
seven different pieces of privacy UI governing these identifiers and
permissions can become just one piece of UI. For instance, a window that lists
the url bar origin for which browser state exists, possibly with a
context-menu option to drill down into specific types of state or permissions.
An example of this simplification can be seen in Figure 1.

</p><div class="figure"><a id="idp5612672"></a><p class="title"><strong>Figure 1. Improving the Privacy UI</strong></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="NewCookieManager.png" align="middle" alt="Improving the Privacy UI" /></div><div class="caption"><p></p>

This example UI is a mock-up of how isolating identifiers to the URL bar
origin can simplify the privacy UI for all data - not just cookies. Once
browser identifiers and site permissions operate on a url bar basis, the same
privacy window can represent browsing history, DOM Storage, HTTP Auth, search
form history, login values, and so on within a context menu for each site.

</div></div></div><br class="figure-break" /><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">Cookies
     <p><span class="command"><strong>Design Goal:</strong></span>

All cookies MUST be double-keyed to the url bar origin and third-party
origin. There exists a <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=565965" target="_top">Mozilla bug</a>
that contains a prototype patch, but it lacks UI, and does not apply to modern
Firefoxes.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

As a stopgap to satisfy our design requirement of unlinkability, we currently
entirely disable 3rd party cookies by setting
<span class="command"><strong>network.cookie.cookieBehavior</strong></span> to 1. We would prefer that
third party content continue to function, but we believe the requirement for 
unlinkability trumps that desire.

</p></li><li class="listitem">Cache
     <p>

Cache is isolated to the url bar origin by using a technique pioneered by
Colin Jackson et al, via their work on <a class="ulink" href="http://www.safecache.com/" target="_top">SafeCache</a>. The technique re-uses the
<a class="ulink" href="https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsICachingChannel" target="_top">nsICachingChannel.cacheKey</a>
attribute that Firefox uses internally to prevent improper caching and reuse
of HTTP POST data.

</p><p>

However, to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3666" target="_top">increase the
security of the isolation</a> and to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3754" target="_top">solve conflicts
with OCSP relying the cacheKey property for reuse of POST requests</a>, we
had to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0004-Add-a-string-based-cacheKey.patch" target="_top">patch
Firefox to provide a cacheDomain cache attribute</a>. We use the fully
qualified url bar domain as input to this field, to avoid the complexities
of heuristically determining the second-level DNS name.

</p><p>

Furthermore, we chose a different
isolation scheme than the Stanford implementation. First, we decoupled the
cache isolation from the third party cookie attribute. Second, we use several
mechanisms to attempt to determine the actual location attribute of the
top-level window (to obtain the url bar FQDN) used to load the page, as
opposed to relying solely on the Referer property.

</p><p>

Therefore, <a class="ulink" href="http://crypto.stanford.edu/sameorigin/safecachetest.html" target="_top">the original
Stanford test cases</a> are expected to fail. Functionality can still be
verified by navigating to <a class="ulink" href="about:cache" target="_top">about:cache</a> and
viewing the key used for each cache entry. Each third party element should
have an additional "domain=string" property prepended, which will list the
FQDN that was used to source the third party element.

</p><p>

Additionally, because the image cache is a separate entity from the content
cache, we had to patch Firefox to also <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0024-Isolate-the-Image-Cache-per-url-bar-domain.patch" target="_top">isolate
this cache per url bar domain</a>.

</p></li><li class="listitem">HTTP Auth
     <p>

HTTP authentication tokens are removed for third party elements using the
<a class="ulink" href="https://developer.mozilla.org/en/Setting_HTTP_request_headers#Observers" target="_top">http-on-modify-request
observer</a> to remove the Authorization headers to prevent <a class="ulink" href="http://jeremiahgrossman.blogspot.com/2007/04/tracking-users-without-cookies.html" target="_top">silent
linkability between domains</a>. 
     </p></li><li class="listitem">DOM Storage
     <p>

DOM storage for third party domains MUST be isolated to the url bar origin,
to prevent linkability between sites. This functionality is provided through a
<a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0026-Isolate-DOM-storage-to-first-party-URI.patch" target="_top">patch
to Firefox</a>.

</p></li><li class="listitem">Flash cookies
     <p><span class="command"><strong>Design Goal:</strong></span>

Users should be able to click-to-play flash objects from trusted sites. To
make this behavior unlinkable, we wish to include a settings file for all platforms that disables flash
cookies using the <a class="ulink" href="http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html" target="_top">Flash
settings manager</a>.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

We are currently <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3974" target="_top">having
difficulties</a> causing Flash player to use this settings
file on Windows, so Flash remains difficult to enable.

</p></li><li class="listitem">SSL+TLS session resumption, HTTP Keep-Alive and SPDY
     <p><span class="command"><strong>Design Goal:</strong></span>

TLS session resumption tickets and SSL Session IDs MUST be limited to the url
bar origin.  HTTP Keep-Alive connections from a third party in one url bar
origin MUST NOT be reused for that same third party in another url bar origin.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

We currently clear SSL Session IDs upon <a class="link" href="#new-identity" title="4.7. Long-Term Unlinkability via &quot;New Identity&quot; button">New
Identity</a>, we disable TLS Session Tickets via the Firefox Pref
<span class="command"><strong>security.enable_tls_session_tickets</strong></span>. We disable SSL Session
IDs via a <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0008-Disable-SSL-Session-ID-tracking.patch" target="_top">patch
to Firefox</a>. To compensate for the increased round trip latency from disabling
these performance optimizations, we also enable
<a class="ulink" href="https://tools.ietf.org/html/draft-bmoeller-tls-falsestart-00" target="_top">TLS
False Start</a> via the Firefox Pref 
<span class="command"><strong>security.ssl.enable_false_start</strong></span>.
    </p><p>

Because of the extreme performance benefits of HTTP Keep-Alive for interactive
web apps, and because of the difficulties of conveying urlbar origin
information down into the Firefox HTTP layer, as a compromise we currently
merely reduce the HTTP Keep-Alive timeout to 20 seconds (which is measured
from the last packet read on the connection) using the Firefox preference
<span class="command"><strong>network.http.keep-alive.timeout</strong></span>.

