projects/torbrowser/design/index.html.en (9e00fe357) - tor-webwml.git

9e00fe357cb9e9004f05ba1e45c920ba7b770a18

Add design doc draft. Mike Perry authored 12 years ago	`projects/en/torbrowser/design/index.html.en 1) <?xml version="1.0" encoding="UTF-8"?> projects/en/torbrowser/design/index.html.en 2) <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">`
Add a couple extra sentence... Mike Perry authored 12 years ago	projects/torbrowser/design/index.html.en 3) <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 V1.75.2" /></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"><<a class="email" href="mailto:mikeperry#torproject org">mikeperry#torproject org</a>></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"><<a class="email" href="mailto:erinn_torproject\org">erinn_torproject\org</a>></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"><<a class="email" href="mailto:sjmurdoch#torproject\org">sjmurdoch#torproject\org</a>></code></p></div></div></div></div><div><p class="pubdate">Sep 29 2011</p></div></div><hr /></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="#id2974058">1. Introduction</a></span></dt><dd><dl><dt><span class="sect2"><a href="#adversary">1.1. Adversary Model</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="#Implementation">3. Implementation</a></span></dt><dd><dl><dt><span class="sect2"><a href="#proxy-obedience">3.1. Proxy Obedience</a></span></dt><dt><span class="sect2"><a href="#state-separation">3.2. State Separation</a></span></dt><dt><span class="sect2"><a href="#disk-avoidance">3.3. Disk Avoidance</a></span></dt><dt><span class="sect2"><a href="#app-data-isolation">3.4. Application Data Isolation</a></span></dt><dt><span class="sect2"><a href="#identifier-linkability">3.5. Cross-Domain Identifier Unlinkability</a></span></dt><dt><span class="sect2"><a href="#fingerprinting-linkability">3.6. Cross-Domain Fingerprinting Unlinkability</a></span></dt><dt><span class="sect2"><a href="#new-identity">3.7. Long-Term Unlinkability via "New Identity" button</a></span></dt><dt><span class="sect2"><a href="#click-to-play">3.8. Click-to-play for plugins and invasive content</a></span></dt><dt><span class="sect2"><a href="#firefox-patches">3.9. Description of Firefox Patches</a></span></dt></dl></dd><dt><span class="sect1"><a href="#Packaging">4. Packaging</a></span></dt><dd><dl><dt><span class="sect2"><a href="#build-security">4.1. Build Process Security</a></span></dt><dt><span class="sect2"><a href="#addons">4.2. External Addons</a></span></dt><dt><span class="sect2"><a href="#prefs">4.3. Pref Changes</a></span></dt><dt><span class="sect2"><a href="#update-mechanism">4.4. Update Security</a></span></dt></dl></dd><dt><span class="sect1"><a href="#Testing">5. Testing</a></span></dt><dd><dl><dt><span class="sect2"><a href="#SingleStateTesting">5.1. Single state testing</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="id2974058"></a>1. Introduction</h2></div></div></div><p>
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 4) projects/en/torbrowser/design/index.html.en 5) This document describes the <a class="link" href="#adversary" title="1.1. Adversary Model">adversary model</a>, projects/en/torbrowser/design/index.html.en 6) <a class="link" href="#DesignRequirements" title="2. Design Requirements and Philosophy">design requirements</a>, projects/en/torbrowser/design/index.html.en 7) <a class="link" href="#Implementation" title="3. Implementation">implementation</a>, <a class="link" href="#Packaging" title="4. Packaging">packaging</a> and <a class="link" href="#Testing" title="5. Testing">testing projects/en/torbrowser/design/index.html.en 8) procedures</a> of the Tor Browser. It is projects/en/torbrowser/design/index.html.en 9) current as of Tor Browser 2.2.32-4. projects/en/torbrowser/design/index.html.en 10) projects/en/torbrowser/design/index.html.en 11) </p><p> projects/en/torbrowser/design/index.html.en 12) projects/en/torbrowser/design/index.html.en 13) This document is also meant to serve as a set of design requirements and to projects/en/torbrowser/design/index.html.en 14) describe a reference implementation of a Private Browsing Mode that defends projects/en/torbrowser/design/index.html.en 15) against both local and network adversaries. projects/en/torbrowser/design/index.html.en 16) projects/en/torbrowser/design/index.html.en 17) </p><div class="sect2" title="1.1. Adversary Model"><div class="titlepage"><div><div><h3 class="title"><a id="adversary"></a>1.1. Adversary Model</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 18) projects/en/torbrowser/design/index.html.en 19) A Tor web browser adversary has a number of goals, capabilities, and attack projects/en/torbrowser/design/index.html.en 20) types that can be used to guide us towards a set of requirements for the projects/en/torbrowser/design/index.html.en 21) Tor Browser. Let's start with the goals. projects/en/torbrowser/design/index.html.en 22) projects/en/torbrowser/design/index.html.en 23) </p><div class="sect3" title="Adversary Goals"><div class="titlepage"><div><div><h4 class="title"><a id="adversarygoals"></a>Adversary Goals</h4></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 projects/en/torbrowser/design/index.html.en 24) Tor, causing the user to directly connect to an IP of the adversary's projects/en/torbrowser/design/index.html.en 25) 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 projects/en/torbrowser/design/index.html.en 26) happily settle for the ability to correlate something a user did via Tor with projects/en/torbrowser/design/index.html.en 27) their non-Tor activity. This can be done with cookies, cache identifiers, projects/en/torbrowser/design/index.html.en 28) javascript events, and even CSS. Sometimes the fact that a user uses Tor may projects/en/torbrowser/design/index.html.en 29) be enough for some authorities.</p></li><li class="listitem"><span class="command"><strong>History disclosure</strong></span><p> projects/en/torbrowser/design/index.html.en 30) The adversary may also be interested in history disclosure: the ability to projects/en/torbrowser/design/index.html.en 31) query a user's history to see if they have issued certain censored search projects/en/torbrowser/design/index.html.en 32) queries, or visited censored sites. projects/en/torbrowser/design/index.html.en 33) </p></li><li class="listitem"><span class="command"><strong>Location information</strong></span><p> projects/en/torbrowser/design/index.html.en 34) projects/en/torbrowser/design/index.html.en 35) Location information such as timezone and locality can be useful for the projects/en/torbrowser/design/index.html.en 36) adversary to determine if a user is in fact originating from one of the projects/en/torbrowser/design/index.html.en 37) regions they are attempting to control, or to zero-in on the geographical projects/en/torbrowser/design/index.html.en 38) location of a particular dissident or whistleblower. projects/en/torbrowser/design/index.html.en 39) projects/en/torbrowser/design/index.html.en 40) </p></li><li class="listitem"><span class="command"><strong>Miscellaneous anonymity set reduction</strong></span><p> projects/en/torbrowser/design/index.html.en 41) projects/en/torbrowser/design/index.html.en 42) Anonymity set reduction is also useful in attempting to zero in on a projects/en/torbrowser/design/index.html.en 43) particular individual. If the dissident or whistleblower is using a rare build projects/en/torbrowser/design/index.html.en 44) of Firefox for an obscure operating system, this can be very useful projects/en/torbrowser/design/index.html.en 45) information for tracking them down, or at least <a class="link" href="#fingerprinting">tracking their activities</a>. projects/en/torbrowser/design/index.html.en 46) projects/en/torbrowser/design/index.html.en 47) </p></li><li class="listitem"><span class="command"><strong>History records and other on-disk projects/en/torbrowser/design/index.html.en 48) information</strong></span><p> projects/en/torbrowser/design/index.html.en 49) In some cases, the adversary may opt for a heavy-handed approach, such as projects/en/torbrowser/design/index.html.en 50) seizing the computers of all Tor users in an area (especially after narrowing projects/en/torbrowser/design/index.html.en 51) the field by the above two pieces of information). History records and cache projects/en/torbrowser/design/index.html.en 52) data are the primary goals here. projects/en/torbrowser/design/index.html.en 53) </p></li></ol></div></div><div class="sect3" title="Adversary Capabilities - Positioning"><div class="titlepage"><div><div><h4 class="title"><a id="adversarypositioning"></a>Adversary Capabilities - Positioning</h4></div></div></div><p> projects/en/torbrowser/design/index.html.en 54) The adversary can position themselves at a number of different locations in projects/en/torbrowser/design/index.html.en 55) order to execute their attacks. projects/en/torbrowser/design/index.html.en 56) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Exit Node or Upstream Router</strong></span><p> projects/en/torbrowser/design/index.html.en 57) The adversary can run exit nodes, or alternatively, they may control routers projects/en/torbrowser/design/index.html.en 58) upstream of exit nodes. Both of these scenarios have been observed in the projects/en/torbrowser/design/index.html.en 59) wild. projects/en/torbrowser/design/index.html.en 60) </p></li><li class="listitem"><span class="command"><strong>Adservers and/or Malicious Websites</strong></span><p> projects/en/torbrowser/design/index.html.en 61) The adversary can also run websites, or more likely, they can contract out projects/en/torbrowser/design/index.html.en 62) ad space from a number of different adservers and inject content that way. For projects/en/torbrowser/design/index.html.en 63) some users, the adversary may be the adservers themselves. It is not projects/en/torbrowser/design/index.html.en 64) inconceivable that adservers may try to subvert or reduce a user's anonymity projects/en/torbrowser/design/index.html.en 65) through Tor for marketing purposes. projects/en/torbrowser/design/index.html.en 66) </p></li><li class="listitem"><span class="command"><strong>Local Network/ISP/Upstream Router</strong></span><p> projects/en/torbrowser/design/index.html.en 67) The adversary can also inject malicious content at the user's upstream router projects/en/torbrowser/design/index.html.en 68) when they have Tor disabled, in an attempt to correlate their Tor and Non-Tor projects/en/torbrowser/design/index.html.en 69) activity. projects/en/torbrowser/design/index.html.en 70) </p></li><li class="listitem"><span class="command"><strong>Physical Access</strong></span><p> projects/en/torbrowser/design/index.html.en 71) Some users face adversaries with intermittent or constant physical access. projects/en/torbrowser/design/index.html.en 72) Users in Internet cafes, for example, face such a threat. In addition, in projects/en/torbrowser/design/index.html.en 73) countries where simply using tools like Tor is illegal, users may face projects/en/torbrowser/design/index.html.en 74) confiscation of their computer equipment for excessive Tor usage or just projects/en/torbrowser/design/index.html.en 75) general suspicion. projects/en/torbrowser/design/index.html.en 76) </p></li></ol></div></div><div class="sect3" title="Adversary Capabilities - Attacks"><div class="titlepage"><div><div><h4 class="title"><a id="attacks"></a>Adversary Capabilities - Attacks</h4></div></div></div><p> projects/en/torbrowser/design/index.html.en 77) projects/en/torbrowser/design/index.html.en 78) The adversary can perform the following attacks from a number of different projects/en/torbrowser/design/index.html.en 79) positions to accomplish various aspects of their goals. It should be noted projects/en/torbrowser/design/index.html.en 80) that many of these attacks (especially those involving IP address leakage) are projects/en/torbrowser/design/index.html.en 81) often performed by accident by websites that simply have Javascript, dynamic projects/en/torbrowser/design/index.html.en 82) CSS elements, and plugins. Others are performed by adservers seeking to projects/en/torbrowser/design/index.html.en 83) correlate users' activity across different IP addresses, and still others are projects/en/torbrowser/design/index.html.en 84) performed by malicious agents on the Tor network and at national firewalls. projects/en/torbrowser/design/index.html.en 85) projects/en/torbrowser/design/index.html.en 86) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Inserting Javascript</strong></span><p> projects/en/torbrowser/design/index.html.en 87) If not properly disabled, Javascript event handlers and timers projects/en/torbrowser/design/index.html.en 88) can cause the browser to perform network activity after Tor has been disabled, projects/en/torbrowser/design/index.html.en 89) thus allowing the adversary to correlate Tor and Non-Tor activity and reveal projects/en/torbrowser/design/index.html.en 90) a user's non-Tor IP address. Javascript projects/en/torbrowser/design/index.html.en 91) also allows the adversary to execute <a class="ulink" href="http://whattheinternetknowsaboutyou.com/" target="_top">history disclosure attacks</a>: projects/en/torbrowser/design/index.html.en 92) to query the history via the different attributes of 'visited' links to search projects/en/torbrowser/design/index.html.en 93) for particular Google queries, sites, or even to <a class="ulink" href="http://www.mikeonads.com/2008/07/13/using-your-browser-url-history-estimate-gender/" target="_top">profile projects/en/torbrowser/design/index.html.en 94) users based on gender and other classifications</a>. Finally, projects/en/torbrowser/design/index.html.en 95) Javascript can be used to query the user's timezone via the projects/en/torbrowser/design/index.html.en 96) <code class="function">Date()</code> object, and to reduce the anonymity set by querying projects/en/torbrowser/design/index.html.en 97) the <code class="function">navigator</code> object for operating system, CPU, locale, projects/en/torbrowser/design/index.html.en 98) and user agent information. projects/en/torbrowser/design/index.html.en 99) </p></li><li class="listitem"><span class="command"><strong>Inserting Plugins</strong></span><p> projects/en/torbrowser/design/index.html.en 100) projects/en/torbrowser/design/index.html.en 101) Plugins are abysmal at obeying the proxy settings of the browser. Every plugin projects/en/torbrowser/design/index.html.en 102) capable of performing network activity that the author has projects/en/torbrowser/design/index.html.en 103) investigated is also capable of performing network activity independent of projects/en/torbrowser/design/index.html.en 104) browser proxy settings - and often independent of its own proxy settings. projects/en/torbrowser/design/index.html.en 105) Sites that have plugin content don't even have to be malicious to obtain a projects/en/torbrowser/design/index.html.en 106) user's projects/en/torbrowser/design/index.html.en 107) Non-Tor IP (it usually leaks by itself), though <a class="ulink" href="http://decloak.net" target="_top">plenty of active projects/en/torbrowser/design/index.html.en 108) exploits</a> are possible as well. In addition, plugins can be used to store unique identifiers that are more projects/en/torbrowser/design/index.html.en 109) difficult to clear than standard cookies. projects/en/torbrowser/design/index.html.en 110) <a class="ulink" href="http://epic.org/privacy/cookies/flash.html" target="_top">Flash-based projects/en/torbrowser/design/index.html.en 111) cookies</a> fall into this category, but there are likely numerous other projects/en/torbrowser/design/index.html.en 112) examples. projects/en/torbrowser/design/index.html.en 113) projects/en/torbrowser/design/index.html.en 114) </p></li><li class="listitem"><span class="command"><strong>Inserting CSS</strong></span><p> projects/en/torbrowser/design/index.html.en 115) projects/en/torbrowser/design/index.html.en 116) CSS can also be used to correlate Tor and Non-Tor activity and reveal a user's projects/en/torbrowser/design/index.html.en 117) Non-Tor IP address, via the usage of projects/en/torbrowser/design/index.html.en 118) <a class="ulink" href="http://www.tjkdesign.com/articles/css%20pop%20ups/" target="_top">CSS projects/en/torbrowser/design/index.html.en 119) popups</a> - essentially CSS-based event handlers that fetch content via projects/en/torbrowser/design/index.html.en 120) CSS's onmouseover attribute. If these popups are allowed to perform network projects/en/torbrowser/design/index.html.en 121) activity in a different Tor state than they were loaded in, they can easily projects/en/torbrowser/design/index.html.en 122) correlate Tor and Non-Tor activity and reveal a user's IP address. In projects/en/torbrowser/design/index.html.en 123) addition, CSS can also be used without Javascript to perform <a class="ulink" href="http://ha.ckers.org/weird/CSS-history.cgi" target="_top">CSS-only history disclosure projects/en/torbrowser/design/index.html.en 124) attacks</a>. projects/en/torbrowser/design/index.html.en 125) </p></li><li class="listitem"><span class="command"><strong>Read and insert cookies</strong></span><p> projects/en/torbrowser/design/index.html.en 126) projects/en/torbrowser/design/index.html.en 127) An adversary in a position to perform MITM content alteration can inject projects/en/torbrowser/design/index.html.en 128) document content elements to both read and inject cookies for arbitrary projects/en/torbrowser/design/index.html.en 129) domains. In fact, many "SSL secured" websites are vulnerable to this sort of projects/en/torbrowser/design/index.html.en 130) <a class="ulink" href="http://seclists.org/bugtraq/2007/Aug/0070.html" target="_top">active projects/en/torbrowser/design/index.html.en 131) sidejacking</a>. In addition, the ad networks of course perform tracking projects/en/torbrowser/design/index.html.en 132) with cookies as well. projects/en/torbrowser/design/index.html.en 133) projects/en/torbrowser/design/index.html.en 134) </p></li><li class="listitem"><span class="command"><strong>Create arbitrary cached content</strong></span><p> projects/en/torbrowser/design/index.html.en 135) projects/en/torbrowser/design/index.html.en 136) Likewise, the browser cache can also be used to <a class="ulink" href="http://crypto.stanford.edu/sameorigin/safecachetest.html" target="_top">store unique projects/en/torbrowser/design/index.html.en 137) identifiers</a>. Since by default the cache has no same-origin policy, projects/en/torbrowser/design/index.html.en 138) these identifiers can be read by any domain, making them an ideal target for projects/en/torbrowser/design/index.html.en 139) ad network-class adversaries. projects/en/torbrowser/design/index.html.en 140) projects/en/torbrowser/design/index.html.en 141) </p></li><li class="listitem"><a id="fingerprinting"></a><span class="command"><strong>Fingerprint users based on browser projects/en/torbrowser/design/index.html.en 142) attributes</strong></span><p> projects/en/torbrowser/design/index.html.en 143) projects/en/torbrowser/design/index.html.en 144) There is an absurd amount of information available to websites via attributes projects/en/torbrowser/design/index.html.en 145) of the browser. This information can be used to reduce anonymity set, or even projects/en/torbrowser/design/index.html.en 146) <a class="ulink" href="http://mandark.fr/0x000000/articles/Total_Recall_On_Firefox..html" target="_top">uniquely projects/en/torbrowser/design/index.html.en 147) fingerprint individual users</a>. </p><p> projects/en/torbrowser/design/index.html.en 148) projects/en/torbrowser/design/index.html.en 149) The <a class="ulink" href="https://wiki.mozilla.org/Fingerprinting#Data" target="_top">Panopticlick study projects/en/torbrowser/design/index.html.en 150) done</a> by the EFF attempts to measure the actual entropy - the number of projects/en/torbrowser/design/index.html.en 151) identifying bits of information encoded in browser properties. Their result projects/en/torbrowser/design/index.html.en 152) data is definitely useful, and the metric is probably the appropriate one for projects/en/torbrowser/design/index.html.en 153) determining how identifying a particular browser property is. However, some projects/en/torbrowser/design/index.html.en 154) quirks of their study means that they do not extract as much information as projects/en/torbrowser/design/index.html.en 155) they could from display information: they only use desktop resolution (which projects/en/torbrowser/design/index.html.en 156) Torbutton reports as the window resolution) and do not attempt to infer the projects/en/torbrowser/design/index.html.en 157) size of toolbars. projects/en/torbrowser/design/index.html.en 158) projects/en/torbrowser/design/index.html.en 159) projects/en/torbrowser/design/index.html.en 160) projects/en/torbrowser/design/index.html.en 161) </p></li><li class="listitem"><span class="command"><strong>Remotely or locally exploit browser and/or projects/en/torbrowser/design/index.html.en 162) OS</strong></span><p> projects/en/torbrowser/design/index.html.en 163) projects/en/torbrowser/design/index.html.en 164) Last, but definitely not least, the adversary can exploit either general projects/en/torbrowser/design/index.html.en 165) browser vulnerabilities, plugin vulnerabilities, or OS vulnerabilities to projects/en/torbrowser/design/index.html.en 166) install malware and surveillance software. An adversary with physical access projects/en/torbrowser/design/index.html.en 167) can perform similar actions. Regrettably, this last attack capability is projects/en/torbrowser/design/index.html.en 168) outside of our ability to defend against, but it is worth mentioning for projects/en/torbrowser/design/index.html.en 169) completeness. <a class="ulink" href="http://tails.boum.org/contribute/design/" target="_top">The Tails projects/en/torbrowser/design/index.html.en 170) system</a> however can provide some limited defenses against this projects/en/torbrowser/design/index.html.en 171) adversary. projects/en/torbrowser/design/index.html.en 172) projects/en/torbrowser/design/index.html.en 173) </p></li></ol></div></div></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> projects/en/torbrowser/design/index.html.en 174) projects/en/torbrowser/design/index.html.en 175) The Tor Browser Design Requirements are meant to describe the properties of a projects/en/torbrowser/design/index.html.en 176) Private Browsing Mode that defends against both network and local adversaries. projects/en/torbrowser/design/index.html.en 177) projects/en/torbrowser/design/index.html.en 178) </p><p> projects/en/torbrowser/design/index.html.en 179) projects/en/torbrowser/design/index.html.en 180) 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 projects/en/torbrowser/design/index.html.en 181) minimum properties in order for a web client platform to be able to support projects/en/torbrowser/design/index.html.en 182) Tor. Privacy requirements are the set of properties that cause us to prefer projects/en/torbrowser/design/index.html.en 183) one platform over another. projects/en/torbrowser/design/index.html.en 184) projects/en/torbrowser/design/index.html.en 185) </p><p> projects/en/torbrowser/design/index.html.en 186) projects/en/torbrowser/design/index.html.en 187) We will maintain an alternate distribution of the web client in order to projects/en/torbrowser/design/index.html.en 188) maintain and/or restore privacy properties to our users. projects/en/torbrowser/design/index.html.en 189) projects/en/torbrowser/design/index.html.en 190) </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> projects/en/torbrowser/design/index.html.en 191) projects/en/torbrowser/design/index.html.en 192) The security requirements are primarily concerned with ensuring the safe use projects/en/torbrowser/design/index.html.en 193) of Tor. Violations in these properties typically result in serious risk for