</p><p>
However, because SPDY can store identifiers and has extremely long keepalive
duration, it is disabled through the Firefox preference
<span class="command"><strong>network.http.spdy.enabled</strong></span>.
     </p></li><li class="listitem">Automated cross-origin redirects MUST NOT store identifiers
    <p><span class="command"><strong>Design Goal:</strong></span>

To prevent attacks aimed at subverting the Cross-Origin Identifier
Unlinkability <a class="link" href="#privacy" title="2.2. Privacy Requirements">privacy requirement</a>, the browser
MUST NOT store any identifiers (cookies, cache, DOM storage, HTTP auth, etc)
for cross-origin redirect intermediaries that do not prompt for user input.
For example, if a user clicks on a bit.ly url that redirects to a
doubleclick.net url that finally redirects to a cnn.com url, only cookies from
cnn.com should be retained after the redirect chain completes.

</p><p>

Non-automated redirect chains that require user input at some step (such as
federated login systems) SHOULD still allow identifiers to persist.

</p><p><span class="command"><strong>Implementation status:</strong></span>

There are numerous ways for the user to be redirected, and the Firefox API
support to detect each of them is poor. We have a <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3600" target="_top">trac bug
open</a> to implement what we can.

</p></li><li class="listitem">window.name
     <p>

<a class="ulink" href="https://developer.mozilla.org/En/DOM/Window.name" target="_top">window.name</a> is
a magical DOM property that for some reason is allowed to retain a persistent value
for the lifespan of a browser tab. It is possible to utilize this property for
<a class="ulink" href="http://www.thomasfrank.se/sessionvars.html" target="_top">identifier
storage</a>.

</p><p>

In order to eliminate non-consensual linkability but still allow for sites
that utilize this property to function, we reset the window.name property of
tabs in Torbutton every time we encounter a blank Referer. This behavior
allows window.name to persist for the duration of a click-driven navigation
session, but as soon as the user enters a new URL or navigates between
https/http schemes, the property is cleared.

</p></li><li class="listitem">Auto form-fill
     <p>

We disable the password saving functionality in the browser as part of our
<a class="link" href="#disk-avoidance" title="4.3. Disk Avoidance">Disk Avoidance</a> requirement. However,
since users may decide to re-enable disk history records and password saving,
we also set the <a class="ulink" href="http://kb.mozillazine.org/Signon.autofillForms" target="_top">signon.autofillForms</a>
preference to false to prevent saved values from immediately populating
fields upon page load. Since Javascript can read these values as soon as they
appear, setting this preference prevents automatic linkability from stored passwords.

</p></li><li class="listitem">HSTS supercookies
      <p>

An extreme (but not impossible) attack to mount is the creation of <a class="ulink" href="http://www.leviathansecurity.com/blog/archives/12-The-Double-Edged-Sword-of-HSTS-Persistence-and-Privacy.html" target="_top">HSTS
supercookies</a>. Since HSTS effectively stores one bit of information per domain
name, an adversary in possession of numerous domains can use them to construct
cookies based on stored HSTS state.

</p><p><span class="command"><strong>Design Goal:</strong></span>

There appears to be three options for us: 1. Disable HSTS entirely, and rely
instead on HTTPS-Everywhere to crawl and ship rules for HSTS sites. 2.
Restrict the number of HSTS-enabled third parties allowed per url bar origin.
3. Prevent third parties from storing HSTS rules. We have not yet decided upon
the best approach.

</p><p><span class="command"><strong>Implementation Status:</strong></span> Currently, HSTS state is
cleared by <a class="link" href="#new-identity" title="4.7. Long-Term Unlinkability via &quot;New Identity&quot; button">New Identity</a>, but we don't
defend against the creation of these cookies between <span class="command"><strong>New
Identity</strong></span> invocations.
      </p></li><li class="listitem">Exit node usage
     <p><span class="command"><strong>Design Goal:</strong></span>

Every distinct navigation session (as defined by a non-blank Referer header)
MUST exit through a fresh Tor circuit in Tor Browser to prevent exit node
observers from linking concurrent browsing activity.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

The Tor feature that supports this ability only exists in the 0.2.3.x-alpha
series. <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3455" target="_top">Ticket
#3455</a> is the Torbutton ticket to make use of the new Tor
functionality.

</p></li></ol></div><p>
For more details on identifier linkability bugs and enhancements, see the <a class="ulink" href="https://trac.torproject.org/projects/tor/query?keywords=~tbb-linkability&amp;status=!closed" target="_top">tbb-linkability tag in our bugtracker</a>
  </p></div><div class="sect2" title="4.6. Cross-Origin Fingerprinting Unlinkability"><div class="titlepage"><div><div><h3 class="title"><a id="fingerprinting-linkability"></a>4.6. Cross-Origin Fingerprinting Unlinkability</h3></div></div></div><p>

In order to properly address the fingerprinting adversary on a technical
level, we need a metric to measure linkability of the various browser
properties beyond any stored origin-related state. <a class="ulink" href="https://panopticlick.eff.org/about.php" target="_top">The Panopticlick Project</a>
by the EFF provides us with a prototype of such a metric. The researchers
conducted a survey of volunteers who were asked to visit an experiment page
that harvested many of the above components. They then computed the Shannon
Entropy of the resulting distribution of each of several key attributes to
determine how many bits of identifying information each attribute provided.

</p><p>

Many browser features have been added since the EFF first ran their experiment
and collected their data. To avoid an infinite sinkhole, we reduce the efforts
for fingerprinting resistance by only concerning ourselves with reducing the
fingerprintable differences <span class="emphasis"><em>among</em></span> Tor Browser users. We
do not believe it is possible to solve cross-browser fingerprinting issues.

</p><p>

Unfortunately, the unsolvable nature of the cross-browser fingerprinting
problem means that the Panopticlick test website itself is not useful for
evaluating the actual effectiveness of our defenses, or the fingerprinting
defenses of any other web browser. Because the Panopticlick dataset is based
on browser data spanning a number of widely deployed browsers over a number of
years, any fingerprinting defenses attempted by browsers today are very likely
to cause Panopticlick to report an <span class="emphasis"><em>increase</em></span> in
fingerprintability and entropy, because those defenses will stand out in sharp
contrast to historical data. We have been <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/6119" target="_top">working to convince
the EFF</a> that it is worthwhile to release the source code to
Panopticlick to allow us to run our own version for this reason.

</p><div class="sect3" title="Fingerprinting defenses in the Tor Browser"><div class="titlepage"><div><div><h4 class="title"><a id="fingerprinting-defenses"></a>Fingerprinting defenses in the Tor Browser</h4></div></div></div><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">Plugins
     <p>

Plugins add to fingerprinting risk via two main vectors: their mere presence in
window.navigator.plugins, as well as their internal functionality.

</p><p><span class="command"><strong>Design Goal:</strong></span>

All plugins that have not been specifically audited or sandboxed MUST be
disabled. To reduce linkability potential, even sandboxed plugins should not
be allowed to load objects until the user has clicked through a click-to-play
barrier.  Additionally, version information should be reduced or obfuscated
until the plugin object is loaded. For flash, we wish to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3974" target="_top">provide a
settings.sol file</a> to disable Flash cookies, and to restrict P2P
features that are likely to bypass proxy settings.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

Currently, we entirely disable all plugins in Tor Browser. However, as a
compromise due to the popularity of Flash, we allow users to re-enable Flash,
and flash objects are blocked behind a click-to-play barrier that is available
only after the user has specifically enabled plugins. Flash is the only plugin
available, the rest are <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0005-Block-all-plugins-except-flash.patch" target="_top">entirely
blocked from loading by a Firefox patch</a>. We also set the Firefox
preference <span class="command"><strong>plugin.expose_full_path</strong></span> to false, to avoid
leaking plugin installation information.

</p></li><li class="listitem">HTML5 Canvas Image Extraction
     <p>

The <a class="ulink" href="https://developer.mozilla.org/en-US/docs/HTML/Canvas" target="_top">HTML5
Canvas</a> is a feature that has been added to major browsers after the
EFF developed their Panopticlick study. After plugins and plugin-provided
information, we believe that the HTML5 Canvas is the single largest
fingerprinting threat browsers face today. <a class="ulink" href="http://www.w2spconf.com/2012/papers/w2sp12-final4.pdf" target="_top">Initial
studies</a> show that the Canvas can provide an easy-access fingerprinting
target: The adversary simply renders WebGL, font, and named color data to a
Canvas element, extracts the image buffer, and computes a hash of that image
data. Subtle differences in the video card, font packs, and even font and
graphics library versions allow the adversary to produce a stable, simple,
high-entropy fingerprint of a computer. In fact, the hash of the rendered
image can be used almost identically to a tracking cookie by the web server.

</p><p>

To reduce the threat from this vector, we have patched Firefox to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0020-Add-canvas-image-extraction-prompt.patch" target="_top">prompt
before returning valid image data</a> to the Canvas APIs. If the user
hasn't previously allowed the site in the URL bar to access Canvas image data,
pure white image data is returned to the Javascript APIs.

</p></li><li class="listitem">WebGL
     <p>

WebGL is fingerprintable both through information that is exposed about the
underlying driver and optimizations, as well as through performance
fingerprinting.

</p><p>

Because of the large amount of potential fingerprinting vectors and the <a class="ulink" href="http://www.contextis.com/resources/blog/webgl/" target="_top">previously unexposed
vulnerability surface</a>, we deploy a similar strategy against WebGL as
for plugins. First, WebGL Canvases have click-to-play placeholders (provided
by NoScript), and do not run until authorized by the user. Second, we
obfuscate driver information by setting the Firefox preferences
<span class="command"><strong>webgl.disable-extensions</strong></span> and
<span class="command"><strong>webgl.min_capability_mode</strong></span>, which reduce the information
provided by the following WebGL API calls: <span class="command"><strong>getParameter()</strong></span>,
<span class="command"><strong>getSupportedExtensions()</strong></span>, and
<span class="command"><strong>getExtension()</strong></span>.

</p></li><li class="listitem">Fonts
     <p>

According to the Panopticlick study, fonts provide the most linkability when
they are provided as an enumerable list in filesystem order, via either the
Flash or Java plugins. However, it is still possible to use CSS and/or
Javascript to query for the existence of specific fonts. With a large enough
pre-built list to query, a large amount of fingerprintable information may
still be available.

</p><p>

The sure-fire way to address font linkability is to ship the browser with a
font for every language, typeface, and style in use in the world, and to only
use those fonts at the exclusion of system fonts.  However, this set may be
impractically large. It is possible that a smaller <a class="ulink" href="https://secure.wikimedia.org/wikipedia/en/wiki/Unicode_typeface#List_of_Unicode_fonts" target="_top">common
subset</a> may be found that provides total coverage. However, we believe
that with strong url bar origin identifier isolation, a simpler approach can reduce the
number of bits available to the adversary while avoiding the rendering and
language issues of supporting a global font set.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

We disable plugins, which prevents font enumeration. Additionally, we limit
both the number of font queries from CSS, as well as the total number of 
fonts that can be used in a document <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0011-Limit-the-number-of-fonts-per-document.patch" target="_top">with
a Firefox patch</a>. We create two prefs,
<span class="command"><strong>browser.display.max_font_attempts</strong></span> and
<span class="command"><strong>browser.display.max_font_count</strong></span> for this purpose. Once these
limits are reached, the browser behaves as if
<span class="command"><strong>browser.display.use_document_fonts</strong></span> was set. We are
still working to determine optimal values for these prefs.

</p><p>

To improve rendering, we exempt remote <a class="ulink" href="https://developer.mozilla.org/en-US/docs/CSS/@font-face" target="_top">@font-face
fonts</a> from these counts, and if a font-family CSS rule lists a remote
font (in any order), we use that font instead of any of the named local fonts.