Add a couple extra sentence... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 194) the user in terms of immediate deanonymization and/or observability. With projects/torbrowser/design/index.html.en 195) respect to platform support, security requirements are the minimum properties projects/torbrowser/design/index.html.en 196) in order for Tor to support the use of a web client platform.`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 197) projects/en/torbrowser/design/index.html.en 198) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Proxy Obedience</strong></span><p>The browser projects/en/torbrowser/design/index.html.en 199) MUST NOT bypass Tor proxy settings for any content.</p></li><li class="listitem"><span class="command"><strong>State Separation</strong></span><p>The browser MUST NOT provide any stored state to the content window projects/en/torbrowser/design/index.html.en 200) from other browsers or other browsing modes, including shared state from projects/en/torbrowser/design/index.html.en 201) plugins, machine identifiers, and TLS session state. projects/en/torbrowser/design/index.html.en 202) </p></li><li class="listitem"><span class="command"><strong>Disk Avoidance</strong></span><p>The projects/en/torbrowser/design/index.html.en 203) browser SHOULD NOT write any browsing history information to disk, or store it projects/en/torbrowser/design/index.html.en 204) in memory beyond the duration of one Tor session, unless the user has projects/en/torbrowser/design/index.html.en 205) explicitly opted to store their browsing history information to projects/en/torbrowser/design/index.html.en 206) disk.</p></li><li class="listitem"><span class="command"><strong>Application Data Isolation</strong></span><p>The browser projects/en/torbrowser/design/index.html.en 207) MUST NOT write or cause the operating system to projects/en/torbrowser/design/index.html.en 208) write <span class="emphasis"><em>any information</em></span> to disk outside of the application projects/en/torbrowser/design/index.html.en 209) directory. All exceptions and shortcomings due to operating system behavior projects/en/torbrowser/design/index.html.en 210) MUST BE documented. projects/en/torbrowser/design/index.html.en 211) projects/en/torbrowser/design/index.html.en 212) </p></li><li class="listitem"><span class="command"><strong>Update Safety</strong></span><p>The browser SHOULD NOT perform unsafe updates or upgrades.</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> projects/en/torbrowser/design/index.html.en 213) projects/en/torbrowser/design/index.html.en 214) The privacy requirements are primarily concerned with reducing linkability:
Add a couple extra sentence... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 215) the ability for a user's activity on one site to be linked with their activity projects/torbrowser/design/index.html.en 216) on another site without their knowledge or explicit consent. With respect to projects/torbrowser/design/index.html.en 217) platform support, privacy requirements are the set of properties that cause us projects/torbrowser/design/index.html.en 218) to prefer one platform over another.`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 219) projects/en/torbrowser/design/index.html.en 220) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Cross-Domain Identifier Unlinkability</strong></span><p> projects/en/torbrowser/design/index.html.en 221) projects/en/torbrowser/design/index.html.en 222) User activity on one url bar domain MUST NOT be linkable to their activity in projects/en/torbrowser/design/index.html.en 223) any other domain by any third party. This property specifically applies to projects/en/torbrowser/design/index.html.en 224) linkability from stored browser identifiers, authentication tokens, and shared projects/en/torbrowser/design/index.html.en 225) state. This functionality SHOULD NOT interfere with federated login in a projects/en/torbrowser/design/index.html.en 226) substantial way. projects/en/torbrowser/design/index.html.en 227) projects/en/torbrowser/design/index.html.en 228) </p></li><li class="listitem"><span class="command"><strong>Cross-Domain Fingerprinting Unlinkability</strong></span><p> projects/en/torbrowser/design/index.html.en 229) projects/en/torbrowser/design/index.html.en 230) User activity on one url bar domain MUST NOT be linkable to their activity in projects/en/torbrowser/design/index.html.en 231) any other domain by any third party. This property specifically applies to projects/en/torbrowser/design/index.html.en 232) linkability from fingerprinting browser behavior. projects/en/torbrowser/design/index.html.en 233) projects/en/torbrowser/design/index.html.en 234) </p></li><li class="listitem"><span class="command"><strong>Long-Term Unlinkability</strong></span><p> projects/en/torbrowser/design/index.html.en 235) projects/en/torbrowser/design/index.html.en 236) The browser SHOULD provide an obvious, easy way to remove all of their authentication projects/en/torbrowser/design/index.html.en 237) tokens and browser state and obtain a fresh identity. Additionally, this projects/en/torbrowser/design/index.html.en 238) should happen by default automatically upon browser restart. projects/en/torbrowser/design/index.html.en 239) projects/en/torbrowser/design/index.html.en 240) </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> projects/en/torbrowser/design/index.html.en 241) projects/en/torbrowser/design/index.html.en 242) In addition to the above design requirements, the technology decisions about projects/en/torbrowser/design/index.html.en 243) Tor Browser are also guided by some philosophical positions about technology. projects/en/torbrowser/design/index.html.en 244) projects/en/torbrowser/design/index.html.en 245) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><span class="command"><strong>Preserve existing user model</strong></span><p> projects/en/torbrowser/design/index.html.en 246) projects/en/torbrowser/design/index.html.en 247) The existing way that the user expects to use a browser must be preserved. If projects/en/torbrowser/design/index.html.en 248) the user has to maintain a different mental model of how the sites they are projects/en/torbrowser/design/index.html.en 249) using behave depending on tab, browser state, or anything else that would not projects/en/torbrowser/design/index.html.en 250) normally be what they experience in their default browser, the user will projects/en/torbrowser/design/index.html.en 251) inevitably be confused. They will make mistakes and reduce their privacy as a projects/en/torbrowser/design/index.html.en 252) result. Worse, they may just stop using the browser, assuming it is broken. projects/en/torbrowser/design/index.html.en 253) projects/en/torbrowser/design/index.html.en 254) </p><p> projects/en/torbrowser/design/index.html.en 255) projects/en/torbrowser/design/index.html.en 256) 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 projects/en/torbrowser/design/index.html.en 257) of Torbutton</a>: Even if users managed to install everything properly, projects/en/torbrowser/design/index.html.en 258) the toggle model was too hard for the average user to understand, especially projects/en/torbrowser/design/index.html.en 259) in the face of accumulating tabs from multiple states crossed with the current projects/en/torbrowser/design/index.html.en 260) tor-state of the browser. projects/en/torbrowser/design/index.html.en 261) projects/en/torbrowser/design/index.html.en 262) </p></li><li class="listitem"><span class="command"><strong>Favor the implementation mechanism least likely to projects/en/torbrowser/design/index.html.en 263) break sites</strong></span><p> projects/en/torbrowser/design/index.html.en 264) projects/en/torbrowser/design/index.html.en 265) In general, we try to find solutions to privacy issues that will not induce projects/en/torbrowser/design/index.html.en 266) site breakage, though this is not always possible. projects/en/torbrowser/design/index.html.en 267) projects/en/torbrowser/design/index.html.en 268) </p></li><li class="listitem"><span class="command"><strong>Plugins must be restricted</strong></span><p> projects/en/torbrowser/design/index.html.en 269) projects/en/torbrowser/design/index.html.en 270) Even if plugins always properly used the browser proxy settings (which none of projects/en/torbrowser/design/index.html.en 271) them do) and could not be induced to bypass them (which all of them can), the projects/en/torbrowser/design/index.html.en 272) activities of closed-source plugins are very difficult to audit and control. projects/en/torbrowser/design/index.html.en 273) They can obtain and transmit all manner of system information to websites, projects/en/torbrowser/design/index.html.en 274) often have their own identifier storage for tracking users, and also projects/en/torbrowser/design/index.html.en 275) contribute to fingerprinting. projects/en/torbrowser/design/index.html.en 276) projects/en/torbrowser/design/index.html.en 277) </p><p> projects/en/torbrowser/design/index.html.en 278) projects/en/torbrowser/design/index.html.en 279) Therefore, if plugins are to be enabled in private browsing modes, they must projects/en/torbrowser/design/index.html.en 280) be restricted from running automatically on every page (via click-to-play projects/en/torbrowser/design/index.html.en 281) placeholders), and/or be sandboxed to restrict the types of system calls they projects/en/torbrowser/design/index.html.en 282) can execute. If the user decides to craft an exemption to allow a plugin to be projects/en/torbrowser/design/index.html.en 283) used, it MUST ONLY apply to the top level urlbar domain, and not to all sites, projects/en/torbrowser/design/index.html.en 284) to reduce linkability. projects/en/torbrowser/design/index.html.en 