</p></li><li class="listitem">Desktop resolution, CSS Media Queries, and System Colors
     <p>

Both CSS and Javascript have access to a lot of information about the screen
resolution, usable desktop size, OS widget size, toolbar size, title bar size,
system theme colors, and other desktop features that are not at all relevant
to rendering and serve only to provide information for fingerprinting.

</p><p><span class="command"><strong>Design Goal:</strong></span>

Our design goal here is to reduce the resolution information down to the bare
minimum required for properly rendering inside a content window. We intend to 
report all rendering information correctly with respect to the size and
properties of the content window, but report an effective size of 0 for all
border material, and also report that the desktop is only as big as the
inner content window. Additionally, new browser windows are sized such that 
their content windows are one of a few fixed sizes based on the user's
desktop resolution.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

We have implemented the above strategy using a window observer to <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/blob/HEAD:/src/chrome/content/torbutton.js#l2004" target="_top">resize
new windows based on desktop resolution</a>. Additionally, we patch
Firefox to use the client content window size <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0022-Do-not-expose-physical-screen-info.-via-window-and-w.patch" target="_top">for
window.screen</a> and <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0010-Limit-device-and-system-specific-CSS-Media-Queries.patch" target="_top">for
CSS Media Queries</a>. Similarly, we <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0021-Return-client-window-coordinates-for-mouse-event-scr.patch" target="_top">patch
DOM events to return content window relative points</a>. We also patch
Firefox to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0023-Do-not-expose-system-colors-to-CSS-or-canvas.patch" target="_top">report
a fixed set of system colors to content window CSS</a>.

</p><p>

To further reduce resolution-based fingerprinting, we are <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/7256" target="_top">investigating
zoom/viewport-based mechanisms</a> that might allow us to always report
the same desktop resolution regardless of the actual size of the content
window, and simply scale to make up the difference. However, the complexity
and rendering impact of such a change is not yet known.

</p></li><li class="listitem">User Agent and HTTP Headers
     <p><span class="command"><strong>Design Goal:</strong></span>

All Tor Browser users MUST provide websites with an identical user agent and
HTTP header set for a given request type. We omit the Firefox minor revision,
and report a popular Windows platform. If the software is kept up to date,
these headers should remain identical across the population even when updated.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

Firefox provides several options for controlling the browser user agent string
which we leverage. We also set similar prefs for controlling the
Accept-Language and Accept-Charset headers, which we spoof to English by default. Additionally, we
<a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0001-Block-Components.interfaces-from-content.patch" target="_top">remove
content script access</a> to Components.interfaces, which <a class="ulink" href="http://pseudo-flaw.net/tor/torbutton/fingerprint-firefox.html" target="_top">can be
used</a> to fingerprint OS, platform, and Firefox minor version.  </p></li><li class="listitem">Timezone and clock offset
     <p><span class="command"><strong>Design Goal:</strong></span>

All Tor Browser users MUST report the same timezone to websites. Currently, we
choose UTC for this purpose, although an equally valid argument could be made
for EDT/EST due to the large English-speaking population density (coupled with
the fact that we spoof a US English user agent).  Additionally, the Tor
software should detect if the users clock is significantly divergent from the
clocks of the relays that it connects to, and use this to reset the clock
values used in Tor Browser to something reasonably accurate.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

We set the timezone using the TZ environment variable, which is supported on
all platforms. Additionally, we plan to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3652" target="_top">obtain a clock
offset from Tor</a>, but this won't be available until Tor 0.2.3.x is in
use.

</p></li><li class="listitem">Javascript performance fingerprinting
     <p>

<a class="ulink" href="http://w2spconf.com/2011/papers/jspriv.pdf" target="_top">Javascript performance
fingerprinting</a> is the act of profiling the performance
of various Javascript functions for the purpose of fingerprinting the
Javascript engine and the CPU.

</p><p><span class="command"><strong>Design Goal:</strong></span>

We have <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3059" target="_top">several potential
mitigation approaches</a> to reduce the accuracy of performance
fingerprinting without risking too much damage to functionality. Our current
favorite is to reduce the resolution of the Event.timeStamp and the Javascript
Date() object, while also introducing jitter. Our goal is to increase the
amount of time it takes to mount a successful attack. <a class="ulink" href="http://w2spconf.com/2011/papers/jspriv.pdf" target="_top">Mowery et al</a> found that
even with the default precision in most browsers, they required up to 120
seconds of amortization and repeated trials to get stable results from their
feature set. We intend to work with the research community to establish the
optimum trade-off between quantization+jitter and amortization time.

</p><p><span class="command"><strong>Implementation Status:</strong></span>

Currently, the only mitigation against performance fingerprinting is to
disable <a class="ulink" href="http://www.w3.org/TR/navigation-timing/" target="_top">Navigation
Timing</a> through the Firefox preference
<span class="command"><strong>dom.enable_performance</strong></span>.

</p></li><li class="listitem">Non-Uniform HTML5 API Implementations
     <p>

At least two HTML5 features have different implementation status across the
major OS vendors: the <a class="ulink" href="https://developer.mozilla.org/en-US/docs/DOM/window.navigator.battery" target="_top">Battery
API</a> and the <a class="ulink" href="https://developer.mozilla.org/en-US/docs/DOM/window.navigator.connection" target="_top">Network
Connection API</a>. We disable these APIs
through the Firefox preferences <span class="command"><strong>dom.battery.enabled</strong></span> and
<span class="command"><strong>dom.network.enabled</strong></span>.

</p></li><li class="listitem">Keystroke fingerprinting
     <p>

Keystroke fingerprinting is the act of measuring key strike time and key
flight time. It is seeing increasing use as a biometric.

</p><p><span class="command"><strong>Design Goal:</strong></span>

We intend to rely on the same mechanisms for defeating Javascript performance
fingerprinting: timestamp quantization and jitter.