285) projects/en/torbrowser/design/index.html.en 286) </p></li><li class="listitem"><span class="command"><strong>Minimize Global Privacy Options</strong></span><p> projects/en/torbrowser/design/index.html.en 287) projects/en/torbrowser/design/index.html.en 288) <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3100" target="_top">Another projects/en/torbrowser/design/index.html.en 289) failure of Torbutton</a> was (and still is) the options panel. Each option projects/en/torbrowser/design/index.html.en 290) that detectably alters browser behavior can be used as a fingerprinting tool. projects/en/torbrowser/design/index.html.en 291) Similarly, all extensions <a class="ulink" href="http://blog.chromium.org/2010/06/extensions-in-incognito.html" target="_top">should be projects/en/torbrowser/design/index.html.en 292) disabled in the mode</a> except as an opt-in basis. We should not load projects/en/torbrowser/design/index.html.en 293) system-wide addons or plugins. projects/en/torbrowser/design/index.html.en 294) projects/en/torbrowser/design/index.html.en 295) </p><p> projects/en/torbrowser/design/index.html.en 296) Instead of global browser privacy options, privacy decisions should be made projects/en/torbrowser/design/index.html.en 297) <a class="ulink" href="https://wiki.mozilla.org/Privacy/Features/Site-based_data_management_UI" target="_top">per projects/en/torbrowser/design/index.html.en 298) top-level url-bar domain</a> to eliminate the possibility of linkability projects/en/torbrowser/design/index.html.en 299) between domains. For example, when a plugin object (or a Javascript access of projects/en/torbrowser/design/index.html.en 300) window.plugins) is present in a page, the user should be given the choice of projects/en/torbrowser/design/index.html.en 301) allowing that plugin object for that top-level url-bar domain only. The same projects/en/torbrowser/design/index.html.en 302) goes for exemptions to third party cookie policy, geo-location, and any other projects/en/torbrowser/design/index.html.en 303) privacy permissions. projects/en/torbrowser/design/index.html.en 304) </p><p> projects/en/torbrowser/design/index.html.en 305) If the user has indicated they do not care about local history storage, these projects/en/torbrowser/design/index.html.en 306) permissions can be written to disk. Otherwise, they should remain memory-only. projects/en/torbrowser/design/index.html.en 307) </p></li><li class="listitem"><span class="command"><strong>No filters</strong></span><p> projects/en/torbrowser/design/index.html.en 308) projects/en/torbrowser/design/index.html.en 309) Filter-based addons such as <a class="ulink" href="https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/" target="_top">AdBlock projects/en/torbrowser/design/index.html.en 310) Plus</a>, <a class="ulink" href="" target="_top">Request Policy</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 projects/en/torbrowser/design/index.html.en 311) avoided. We believe that these addons do not add any real privacy to a proper projects/en/torbrowser/design/index.html.en 312) <a class="link" href="#Implementation" title="3. Implementation">implementation</a> of the above <a class="link" href="#privacy" title="2.2. Privacy Requirements">privacy requirements</a>, as all third parties are projects/en/torbrowser/design/index.html.en 313) prevented from tracking users between sites by the implementation. projects/en/torbrowser/design/index.html.en 314) Filter-based addons can also introduce strange breakage and cause usability projects/en/torbrowser/design/index.html.en 315) nightmares, and will also fail to do their job if an adversary simply projects/en/torbrowser/design/index.html.en 316) registers a new domain or creates a new url path. Worse still, the unique
Fix a typo and some links i... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 317) filter sets that each user creates or installs will provide a wealth`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 318) of fingerprinting targets. projects/en/torbrowser/design/index.html.en 319) projects/en/torbrowser/design/index.html.en 320) </p><p> projects/en/torbrowser/design/index.html.en 321) projects/en/torbrowser/design/index.html.en 322) As a general matter, we are also generally opposed to shipping an always-on Ad projects/en/torbrowser/design/index.html.en 323) blocker with Tor Browser. We feel that this would damage our credibility in projects/en/torbrowser/design/index.html.en 324) terms of demonstrating that we are providing privacy through a sound design projects/en/torbrowser/design/index.html.en 325) alone, as well as damage the acceptance of Tor users by sites who support projects/en/torbrowser/design/index.html.en 326) themselves through advertising revenue. projects/en/torbrowser/design/index.html.en 327) projects/en/torbrowser/design/index.html.en 328) </p><p> projects/en/torbrowser/design/index.html.en 329) Users are free to install these addons if they wish, but doing projects/en/torbrowser/design/index.html.en 330) so is not recommended, as it will alter the browser request fingerprint. projects/en/torbrowser/design/index.html.en 331) </p></li><li class="listitem"><span class="command"><strong>Stay Current</strong></span><p> projects/en/torbrowser/design/index.html.en 332) We believe that if we do not stay current with the support of new web projects/en/torbrowser/design/index.html.en 333) technologies, we cannot hope to substantially influence or be involved in projects/en/torbrowser/design/index.html.en 334) their proper deployment or privacy realization. However, we will likely disable projects/en/torbrowser/design/index.html.en 335) certain new features (where possible) pending analysis and audit. projects/en/torbrowser/design/index.html.en 336) </p></li></ol></div></div></div><div class="sect1" title="3. Implementation"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Implementation"></a>3. Implementation</h2></div></div></div><p> projects/en/torbrowser/design/index.html.en 337) </p><div class="sect2" title="3.1. Proxy Obedience"><div class="titlepage"><div><div><h3 class="title"><a id="proxy-obedience"></a>3.1. Proxy Obedience</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 338) projects/en/torbrowser/design/index.html.en 339) Proxy obedience is assured through the following: projects/en/torbrowser/design/index.html.en 340) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">Firefox Proxy settings projects/en/torbrowser/design/index.html.en 341) <p> projects/en/torbrowser/design/index.html.en 342) The Torbutton xpi sets the Firefox proxy settings to use Tor directly as a projects/en/torbrowser/design/index.html.en 343) SOCKS proxy. It sets <span class="command"><strong>network.proxy.socks_remote_dns</strong></span>, projects/en/torbrowser/design/index.html.en 344) <span class="command"><strong>network.proxy.socks_version</strong></span>, and projects/en/torbrowser/design/index.html.en 345) <span class="command"><strong>network.proxy.socks_port</strong></span>. projects/en/torbrowser/design/index.html.en 346) </p></li><li class="listitem">Disabling plugins projects/en/torbrowser/design/index.html.en 347) <p> projects/en/torbrowser/design/index.html.en 348) Plugins have the ability to make arbitrary OS system calls. This includes projects/en/torbrowser/design/index.html.en 349) the ability to make UDP sockets and send arbitrary data independent of the projects/en/torbrowser/design/index.html.en 350) browser proxy settings. projects/en/torbrowser/design/index.html.en 351) </p><p> projects/en/torbrowser/design/index.html.en 352) Torbutton disables plugins by using the projects/en/torbrowser/design/index.html.en 353) <span class="command"><strong>@mozilla.org/plugin/host;1</strong></span> service to mark the plugin tags projects/en/torbrowser/design/index.html.en 354) as disabled. Additionally, we set projects/en/torbrowser/design/index.html.en 355) <span class="command"><strong>plugin.disable_full_page_plugin_for_types</strong></span> to the list of projects/en/torbrowser/design/index.html.en 356) supported mime types for all currently installed plugins. projects/en/torbrowser/design/index.html.en 357) </p><p> projects/en/torbrowser/design/index.html.en 358) In addition, to prevent any unproxied activity by plugins at load time, we
Fix a typo and some links i... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 359) also patch the Firefox source code to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0007-Block-all-plugins-except-flash.patch" target="_top">prevent the load of any plugins except`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 360) for Flash and Gnash</a>. projects/en/torbrowser/design/index.html.en 361) projects/en/torbrowser/design/index.html.en 362) </p></li><li class="listitem">External App Blocking projects/en/torbrowser/design/index.html.en 363) <p> projects/en/torbrowser/design/index.html.en 364) External apps, if launched automatically, can be induced to load files that projects/en/torbrowser/design/index.html.en 365) perform network activity. In order to prevent this, Torbutton installs a projects/en/torbrowser/design/index.html.en 366) component to
Fix a typo and some links i... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 367) <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/blob_plain/HEAD:/src/components/external-app-blocker.js" target="_top">`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 368) provide the user with a popup</a> whenever the browser attempts to projects/en/torbrowser/design/index.html.en 369) launch a helper app. projects/en/torbrowser/design/index.html.en 370) </p></li></ol></div></div><div class="sect2" title="3.2. State Separation"><div class="titlepage"><div><div><h3 class="title"><a id="state-separation"></a>3.2. State Separation</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 371) Tor Browser State is separated from existing browser state through use of a projects/en/torbrowser/design/index.html.en 372) custom Firefox profile. Furthermore, plugins are disabled, which prevents projects/en/torbrowser/design/index.html.en 373) Flash cookies from leaking from a pre-existing Flash directory.
Add a couple extra sentence... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 374) </p></div><div class="sect2" title="3.3. Disk Avoidance"><div class="titlepage"><div><div><h3 class="title"><a id="disk-avoidance"></a>3.3. Disk Avoidance</h3></div></div></div><div class="sect3" title="Design Goal:"><div class="titlepage"><div><div><h4 class="title"><a id="id2980587"></a>Design Goal:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote">`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 375) Tor Browser should optionally prevent all disk records of browser activity. projects/en/torbrowser/design/index.html.en 376) The user should be able to optionally enable URL history and other history projects/en/torbrowser/design/index.html.en 377) features if they so desire. Once we <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3100" target="_top">simplify the projects/en/torbrowser/design/index.html.en 378) preferences interface</a>, we will likely just enable Private Browsing projects/en/torbrowser/design/index.html.en 379) mode by default to handle this goal.