</p><p><span class="command"><strong>Implementation Status:</strong></span>
We have no implementation as of yet.
     </p></li></ol></div></div><p>
For more details on identifier linkability bugs and enhancements, see the <a class="ulink" href="https://trac.torproject.org/projects/tor/query?keywords=~tbb-fingerprinting&amp;status=!closed" target="_top">tbb-fingerprinting tag in our bugtracker</a>
  </p></div><div class="sect2" title="4.7. Long-Term Unlinkability via &quot;New Identity&quot; button"><div class="titlepage"><div><div><h3 class="title"><a id="new-identity"></a>4.7. Long-Term Unlinkability via "New Identity" button</h3></div></div></div><p>

In order to avoid long-term linkability, we provide a "New Identity" context
menu option in Torbutton. This context menu option is active if Torbutton can
read the environment variables $TOR_CONTROL_PASSWD and $TOR_CONTROL_PORT.

</p><div class="sect3" title="Design Goal:"><div class="titlepage"><div><div><h4 class="title"><a id="idp5731056"></a>Design Goal:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote">

All linkable identifiers and browser state MUST be cleared by this feature.

</blockquote></div></div><div class="sect3" title="Implementation Status:"><div class="titlepage"><div><div><h4 class="title"><a id="idp5732304"></a>Implementation Status:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote"><p>

First, Torbutton disables Javascript in all open tabs and windows by using
both the <a class="ulink" href="https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsIDocShell#Attributes" target="_top">browser.docShell.allowJavascript</a>
attribute as well as <a class="ulink" href="https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsIDOMWindowUtils#suppressEventHandling%28%29" target="_top">nsIDOMWindowUtil.suppressEventHandling()</a>.
We then stop all page activity for each tab using <a class="ulink" href="https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsIWebNavigation#stop%28%29" target="_top">browser.webNavigation.stop(nsIWebNavigation.STOP_ALL)</a>.
We then clear the site-specific Zoom by temporarily disabling the preference
<span class="command"><strong>browser.zoom.siteSpecific</strong></span>, and clear the GeoIP wifi token URL
<span class="command"><strong>geo.wifi.access_token</strong></span> and the last opened URL prefs (if
they exist). Each tab is then closed.

</p><p>

After closing all tabs, we then emit "<a class="ulink" href="https://developer.mozilla.org/en-US/docs/Supporting_private_browsing_mode#Private_browsing_notifications" target="_top">browser:purge-session-history</a>"
(which instructs addons and various Firefox components to clear their session
state), and then manually clear the following state: searchbox and findbox
text, HTTP auth, SSL state, OCSP state, site-specific content preferences
(including HSTS state), content and image cache, offline cache, Cookies, DOM
storage, DOM local storage, the safe browsing key, and the Google wifi geolocation
token (if it exists).

</p><p>

After the state is cleared, we then close all remaining HTTP keep-alive
connections and then send the NEWNYM signal to the Tor control port to cause a
new circuit to be created.
     </p><p>
Finally, a fresh browser window is opened, and the current browser window is
closed (this does not spawn a new Firefox process, only a new window).
     </p></blockquote></div><div class="blockquote"><blockquote class="blockquote">
If the user chose to "protect" any cookies by using the Torbutton Cookie
Protections UI, those cookies are not cleared as part of the above.
    </blockquote></div></div></div><div class="sect2" title="4.8. Other Security Measures"><div class="titlepage"><div><div><h3 class="title"><a id="other-security"></a>4.8. Other Security Measures</h3></div></div></div><p>

In addition to the above mechanisms that are devoted to preserving privacy
while browsing, we also have a number of technical mechanisms to address other
privacy and security issues.

</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><a id="traffic-fingerprinting-defenses"></a><span class="command"><strong>Website Traffic Fingerprinting Defenses</strong></span><p>

<a class="link" href="#website-traffic-fingerprinting">Website Traffic
Fingerprinting</a> is a statistical attack to attempt to recognize specific
encrypted website activity.

</p><div class="sect3" title="Design Goal:"><div class="titlepage"><div><div><h4 class="title"><a id="idp5746320"></a>Design Goal:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote"><p>

We want to deploy a mechanism that reduces the accuracy of <a class="ulink" href="https://en.wikipedia.org/wiki/Feature_selection" target="_top">useful features</a> available
for classification. This mechanism would either impact the true and false
positive accuracy rates, <span class="emphasis"><em>or</em></span> reduce the number of webpages
that could be classified at a given accuracy rate.

</p><p>

Ideally, this mechanism would be as light-weight as possible, and would be
tunable in terms of overhead. We suspect that it may even be possible to
deploy a mechanism that reduces feature extraction resolution without any
network overhead. In the no-overhead category, we have <a class="ulink" href="http://freehaven.net/anonbib/cache/LZCLCP_NDSS11.pdf" target="_top">HTTPOS</a> and
<a class="ulink" href="https://blog.torproject.org/blog/experimental-defense-website-traffic-fingerprinting" target="_top">better
use of HTTP pipelining and/or SPDY</a>. 
In the tunable/low-overhead
category, we have <a class="ulink" href="http://freehaven.net/anonbib/cache/ShWa-Timing06.pdf" target="_top">Adaptive
Padding</a> and <a class="ulink" href="http://www.cs.sunysb.edu/~xcai/fp.pdf" target="_top">
Congestion-Sensitive BUFLO</a>. It may be also possible to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/7028" target="_top">tune such
defenses</a> such that they only use existing spare Guard bandwidth capacity in the Tor
network, making them also effectively no-overhead.

</p></blockquote></div></div><div class="sect3" title="Implementation Status:"><div class="titlepage"><div><div><h4 class="title"><a id="idp5753216"></a>Implementation Status:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote"><p>
Currently, we patch Firefox to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0017-Randomize-HTTP-request-order-and-pipeline-depth.patch" target="_top">randomize
pipeline order and depth</a>. Unfortunately, pipelining is very fragile.
Many sites do not support it, and even sites that advertise support for
pipelining may simply return error codes for successive requests, effectively
forcing the browser into non-pipelined behavior. Firefox also has code to back
off and reduce or eliminate the pipeline if this happens. These
shortcomings and fallback behaviors are the primary reason that Google
developed SPDY as opposed simply extending HTTP to improve pipelining. It
turns out that we could actually deploy exit-side proxies that allow us to
<a class="ulink" href="https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/ideas/xxx-using-spdy.txt" target="_top">use
SPDY from the client to the exit node</a>. This would make our defense not
only free, but one that actually <span class="emphasis"><em>improves</em></span> performance.

</p><p>

Knowing this, we created this defense as an <a class="ulink" href="https://blog.torproject.org/blog/experimental-defense-website-traffic-fingerprinting" target="_top">experimental
research prototype</a> to help evaluate what could be done in the best
case with full server support. Unfortunately, the bias in favor of compelling
attack papers has caused academia to ignore this request thus far, instead
publishing only cursory (yet "devastating") evaluations that fail to provide
even simple statistics such as the rates of actual pipeline utilization during
their evaluations, in addition to the other shortcomings and shortcuts <a class="link" href="#website-traffic-fingerprinting">mentioned earlier</a>. We can
accept that our defense might fail to work as well as others (in fact we
expect it), but unfortunately the very same shortcuts that provide excellent
attack results also allow the conclusion that all defenses are broken forever.
So sadly, we are still left in the dark on this point.

</p></blockquote></div></div></li><li class="listitem"><span class="command"><strong>Privacy-preserving update notification</strong></span><p>

In order to inform the user when their Tor Browser is out of date, we perform a
privacy-preserving update check asynchronously in the background. The
check uses Tor to download the file <a class="ulink" href="https://check.torproject.org/RecommendedTBBVersions" target="_top">https://check.torproject.org/RecommendedTBBVersions</a>
and searches that version list for the current value for the local preference
<span class="command"><strong>torbrowser.version</strong></span>. If the value from our preference is
present in the recommended version list, the check is considered to have
succeeded and the user is up to date. If not, it is considered to have failed
and an update is needed. The check is triggered upon browser launch, new
window, and new tab, but is rate limited so as to happen no more frequently
than once every 1.5 hours.

</p><p>

If the check fails, we cache this fact, and update the Torbutton graphic to
display a flashing warning icon and insert a menu option that provides a link
to our download page. Additionally, we reset the value for the browser
homepage to point to a <a class="ulink" href="https://check.torproject.org/?lang=en-US&amp;small=1&amp;uptodate=0" target="_top">page that
informs the user</a> that their browser is out of
date.

</p></li></ol></div></div><div class="sect2" title="4.9. Description of Firefox Patches"><div class="titlepage"><div><div><h3 class="title"><a id="firefox-patches"></a>4.9. Description of Firefox Patches</h3></div></div></div><p>

The set of patches we have against Firefox can be found in the <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/tree/maint-2.4:/src/current-patches/firefox" target="_top">current-patches directory of the torbrowser git repository</a>. They are:

</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0001-Block-Components.interfaces-from-content.patch" target="_top">Block
Components.interfaces</a><p>

In order to reduce fingerprinting, we block access to this interface from
content script. Components.interfaces can be used for fingerprinting the
platform, OS, and Firebox version, but not much else.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0002-Make-Permissions-Manager-memory-only.patch" target="_top">Make
Permissions Manager memory only</a><p>

This patch exposes a pref 'permissions.memory_only' that properly isolates the
permissions manager to memory, which is responsible for all user specified
site permissions, as well as stored <a class="ulink" href="https://secure.wikimedia.org/wikipedia/en/wiki/HTTP_Strict_Transport_Security" target="_top">HSTS</a>
policy from visited sites.

The pref does successfully clear the permissions manager memory if toggled. It
does not need to be set in prefs.js, and can be handled by Torbutton.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0003-Make-Intermediate-Cert-Store-memory-only.patch" target="_top">Make
Intermediate Cert Store memory-only</a><p>

The intermediate certificate store records the intermediate SSL certificates
the browser has seen to date. Because these intermediate certificates are used 
by a limited number of domains (and in some cases, only a single domain),
the intermediate certificate store can serve as a low-resolution record of
browsing history.

</p><p><span class="command"><strong>Design Goal:</strong></span>

As an additional design goal, we would like to later alter this patch to allow this
information to be cleared from memory. The implementation does not currently
allow this.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0004-Add-a-string-based-cacheKey.patch" target="_top">Add
a string-based cacheKey property for domain isolation</a><p>

To <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3666" target="_top">increase the
security of cache isolation</a> and to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3754" target="_top">solve strange and
unknown conflicts with OCSP</a>, we had to patch
Firefox to provide a cacheDomain cache attribute. We use the url bar
FQDN as input to this field.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0005-Block-all-plugins-except-flash.patch" target="_top">Block
all plugins except flash</a><p>
We cannot use the <a class="ulink" href="http://www.oxymoronical.com/experiments/xpcomref/applications/Firefox/3.5/components/@mozilla.org/extensions/blocklist%3B1" target="_top">
@mozilla.org/extensions/blocklist;1</a> service, because we
actually want to stop plugins from ever entering the browser's process space
and/or executing code (for example, AV plugins that collect statistics/analyze
URLs, magical toolbars that phone home or "help" the user, Skype buttons that
ruin our day, and censorship filters). Hence we rolled our own.
     </p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0006-Make-content-pref-service-memory-only-clearable.patch" target="_top">Make content-prefs service memory only</a><p>
This patch prevents random URLs from being inserted into content-prefs.sqlite in
the profile directory as content prefs change (includes site-zoom and perhaps
other site prefs?).
     </p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0007-Make-Tor-Browser-exit-when-not-launched-from-Vidalia.patch" target="_top">Make Tor Browser exit when not launched from Vidalia</a><p>