Add a couple extra sentence... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 380) </blockquote></div></div><div class="sect3" title="Implementation Status:"><div class="titlepage"><div><div><h4 class="title"><a id="id3006806"></a>Implementation Status:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote">`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 381) For now, Tor Browser blocks write access to the disk through Torbutton projects/en/torbrowser/design/index.html.en 382) using several Firefox preferences. projects/en/torbrowser/design/index.html.en 383) projects/en/torbrowser/design/index.html.en 384) projects/en/torbrowser/design/index.html.en 385) projects/en/torbrowser/design/index.html.en 386) The set of prefs is: projects/en/torbrowser/design/index.html.en 387) <span class="command"><strong>dom.storage.enabled</strong></span>, projects/en/torbrowser/design/index.html.en 388) <span class="command"><strong>browser.cache.memory.enable</strong></span>, projects/en/torbrowser/design/index.html.en 389) <span class="command"><strong>network.http.use-cache</strong></span>, projects/en/torbrowser/design/index.html.en 390) <span class="command"><strong>browser.cache.disk.enable</strong></span>, projects/en/torbrowser/design/index.html.en 391) <span class="command"><strong>browser.cache.offline.enable</strong></span>, projects/en/torbrowser/design/index.html.en 392) <span class="command"><strong>general.open_location.last_url</strong></span>, projects/en/torbrowser/design/index.html.en 393) <span class="command"><strong>places.history.enabled</strong></span>, projects/en/torbrowser/design/index.html.en 394) <span class="command"><strong>browser.formfill.enable</strong></span>, projects/en/torbrowser/design/index.html.en 395) <span class="command"><strong>signon.rememberSignons</strong></span>, projects/en/torbrowser/design/index.html.en 396) <span class="command"><strong>browser.download.manager.retention</strong></span>, projects/en/torbrowser/design/index.html.en 397) and <span class="command"><strong>network.cookie.lifetimePolicy</strong></span>. projects/en/torbrowser/design/index.html.en 398) </blockquote></div></div><p> projects/en/torbrowser/design/index.html.en 399) In addition, three Firefox patches are needed to prevent disk writes, even if projects/en/torbrowser/design/index.html.en 400) Private Browsing Mode is enabled. We need to projects/en/torbrowser/design/index.html.en 401) projects/en/torbrowser/design/index.html.en 402) <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0002-Make-Permissions-Manager-memory-only.patch" target="_top">prevent projects/en/torbrowser/design/index.html.en 403) the permissions manager from recording HTTPS STS state</a>, projects/en/torbrowser/design/index.html.en 404) <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0003-Make-Intermediate-Cert-Store-memory-only.patch" target="_top">prevent projects/en/torbrowser/design/index.html.en 405) intermediate SSL certificates from being recorded</a>, and projects/en/torbrowser/design/index.html.en 406) <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0008-Make-content-pref-service-memory-only-clearable.patch" target="_top">prevent projects/en/torbrowser/design/index.html.en 407) the content preferences service from recording site zoom</a>. projects/en/torbrowser/design/index.html.en 408) projects/en/torbrowser/design/index.html.en 409) For more details on these patches, <a class="link" href="#firefox-patches" title="3.9. Description of Firefox Patches">see the projects/en/torbrowser/design/index.html.en 410) Firefox Patches section</a>. projects/en/torbrowser/design/index.html.en 411) projects/en/torbrowser/design/index.html.en 412) </p></div><div class="sect2" title="3.4. Application Data Isolation"><div class="titlepage"><div><div><h3 class="title"><a id="app-data-isolation"></a>3.4. Application Data Isolation</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 413) projects/en/torbrowser/design/index.html.en 414) Tor Browser Bundle MUST NOT cause any information to be written outside of the projects/en/torbrowser/design/index.html.en 415) bundle directory. This is to ensure that the user is able to completely and projects/en/torbrowser/design/index.html.en 416) safely remove the bundle without leaving other traces of Tor usage on their projects/en/torbrowser/design/index.html.en 417) computer. projects/en/torbrowser/design/index.html.en 418) projects/en/torbrowser/design/index.html.en 419) </p><p>XXX: sjmurdoch, Erinn: explain what magic we do to satisfy this, projects/en/torbrowser/design/index.html.en 420) and/or what additional work or auditing needs to be done. projects/en/torbrowser/design/index.html.en 421) </p></div><div class="sect2" title="3.5. Cross-Domain Identifier Unlinkability"><div class="titlepage"><div><div><h3 class="title"><a id="identifier-linkability"></a>3.5. Cross-Domain Identifier Unlinkability</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 422) projects/en/torbrowser/design/index.html.en 423) The Tor Browser MUST prevent a user's activity on one site from being linked projects/en/torbrowser/design/index.html.en 424) to their activity on another site. When this goal cannot yet be met with an projects/en/torbrowser/design/index.html.en 425) existing web technology, that technology or functionality is disabled. Our projects/en/torbrowser/design/index.html.en 426) <a class="link" href="#privacy" title="2.2. Privacy Requirements">design goal</a> is to ultimately eliminate the need to disable arbitrary projects/en/torbrowser/design/index.html.en 427) technologies, and instead simply alter them in ways that allows them to projects/en/torbrowser/design/index.html.en 428) function in a backwards-compatible way while avoiding linkability. Users projects/en/torbrowser/design/index.html.en 429) should be able to use federated login of various kinds to explicitly inform projects/en/torbrowser/design/index.html.en 430) sites who they are, but that information should not transparently allow a projects/en/torbrowser/design/index.html.en 431) third party to record their activity from site to site without their prior projects/en/torbrowser/design/index.html.en 432) consent. projects/en/torbrowser/design/index.html.en 433) projects/en/torbrowser/design/index.html.en 434) </p><p> projects/en/torbrowser/design/index.html.en 435) projects/en/torbrowser/design/index.html.en 436) The benefit of this approach comes not only in the form of reduced projects/en/torbrowser/design/index.html.en 437) linkability, but also in terms of simplified privacy UI. If all stored browser projects/en/torbrowser/design/index.html.en 438) state and permissions become associated with the top-level url-bar domain, the projects/en/torbrowser/design/index.html.en 439) six or seven different pieces of privacy UI governing these identifiers and projects/en/torbrowser/design/index.html.en 440) permissions can become just one piece of UI. For instance, a window that lists projects/en/torbrowser/design/index.html.en 441) the top-level url bar domains for which browser state exists with the ability projects/en/torbrowser/design/index.html.en 442) to clear and/or block them, possibly with a context-menu option to drill down projects/en/torbrowser/design/index.html.en 443) into specific types of state. An exmaple of this simplifcation can be seen in projects/en/torbrowser/design/index.html.en 444) Figure 1. projects/en/torbrowser/design/index.html.en 445)
Add a couple extra sentence... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 446) </p><div class="figure"><a id="id2962771"></a><p class="title"><b>Figure 1. Improving the Privacy UI</b></p><div class="figure-contents"><div class="mediaobject" align="center"><img src="CookieManagers.png" align="middle" alt="Improving the Privacy UI" /></div><div class="caption"><p></p>`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 447) projects/en/torbrowser/design/index.html.en 448) On the left is the standard Firefox cookie manager. On the right is a mock-up projects/en/torbrowser/design/index.html.en 449) of how isolating identifiers to the URL bar domain might simplify the privacy projects/en/torbrowser/design/index.html.en 450) UI for all data - not just cookies. Both windows represent the set of projects/en/torbrowser/design/index.html.en 451) Cookies accomulated after visiting just five sites, but the window on the projects/en/torbrowser/design/index.html.en 452) right has the option of also representing history, DOM Storage, HTTP Auth, projects/en/torbrowser/design/index.html.en 453) search form history, login values, and so on within a context menu for each projects/en/torbrowser/design/index.html.en 454) site. projects/en/torbrowser/design/index.html.en 455) projects/en/torbrowser/design/index.html.en 456) </div></div></div><br class="figure-break" /><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">Cookies projects/en/torbrowser/design/index.html.en 457) <p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 458) projects/en/torbrowser/design/index.html.en 459) All cookies should be double-keyed to the top-level domain. There exists a
Fix a typo and some links i... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 460) <a class="ulink" href="https://bugzilla.mozilla.org/show_bug.cgi?id=565965" target="_top">Mozilla`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 461) bug</a> that contains a prototype patch, but it lacks UI, and does not projects/en/torbrowser/design/index.html.en 462) apply to modern Firefoxes. projects/en/torbrowser/design/index.html.en 463) projects/en/torbrowser/design/index.html.en 464) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 465) projects/en/torbrowser/design/index.html.en 466) As a stopgap to satisfy our design requirement of unlinkability, we currently projects/en/torbrowser/design/index.html.en 467) entirely disable 3rd party cookies by setting projects/en/torbrowser/design/index.html.en 468) <span class="command"><strong>network.cookie.cookieBehavior</strong></span> to 1. We would prefer that projects/en/torbrowser/design/index.html.en 469) third party content continue to function , but we believe the requirement for projects/en/torbrowser/design/index.html.en 470) unlinkability trumps that desire. projects/en/torbrowser/design/index.html.en 471) projects/en/torbrowser/design/index.html.en 472) </p></li><li class="listitem">Cache projects/en/torbrowser/design/index.html.en 473) <p> projects/en/torbrowser/design/index.html.en 474) Cache is isolated to the top-level url bar domain by using a technique projects/en/torbrowser/design/index.html.en 475) 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 projects/en/torbrowser/design/index.html.en 476) <a class="ulink" href="https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsICachingChannel" target="_top">nsICachingChannel.cacheKey</a> projects/en/torbrowser/design/index.html.en 477) attribute that Firefox uses internally to prevent improper caching of HTTP POST data. projects/en/torbrowser/design/index.html.en 478) </p><p> projects/en/torbrowser/design/index.html.en 479) However, to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3666" target="_top">increase the projects/en/torbrowser/design/index.html.en 480) security of the isolation</a> and to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3754" target="_top">solve strange and projects/en/torbrowser/design/index.html.en 481) unknown conflicts with OCSP</a>, we had