It turns out that on Windows 7 and later systems, the Taskbar attempts to
automatically learn the most frequent apps used by the user, and it recognizes
Tor Browser as a separate app from Vidalia. This can cause users to try to
launch Tor Browser without Vidalia or a Tor instance running. Worse, the Tor
Browser will automatically find their default Firefox profile, and properly
connect directly without using Tor. This patch is a simple hack to cause Tor
Browser to immediately exit in this case.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0008-Disable-SSL-Session-ID-tracking.patch" target="_top">Disable SSL Session ID tracking</a><p>

This patch is a simple 1-line hack to prevent SSL connections from caching
(and then later transmitting) their Session IDs. There was no preference to
govern this behavior, so we had to hack it by altering the SSL new connection
defaults.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0009-Provide-an-observer-event-to-close-persistent-connec.patch" target="_top">Provide an observer event to close persistent connections</a><p>

This patch creates an observer event in the HTTP connection manager to close
all keep-alive connections that still happen to be open. This event is emitted
by the <a class="link" href="#new-identity" title="4.7. Long-Term Unlinkability via &quot;New Identity&quot; button">New Identity</a> button.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0010-Limit-device-and-system-specific-CSS-Media-Queries.patch" target="_top">Limit Device and System Specific Media Queries</a><p>

<a class="ulink" href="https://developer.mozilla.org/en-US/docs/CSS/Media_queries" target="_top">CSS
Media Queries</a> have a fingerprinting capability approaching that of
Javascript. This patch causes such Media Queries to evaluate as if the device
resolution was equal to the content window resolution.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0011-Limit-the-number-of-fonts-per-document.patch" target="_top">Limit the number of fonts per document</a><p>

Font availability can be <a class="ulink" href="http://flippingtypical.com/" target="_top">queried by
CSS and Javascript</a> and is a fingerprinting vector. This patch limits
the number of times CSS and Javascript can cause font-family rules to
evaluate. Remote @font-face fonts are exempt from the limits imposed by this
patch, and remote fonts are given priority over local fonts whenever both
appear in the same font-family rule. We do this by explicitly altering the
nsRuleNode rule represenation itself to remove the local font families before
the rule hits the font renderer.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0012-Rebrand-Firefox-to-TorBrowser.patch" target="_top">Rebrand Firefox to Tor Browser</a><p>

This patch updates our branding in compliance with Mozilla's trademark policy.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0013-Make-Download-manager-memory-only.patch" target="_top">Make Download Manager Memory Only</a><p>

This patch prevents disk leaks from the download manager. The original
behavior is to write the download history to disk and then delete it, even if
you disable download history from your Firefox preferences.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0014-Add-DDG-and-StartPage-to-Omnibox.patch" target="_top">Add DDG and StartPage to Omnibox</a><p>

This patch adds DuckDuckGo and StartPage to the Search Box, and sets our
default search engine to StartPage. We deployed this patch due to excessive
Captchas and complete 403 bans from Google.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0015-Make-nsICacheService.EvictEntries-synchronous.patch" target="_top">Make nsICacheService.EvictEntries() Synchronous</a><p>

This patch eliminates a race condition with "New Identity". Without it,
cache-based Evercookies survive for up to a minute after clearing the cache
on some platforms.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0016-Prevent-WebSocket-DNS-leak.patch" target="_top">Prevent WebSockets DNS Leak</a><p>

This patch prevents a DNS leak when using WebSockets. It also prevents other
similar types of DNS leaks.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0017-Randomize-HTTP-request-order-and-pipeline-depth.patch" target="_top">Randomize HTTP pipeline order and depth</a><p>
As an 
<a class="ulink" href="https://blog.torproject.org/blog/experimental-defense-website-traffic-fingerprinting" target="_top">experimental
defense against Website Traffic Fingerprinting</a>, we patch the standard
HTTP pipelining code to randomize the number of requests in a
pipeline, as well as their order.
     </p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0018-Emit-observer-event-to-filter-the-Drag-Drop-url-list.patch" target="_top">Emit
an observer event to filter the Drag and Drop URL list</a><p>

This patch allows us to block external Drag and Drop events from Torbutton.
We need to block Drag and Drop because Mac OS and Ubuntu both immediately load
any URLs they find in your drag buffer before you even drop them (without
using your browser's proxy settings, of course). This can lead to proxy bypass
during user activity that is as basic as holding down the mouse button for
slightly too long while clicking on an image link.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0019-Add-mozIThirdPartyUtil.getFirstPartyURI-API.patch" target="_top">Add mozIThirdPartyUtil.getFirstPartyURI() API</a><p>

This patch provides an API that allows us to more easily isolate identifiers
to the URL bar domain.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0020-Add-canvas-image-extraction-prompt.patch" target="_top">Add canvas image extraction prompt</a><p>

This patch prompts the user before returning canvas image data. Canvas image
data can be used to create an extremely stable, high-entropy fingerprint based
on the unique rendering behavior of video cards, OpenGL behavior,
system fonts, and supporting library versions.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0021-Return-client-window-coordinates-for-mouse-event-scr.patch" target="_top">Return client window coordinates for mouse events</a><p>

This patch causes mouse events to return coordinates relative to the content
window instead of the desktop.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0022-Do-not-expose-physical-screen-info.-via-window-and-w.patch" target="_top">Do not expose physical screen info to window.screen</a><p>

This patch causes window.screen to return the display resolution size of the
content window instead of the desktop resolution size.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0023-Do-not-expose-system-colors-to-CSS-or-canvas.patch" target="_top">Do not expose system colors to CSS or canvas</a><p>

This patch prevents CSS and Javascript from discovering your desktop color
scheme and/or theme.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0024-Isolate-the-Image-Cache-per-url-bar-domain.patch" target="_top">Isolate the Image Cache per url bar domain</a><p>

This patch prevents cached images from being used to store third party tracking
identifiers.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0025-nsIHTTPChannel.redirectTo-API.patch" target="_top">nsIHTTPChannel.redirectTo() API</a><p>

This patch provides HTTPS-Everywhere with an API to perform redirections more
securely and without addon conflicts.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0026-Isolate-DOM-storage-to-first-party-URI.patch" target="_top">Isolate DOM Storage to first party URI</a><p>

This patch prevents DOM Storage from being used to store third party tracking
identifiers.