to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0005-Add-a-string-based-cacheKey.patch" target="_top">patch projects/en/torbrowser/design/index.html.en 482) Firefox to provide a cacheDomain cache attribute</a>. We use the full projects/en/torbrowser/design/index.html.en 483) url bar domain as input to this field. projects/en/torbrowser/design/index.html.en 484) </p><p> projects/en/torbrowser/design/index.html.en 485) projects/en/torbrowser/design/index.html.en 486) projects/en/torbrowser/design/index.html.en 487) Furthermore, we chose a different isolation scheme than the Stanford projects/en/torbrowser/design/index.html.en 488) implementation. First, we decoupled the cache isolation from the third party projects/en/torbrowser/design/index.html.en 489) cookie attribute. Second, we use several mechanisms to attempt to determine projects/en/torbrowser/design/index.html.en 490) the actual location attribute of the top-level window (the url bar domain) projects/en/torbrowser/design/index.html.en 491) used to load the page, as opposed to relying solely on the referer property. projects/en/torbrowser/design/index.html.en 492) </p><p> projects/en/torbrowser/design/index.html.en 493) Therefore, <a class="ulink" href="http://crypto.stanford.edu/sameorigin/safecachetest.html" target="_top">the original projects/en/torbrowser/design/index.html.en 494) Stanford test projects/en/torbrowser/design/index.html.en 495) cases</a> are expected to fail. Functionality can still be verified by projects/en/torbrowser/design/index.html.en 496) navigating to <a class="ulink" href="about:cache" target="_top">about:cache</a> and viewing the key projects/en/torbrowser/design/index.html.en 497) used for each cache entry. Each third party element should have an additional projects/en/torbrowser/design/index.html.en 498) "domain=string" property prepended, which will list the top-level urlbar projects/en/torbrowser/design/index.html.en 499) domain that was used to source the third party element. projects/en/torbrowser/design/index.html.en 500) </p></li><li class="listitem">HTTP Auth projects/en/torbrowser/design/index.html.en 501) <p> projects/en/torbrowser/design/index.html.en 502) projects/en/torbrowser/design/index.html.en 503) HTTP authentication tokens are removed for third party elements using the projects/en/torbrowser/design/index.html.en 504) <a class="ulink" href="https://developer.mozilla.org/en/Setting_HTTP_request_headers#Observers" target="_top">http-on-modify-request projects/en/torbrowser/design/index.html.en 505) 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 projects/en/torbrowser/design/index.html.en 506) linkability between domains</a>. We also needed to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0004-Add-HTTP-auth-headers-before-the-modify-request-obse.patch" target="_top">patch projects/en/torbrowser/design/index.html.en 507) Firefox to cause the headers to get added early enough</a> to allow the projects/en/torbrowser/design/index.html.en 508) observer to modify it. projects/en/torbrowser/design/index.html.en 509) projects/en/torbrowser/design/index.html.en 510) </p></li><li class="listitem">DOM Storage projects/en/torbrowser/design/index.html.en 511) <p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 512) projects/en/torbrowser/design/index.html.en 513) DOM storage for third party domains MUST BE isolated to the url bar domain, projects/en/torbrowser/design/index.html.en 514) to prevent linkability between sites. projects/en/torbrowser/design/index.html.en 515) projects/en/torbrowser/design/index.html.en 516) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 517) projects/en/torbrowser/design/index.html.en 518) Because it is isolated to third party domain as opposed to top level url bar projects/en/torbrowser/design/index.html.en 519) domain, we entirely disable DOM storage as a stopgap to ensure unlinkability. projects/en/torbrowser/design/index.html.en 520) projects/en/torbrowser/design/index.html.en 521) </p></li><li class="listitem">TLS session resumption and HTTP Keep-Alive projects/en/torbrowser/design/index.html.en 522) <p> projects/en/torbrowser/design/index.html.en 523) TLS session resumption and HTTP Keep-Alive must not allow third party origins projects/en/torbrowser/design/index.html.en 524) to track users via either TLS session IDs, or the fact that different requests projects/en/torbrowser/design/index.html.en 525) arrive on the same TCP connection. projects/en/torbrowser/design/index.html.en 526) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 527) projects/en/torbrowser/design/index.html.en 528) TLS session resumption IDs must be limited to the top-level url bar domain. projects/en/torbrowser/design/index.html.en 529) HTTP Keep-Alive connections from a third party in one top-level domain must projects/en/torbrowser/design/index.html.en 530) not be reused for that same third party in another top-level domain. projects/en/torbrowser/design/index.html.en 531) projects/en/torbrowser/design/index.html.en 532) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 533) projects/en/torbrowser/design/index.html.en 534) We <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/4099" target="_top">plan to projects/en/torbrowser/design/index.html.en 535) disable</a> TLS session resumption, and limit HTTP Keep-alive duration. projects/en/torbrowser/design/index.html.en 536) projects/en/torbrowser/design/index.html.en 537) </p></li><li class="listitem">window.name projects/en/torbrowser/design/index.html.en 538) <p> projects/en/torbrowser/design/index.html.en 539) projects/en/torbrowser/design/index.html.en 540) <a class="ulink" href="https://developer.mozilla.org/En/DOM/Window.name" target="_top">window.name</a> is projects/en/torbrowser/design/index.html.en 541) a magical DOM property that for some reason is allowed to retain a persistent value projects/en/torbrowser/design/index.html.en 542) for the lifespan of a browser tab. It is possible to utilize this property for projects/en/torbrowser/design/index.html.en 543) <a class="ulink" href="http://www.thomasfrank.se/sessionvars.html" target="_top">identifier projects/en/torbrowser/design/index.html.en 544) storage</a>. projects/en/torbrowser/design/index.html.en 545) projects/en/torbrowser/design/index.html.en 546) </p><p> projects/en/torbrowser/design/index.html.en 547) projects/en/torbrowser/design/index.html.en 548) In order to eliminate linkability but still allow for sites that utilize this projects/en/torbrowser/design/index.html.en 549) property to function, we reset the window.name property of tabs in Torbutton every projects/en/torbrowser/design/index.html.en 550) time we encounter a blank referer. This behavior allows window.name to persist projects/en/torbrowser/design/index.html.en 551) for the duration of a link-driven navigation session, but as soon as the user projects/en/torbrowser/design/index.html.en 552) enters a new URL or navigates between https/http schemes, the property is cleared. projects/en/torbrowser/design/index.html.en 553) projects/en/torbrowser/design/index.html.en 554) </p></li><li class="listitem">Exit node usage projects/en/torbrowser/design/index.html.en 555) <p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 556) projects/en/torbrowser/design/index.html.en 557) Every distinct navigation session (as defined by a non-blank referer header) projects/en/torbrowser/design/index.html.en 558) MUST exit through a fresh Tor circuit in Tor Browser to prevent exit node projects/en/torbrowser/design/index.html.en 559) observers from linking concurrent browsing activity. projects/en/torbrowser/design/index.html.en 560) projects/en/torbrowser/design/index.html.en 561) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 562) projects/en/torbrowser/design/index.html.en 563) The Tor feature that supports this ability only exists in the 0.2.3.x-alpha projects/en/torbrowser/design/index.html.en 564) series. <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3455" target="_top">Ticket projects/en/torbrowser/design/index.html.en 565) #3455</a> is the Torbutton ticket to make use of the new Tor projects/en/torbrowser/design/index.html.en 566) functionality. projects/en/torbrowser/design/index.html.en 567) projects/en/torbrowser/design/index.html.en 568) </p></li></ol></div></div><div class="sect2" title="3.6. Cross-Domain Fingerprinting Unlinkability"><div class="titlepage"><div><div><h3 class="title"><a id="fingerprinting-linkability"></a>3.6. Cross-Domain Fingerprinting Unlinkability</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 569) projects/en/torbrowser/design/index.html.en 570) In order to properly address the fingerprinting adversary on a technical projects/en/torbrowser/design/index.html.en 571) level, we need a metric to measure linkability of the various browser projects/en/torbrowser/design/index.html.en 572) properties that extend beyond any stored origin-related state. <a class="ulink" href="https://panopticlick.eff.org/about.php" target="_top">The Panopticlick Project</a> projects/en/torbrowser/design/index.html.en 573) by the EFF provides us with exactly this metric. The researchers conducted a projects/en/torbrowser/design/index.html.en 574) survey of volunteers who were asked to visit an experiment page that harvested projects/en/torbrowser/design/index.html.en 575) many of the above components. They then computed the Shannon Entropy of the projects/en/torbrowser/design/index.html.en 576) resulting distribution of each of several key attributes to determine how many projects/en/torbrowser/design/index.html.en 577) bits of identifying information each attribute provided. projects/en/torbrowser/design/index.html.en 578) projects/en/torbrowser/design/index.html.en 579) </p><p> projects/en/torbrowser/design/index.html.en 580) projects/en/torbrowser/design/index.html.en 581) The study is not exhaustive, though. In particular, the test does not take in projects/en/torbrowser/design/index.html.en 582) all aspects of resolution information. It did not calculate the size of projects/en/torbrowser/design/index.html.en 583) widgets, window decoration, or toolbar size, which we believe may add high projects/en/torbrowser/design/index.html.en 584) amounts of entropy. It also did not measure clock offset and other time-based projects/en/torbrowser/design/index.html.en 585) fingerprints. Furthermore, as new browser features are added, this experiment projects/en/torbrowser/design/index.html.en 586) should be repeated to include them. projects/en/torbrowser/design/index.html.en 587) projects/en/torbrowser/design/index.html.en 588) </p><p> projects/en/torbrowser/design/index.html.en 589) projects/en/torbrowser/design/index.html.en 590) On the other hand, to avoid an infinite sinkhole, we reduce the efforts for projects/en/torbrowser/design/index.html.en 591) fingerprinting resistance by only concerning ourselves with reducing the projects/en/torbrowser/design/index.html.en 592) fingerprintable differences <span class="emphasis"><em>among</em></span> Tor Browser users. We projects/en/torbrowser/design/index.html.en 593) do not believe it is productive to concern ourselves with cross-browser projects/en/torbrowser/design/index.html.en 594) fingerprinting issues, at least not at this stage. projects/en/torbrowser/design/index.html.en 595) projects/en/torbrowser/design/index.html.en 596) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">Plugins projects/en/torbrowser/design/index.html.en 597) <p> projects/en/torbrowser/design/index.html.en 598) projects/en/torbrowser/design/index.html.en 599) Plugins add to fingerprinting risk via two main vectors: their mere presence in projects/en/torbrowser/design/index.html.en 600) window.navigator.plugins, as well as their internal functionality. projects/en/torbrowser/design/index.html.en 601) projects/en/torbrowser/design/index.html.en 602) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 603) projects/en/torbrowser/design/index.html.en 604) All plugins that have not been specifically audited or sandboxed must be projects/en/torbrowser/design/index.html.en 605) disabled. To reduce linkability potential, even sandboxed plugins should not projects/en/torbrowser/design/index.html.en 606) be allowed to load objects until the user has clicked through a click-to-play projects/en/torbrowser/design/index.html.en 607) barrier. Additionally, version information should be reduced or obfuscated projects/en/torbrowser/design/index.html.en 608) until the plugin object is loaded. projects/en/torbrowser/design/index.html.en 609) projects/en/torbrowser/design/index.html.en 610) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 611) projects/en/torbrowser/design/index.html.en 612) Currently, we entirely disable all plugins in Tor Browser. However, as a projects/en/torbrowser/design/index.html.en 613) compromise due to the popularity of Flash, we intend to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3974" target="_top">work projects/en/torbrowser/design/index.html.en 614) towards</a> a projects/en/torbrowser/design/index.html.en 615) click-to-play barrier using NoScript that is available only after the user has projects/en/torbrowser/design/index.html.en 616) specifically enabled plugins. Flash will be the only plugin available, and we projects/en/torbrowser/design/index.html.en 617) will ship a settings.sol file to disable Flash cookies, and to restrict P2P projects/en/torbrowser/design/index.html.en 618) features that likely bypass proxy settings. projects/en/torbrowser/design/index.html.en 619) projects/en/torbrowser/design/index.html.en 620) </p></li><li class="listitem">Fonts projects/en/torbrowser/design/index.html.en 621) <p> projects/en/torbrowser/design/index.html.en 622) projects/en/torbrowser/design/index.html.en 623) According to the Panopticlick study, fonts provide the most linkability when projects/en/torbrowser/design/index.html.en 624) they are provided as an enumerable list in filesystem order, via either the projects/en/torbrowser/design/index.html.en 625) Flash or Java plugins. However, it is still possible to use CSS and/or projects/en/torbrowser/design/index.html.en 626) Javascript to query for the existence of specific fonts. With a large enough projects/en/torbrowser/design/index.html.en 627) pre-built list to query, a large amount of fingerprintable information may projects/en/torbrowser/design/index.html.en 628) still be available. projects/en/torbrowser/design/index.html.en 629) projects/en/torbrowser/design/index.html.en 630) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 631) projects/en/torbrowser/design/index.html.en 632) To address the Javascript issue, we intend to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/2872" target="_top">limit the number of projects/en/torbrowser/design/index.html.en 633) fonts</a> an origin can load, gracefully degrading to built-in and/or projects/en/torbrowser/design/index.html.en 634) remote fonts once the limit is reached. projects/en/torbrowser/design/index.html.en 635) projects/en/torbrowser/design/index.html.en 636) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 637) projects/en/torbrowser/design/index.html.en 638) Aside from disabling plugins to prevent enumeration, we have not yet projects/en/torbrowser/design/index.html.en 639) implemented any defense against CSS or Javascript fonts. projects/en/torbrowser/design/index.html.en 640) projects/en/torbrowser/design/index.html.en 641) </p></li><li class="listitem">User Agent and HTTP Headers projects/en/torbrowser/design/index.html.en 642) <p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 643) projects/en/torbrowser/design/index.html.en 644) All Tor Browser users should provide websites with an identical user agent and projects/en/torbrowser/design/index.html.en 645) HTTP header set for a given request type. We omit the Firefox minor revision, projects/en/torbrowser/design/index.html.en 646) and report a popular Windows platform. If the software is kept up to date, projects/en/torbrowser/design/index.html.en 647) these headers should remain identical across the population even when updated. projects/en/torbrowser/design/index.html.en 648) projects/en/torbrowser/design/index.html.en 649) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 650) projects/en/torbrowser/design/index.html.en 651) Firefox provides several options for controlling the browser user agent string projects/en/torbrowser/design/index.html.en 652) which we leverage. We also set similar prefs for controlling the projects/en/torbrowser/design/index.html.en 653) Accept-Language and Accept-Charset headers, which we spoof to English by default. Additionally, we projects/en/torbrowser/design/index.html.en 654) <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0001-Block-Components.interfaces-lookupMethod-from-conten.patch" target="_top">remove projects/en/torbrowser/design/index.html.en 655) 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 projects/en/torbrowser/design/index.html.en 656) used</a> to fingerprint OS, platform, and Firefox minor version. </p></li><li class="listitem">Desktop resolution and CSS Media Queries projects/en/torbrowser/design/index.html.en 657) <p> projects/en/torbrowser/design/index.html.en 658) projects/en/torbrowser/design/index.html.en 659) Both CSS and Javascript have a lot of irrelevant information about the screen projects/en/torbrowser/design/index.html.en 660) resolution, usable desktop size, OS widget size, toolbar size, title bar size, and projects/en/torbrowser/design/index.html.en 661) other desktop features that are not at all relevant to rendering and serve projects/en/torbrowser/design/index.html.en 662) only to provide information for fingerprinting. projects/en/torbrowser/design/index.html.en 663) projects/en/torbrowser/design/index.html.en 664) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 665) projects/en/torbrowser/design/index.html.en 666) Our design goal here is to reduce the resolution information down to the bare projects/en/torbrowser/design/index.html.en 667) minimum required for properly rendering inside a content window. We intend to projects/en/torbrowser/design/index.html.en 668) report all rendering information correctly with respect to the size and projects/en/torbrowser/design/index.html.en 669) properties of the content window, but report an effective size of 0 for all projects/en/torbrowser/design/index.html.en 670) border material, and also report that the desktop is only as big as the projects/en/torbrowser/design/index.html.en 671) inner content window. Additionally, new browser windows are sized such that projects/en/torbrowser/design/index.html.en 672) their content windows are one of ~5 fixed sizes based on the user's projects/en/torbrowser/design/index.html.en 673) desktop resolution. projects/en/torbrowser/design/index.html.en 674) projects/en/torbrowser/design/index.html.en 675) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 676) projects/en/torbrowser/design/index.html.en 677) We have implemented the above strategy for Javascript using Torbutton's <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/blob/HEAD:/src/chrome/content/jshooks4.js" target="_top">JavaScript projects/en/torbrowser/design/index.html.en 678) hooks</a> as well as a window observer to <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/blob/HEAD:/src/chrome/content/torbutton.js#l4002" target="_top">resize projects/en/torbrowser/design/index.html.en 679) new windows based on desktop resolution</a>. However, CSS Media Queries projects/en/torbrowser/design/index.html.en 680) still <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/2875" target="_top">need projects/en/torbrowser/design/index.html.en 681) to be dealt with</a>. projects/en/torbrowser/design/index.html.en 682) projects/en/torbrowser/design/index.html.en 683) </p></li><li class="listitem">Timezone and clock offset projects/en/torbrowser/design/index.html.en 684) <p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 685) projects/en/torbrowser/design/index.html.en 686) All Tor Browser users should report the same timezone to websites. Currently, projects/en/torbrowser/design/index.html.en 687) we choose UTC for this purpose, although an equally valid argument could be projects/en/torbrowser/design/index.html.en 688) made for EDT/EST due to the large English-speaking population density. projects/en/torbrowser/design/index.html.en 689) Additionally, the Tor software should detect if the users clock is projects/en/torbrowser/design/index.html.en 690) significantly divergent from the clocks of the relays that it connects to, and projects/en/torbrowser/design/index.html.en 691) use this to reset the clock values used in Tor Browser to something reasonably projects/en/torbrowser/design/index.html.en 692) accurate. projects/en/torbrowser/design/index.html.en 693) projects/en/torbrowser/design/index.html.en 694) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 695) projects/en/torbrowser/design/index.html.en 696) We set the timezone using the TZ environment variable, which is supported on projects/en/torbrowser/design/index.html.en 697) all platforms. Additionally, we plan to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3652" target="_top">obtain a clock projects/en/torbrowser/design/index.html.en 698) offset from Tor</a>, but this won't be available until Tor 0.2.3.x is in projects/en/torbrowser/design/index.html.en 699) use. projects/en/torbrowser/design/index.html.en 700) projects/en/torbrowser/design/index.html.en 701) </p></li><li class="listitem">Javascript performance fingerprinting projects/en/torbrowser/design/index.html.en 702) <p> projects/en/torbrowser/design/index.html.en 703) projects/en/torbrowser/design/index.html.en 704) <a class="ulink" href="http://w2spconf.com/2011/papers/jspriv.pdf" target="_top">Javascript performance projects/en/torbrowser/design/index.html.en 705) fingerprinting</a> is the act of profiling the performance projects/en/torbrowser/design/index.html.en 706) of various Javascript functions for the purpose of fingerprinting the projects/en/torbrowser/design/index.html.en 707) Javascript engine and the CPU. projects/en/torbrowser/design/index.html.en 708) projects/en/torbrowser/design/index.html.en 709) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 710) projects/en/torbrowser/design/index.html.en 711) We have <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3059" target="_top">several potential projects/en/torbrowser/design/index.html.en 712) mitigation approaches</a> to reduce the accuracy of performance projects/en/torbrowser/design/index.html.en 713) fingerprinting without risking too much damage to functionality. Our current projects/en/torbrowser/design/index.html.en 714) favorite is to reduce the resolution of the Event.timeStamp and the Javascript projects/en/torbrowser/design/index.html.en 715) Date() object, while also introducing jitter. Our goal