</p></li><li class="listitem"><a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/maint-2.4:/src/current-patches/firefox/0027-Remove-This-plugin-is-disabled-barrier.patch" target="_top">Remove
"This plugin is disabled" barrier</a><p>

This patch removes a barrier that was informing users that plugins were
disabled and providing them with a link to enable them. We felt this was poor
user experience, especially since the barrier was displayed even for sites
with dual Flash+HTML5 video players, such as YouTube.

</p></li></ol></div></div></div><div class="appendix" title="A. Towards Transparency in Navigation Tracking"><h2 class="title" style="clear: both"><a id="Transparency"></a>A. Towards Transparency in Navigation Tracking</h2><p>

The <a class="link" href="#privacy" title="2.2. Privacy Requirements">privacy properties</a> of Tor Browser are based
upon the assumption that link-click navigation indicates user consent to
tracking between the linking site and the destination site.  While this
definition is sufficient to allow us to eliminate cross-site third party
tracking with only minimal site breakage, it is our long-term goal to further
reduce cross-origin click navigation tracking to mechanisms that are
detectable by attentive users, so they can alert the general public if
cross-origin click navigation tracking is happening where it should not be.

</p><p>

In an ideal world, the mechanisms of tracking that can be employed during a
link click would be limited to the contents of URL parameters and other
properties that are fully visible to the user before they click. However, the
entrenched nature of certain archaic web features make it impossible for us to
achieve this transparency goal by ourselves without substantial site breakage.
So, instead we maintain a <a class="link" href="#deprecate" title="A.1. Deprecation Wishlist">Deprecation
Wishlist</a> of archaic web technologies that are currently being (ab)used
to facilitate federated login and other legitimate click-driven cross-domain
activity but that can one day be replaced with more privacy friendly,
auditable alternatives.

</p><p>

Because the total elimination of side channels during cross-origin navigation
will undoubtedly break federated login as well as destroy ad revenue, we
also describe auditable alternatives and promising web draft standards that would
preserve this functionality while still providing transparency when tracking is
occurring.

</p><div class="sect1" title="A.1. Deprecation Wishlist"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="deprecate"></a>A.1. Deprecation Wishlist</h2></div></div></div><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">The Referer Header
  <p>

We haven't disabled or restricted the Referer ourselves because of the
non-trivial number of sites that rely on the Referer header to "authenticate"
image requests and deep-link navigation on their sites. Furthermore, there
seems to be no real privacy benefit to taking this action by itself in a
vacuum, because many sites have begun encoding Referer URL information into
GET parameters when they need it to cross http to https scheme transitions.
Google's +1 buttons are the best example of this activity.

</p><p>

Because of the availability of these other explicit vectors, we believe the
main risk of the Referer header is through inadvertent and/or covert data
leakage.  In fact, <a class="ulink" href="http://www2.research.att.com/~bala/papers/wosn09.pdf" target="_top">a great deal of
personal data</a> is inadvertently leaked to third parties through the
source URL parameters.

</p><p>

We believe the Referer header should be made explicit. If a site wishes to
transmit its URL to third party content elements during load or during
link-click, it should have to specify this as a property of the associated HTML
tag. With an explicit property, it would then be possible for the user agent to
inform the user if they are about to click on a link that will transmit Referer
information (perhaps through something as subtle as a different color in the
lower toolbar for the destination URL). This same UI notification can also be
used for links with the <a class="ulink" href="https://developer.mozilla.org/en-US/docs/HTML/Element/a#Attributes" target="_top">"ping"</a>
attribute.

</p></li><li class="listitem">window.name
   <p>
<a class="ulink" href="https://developer.mozilla.org/En/DOM/Window.name" target="_top">window.name</a> is
a DOM property that for some reason is allowed to retain a persistent value
for the lifespan of a browser tab. It is possible to utilize this property for
<a class="ulink" href="http://www.thomasfrank.se/sessionvars.html" target="_top">identifier
storage</a> during click navigation. This is sometimes used for additional
XSRF protection and federated login.
   </p><p>

It's our opinion that the contents of window.name should not be preserved for
cross-origin navigation, but doing so may break federated login for some sites.

</p></li><li class="listitem">Javascript link rewriting
   <p>

In general, it should not be possible for onclick handlers to alter the
navigation destination of 'a' tags, silently transform them into POST
requests, or otherwise create situations where a user believes they are
clicking on a link leading to one URL that ends up on another. This
functionality is deceptive and is frequently a vector for malware and phishing
attacks. Unfortunately, many legitimate sites also employ such transparent
link rewriting, and blanket disabling this functionality ourselves will simply
cause Tor Browser to fail to navigate properly on these sites.

</p><p>

Automated cross-origin redirects are one form of this behavior that is
possible for us to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3600" target="_top">address
ourselves</a>, as they are comparatively rare and can be handled with site
permissions.

</p></li></ol></div></div><div class="sect1" title="A.2. Promising Standards"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="idp5843792"></a>A.2. Promising Standards</h2></div></div></div><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><a class="ulink" href="http://web-send.org" target="_top">Web-Send Introducer</a><p>

Web-Send is a browser-based link sharing and federated login widget that is
designed to operate without relying on third-party tracking or abusing other
cross-origin link-click side channels. It has a compelling list of <a class="ulink" href="http://web-send.org/features.html" target="_top">privacy and security features</a>,
especially if used as a "Like button" replacement.

</p></li><li class="listitem"><a class="ulink" href="https://developer.mozilla.org/en-US/docs/Persona" target="_top">Mozilla Persona</a><p>

Mozilla's Persona is designed to provide decentralized, cryptographically
authenticated federated login in a way that does not expose the user to third
party tracking or require browser redirects or side channels. While it does
not directly provide the link sharing capabilities that Web-Send does, it is a
better solution to the privacy issues associated with federated login than
Web-Send is.

</p></li></ol></div></div></div></div></body></html>

tor-webwml.git

TBB Design Doc: Mention use of SPDY at exits.