is to increase the projects/en/torbrowser/design/index.html.en 716) 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 projects/en/torbrowser/design/index.html.en 717) even with the default precision in most browsers, they required up to 120 projects/en/torbrowser/design/index.html.en 718) seconds of amortization and repeated trials to get stable results from their projects/en/torbrowser/design/index.html.en 719) feature set. We intend to work with the research community to establish the projects/en/torbrowser/design/index.html.en 720) optimum tradeoff between quantization+jitter and amortization time. projects/en/torbrowser/design/index.html.en 721) projects/en/torbrowser/design/index.html.en 722) projects/en/torbrowser/design/index.html.en 723) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 724) projects/en/torbrowser/design/index.html.en 725) We have no implementation as of yet. projects/en/torbrowser/design/index.html.en 726) projects/en/torbrowser/design/index.html.en 727) </p></li><li class="listitem">Keystroke fingerprinting projects/en/torbrowser/design/index.html.en 728) <p> projects/en/torbrowser/design/index.html.en 729) projects/en/torbrowser/design/index.html.en 730) Keystroke fingerprinting is the act of measuring key strike time and key projects/en/torbrowser/design/index.html.en 731) flight time. It is seeing increasing use as a biometric. projects/en/torbrowser/design/index.html.en 732) projects/en/torbrowser/design/index.html.en 733) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 734) projects/en/torbrowser/design/index.html.en 735) We intend to rely on the same mechanisms for defeating Javascript performance projects/en/torbrowser/design/index.html.en 736) fingerprinting: timestamp quantization and jitter. projects/en/torbrowser/design/index.html.en 737) projects/en/torbrowser/design/index.html.en 738) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 739) We have no implementation as of yet. projects/en/torbrowser/design/index.html.en 740) </p></li><li class="listitem">WebGL projects/en/torbrowser/design/index.html.en 741) <p> projects/en/torbrowser/design/index.html.en 742) projects/en/torbrowser/design/index.html.en 743) WebGL is fingerprintable both through information that is exposed about the projects/en/torbrowser/design/index.html.en 744) underlying driver and optimizations, as well as through performance projects/en/torbrowser/design/index.html.en 745) fingerprinting. projects/en/torbrowser/design/index.html.en 746) projects/en/torbrowser/design/index.html.en 747) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 748) projects/en/torbrowser/design/index.html.en 749) Because of the large amount of potential fingerprinting vectors, we intend to projects/en/torbrowser/design/index.html.en 750) deploy a similar strategy against WebGL as for plugins. First, WebGL canvases projects/en/torbrowser/design/index.html.en 751) will have click-to-play placeholders, and will not run until authorized by the projects/en/torbrowser/design/index.html.en 752) user. Second, we intend to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3323" target="_top">obfuscate driver projects/en/torbrowser/design/index.html.en 753) information</a> by hooking projects/en/torbrowser/design/index.html.en 754) <span class="command"><strong>getParameter()</strong></span>, projects/en/torbrowser/design/index.html.en 755) <span class="command"><strong>getSupportedExtensions()</strong></span>, projects/en/torbrowser/design/index.html.en 756) <span class="command"><strong>getExtension()</strong></span>, and projects/en/torbrowser/design/index.html.en 757) <span class="command"><strong>getContextAttributes()</strong></span> to provide standard minimal, projects/en/torbrowser/design/index.html.en 758) driver-neutral information. projects/en/torbrowser/design/index.html.en 759) projects/en/torbrowser/design/index.html.en 760) </p><p><span class="command"><strong>Implementation Status:</strong></span> projects/en/torbrowser/design/index.html.en 761) projects/en/torbrowser/design/index.html.en 762) Currently we simply disable WebGL. projects/en/torbrowser/design/index.html.en 763) projects/en/torbrowser/design/index.html.en 764) </p></li></ol></div></div><div class="sect2" title="3.7. Long-Term Unlinkability via "New Identity" button"><div class="titlepage"><div><div><h3 class="title"><a id="new-identity"></a>3.7. Long-Term Unlinkability via "New Identity" button</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 765) In order to avoid long-term linkability, we provide a "New Identity" context projects/en/torbrowser/design/index.html.en 766) menu option in Torbutton.
Add a couple extra sentence... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 767) </p><div class="sect3" title="Design Goal:"><div class="titlepage"><div><div><h4 class="title"><a id="id2991890"></a>Design Goal:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote">`
Add design doc draft. Mike Perry authored 12 years ago	`projects/en/torbrowser/design/index.html.en 768) projects/en/torbrowser/design/index.html.en 769) All linkable identifiers and browser state should be cleared by this feature. projects/en/torbrowser/design/index.html.en 770)`
Add a couple extra sentence... Mike Perry authored 12 years ago	`projects/torbrowser/design/index.html.en 771) </blockquote></div></div><div class="sect3" title="Implementation Status:"><div class="titlepage"><div><div><h4 class="title"><a id="id3007443"></a>Implementation Status:</h4></div></div></div><div class="blockquote"><blockquote class="blockquote">`
Add design doc draft. Mike Perry authored 12 years ago	projects/en/torbrowser/design/index.html.en 772) First, Torbutton disables projects/en/torbrowser/design/index.html.en 773) all open tabs and windows via nsIContentPolicy blocking, and then closes each projects/en/torbrowser/design/index.html.en 774) tab and window. The extra step for blocking tabs is done as a precaution to projects/en/torbrowser/design/index.html.en 775) ensure that any asynchronous Javascript is in fact properly disabled. After projects/en/torbrowser/design/index.html.en 776) closing all of the windows, we then clear the following state: OCSP (by projects/en/torbrowser/design/index.html.en 777) toggling security.OCSP.enabled), cache, site-specific zoom and content projects/en/torbrowser/design/index.html.en 778) preferences, Cookies, DOM storage, safe browsing key, the Google wifi projects/en/torbrowser/design/index.html.en 779) geolocation token (if exists), HTTP auth, SSL Session IDs, and the last opened URL projects/en/torbrowser/design/index.html.en 780) field (via the pref general.open_location.last_url). After clearing the projects/en/torbrowser/design/index.html.en 781) browser state, we then send the NEWNYM signal to the Tor control port to cause projects/en/torbrowser/design/index.html.en 782) a new circuit to be created. projects/en/torbrowser/design/index.html.en 783) </blockquote></div></div></div><div class="sect2" title="3.8. Click-to-play for plugins and invasive content"><div class="titlepage"><div><div><h3 class="title"><a id="click-to-play"></a>3.8. Click-to-play for plugins and invasive content</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 784) Some content types are too invasive and/or too opaque for us to properly projects/en/torbrowser/design/index.html.en 785) eliminate their linkability properties. For these content types, we use projects/en/torbrowser/design/index.html.en 786) NoScript to provide click-to-play placeholders that do not activate the projects/en/torbrowser/design/index.html.en 787) content until the user clicks on it. This will eliminate the ability for an projects/en/torbrowser/design/index.html.en 788) adversary to use such content types to link users in a dragnet fashion across projects/en/torbrowser/design/index.html.en 789) arbitrary sites. projects/en/torbrowser/design/index.html.en 790) </p><p> projects/en/torbrowser/design/index.html.en 791) Currently, the content types isolated in this way include Flash, WebGL, and projects/en/torbrowser/design/index.html.en 792) audio and video objects. projects/en/torbrowser/design/index.html.en 793) </p></div><div class="sect2" title="3.9. Description of Firefox Patches"><div class="titlepage"><div><div><h3 class="title"><a id="firefox-patches"></a>3.9. Description of Firefox Patches</h3></div></div></div><p> projects/en/torbrowser/design/index.html.en 794) The set of patches we have against Firefox can be found in the <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/tree/refs/heads/maint-2.2:/src/current-patches" target="_top">current-patches projects/en/torbrowser/design/index.html.en 795) directory of the torbrowser git repository</a>. They are: projects/en/torbrowser/design/index.html.en 796) </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem">Block Components.interfaces and Components.lookupMethod projects/en/torbrowser/design/index.html.en 797) <p> projects/en/torbrowser/design/index.html.en 798) projects/en/torbrowser/design/index.html.en 799) In order to reduce fingerprinting, we block access to these two interfaces projects/en/torbrowser/design/index.html.en 800) from content script. Components.lookupMethod can undo our <a class="ulink" href="https://gitweb.torproject.org/torbutton.git/blob/HEAD:/src/chrome/content/jshooks4.js" target="_top">Javascript projects/en/torbrowser/design/index.html.en 801) hooks</a>, projects/en/torbrowser/design/index.html.en 802) and Components.interfaces can be used for fingerprinting the platform, OS, and projects/en/torbrowser/design/index.html.en 803) Firebox version, but not much else. projects/en/torbrowser/design/index.html.en 804) projects/en/torbrowser/design/index.html.en 805) </p></li><li class="listitem">Make Permissions Manager memory only projects/en/torbrowser/design/index.html.en 806) <p> projects/en/torbrowser/design/index.html.en 807) projects/en/torbrowser/design/index.html.en 808) This patch exposes a pref 'permissions.memory_only' that properly isolates the projects/en/torbrowser/design/index.html.en 809) permissions manager to memory, which is responsible for all user specified projects/en/torbrowser/design/index.html.en 810) site permissions, as well as stored HTTPS STS policy from visited sites. projects/en/torbrowser/design/index.html.en 811) projects/en/torbrowser/design/index.html.en 812) The pref does successfully clear the permissions manager memory if toggled. It projects/en/torbrowser/design/index.html.en 813) does not need to be set in prefs.js, and can be handled by Torbutton. projects/en/torbrowser/design/index.html.en 814) projects/en/torbrowser/design/index.html.en 815) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 816) projects/en/torbrowser/design/index.html.en 817) As an additional design goal, we would like to later alter this patch to allow this projects/en/torbrowser/design/index.html.en 818) information to be cleared from memory. The implementation does not currently projects/en/torbrowser/design/index.html.en 819) allow this. projects/en/torbrowser/design/index.html.en 820) projects/en/torbrowser/design/index.html.en 821) </p></li><li class="listitem">Make Intermediate Cert Store memory-only projects/en/torbrowser/design/index.html.en 822) <p> projects/en/torbrowser/design/index.html.en 823) projects/en/torbrowser/design/index.html.en 824) The intermediate certificate store holds information about SSL certificates projects/en/torbrowser/design/index.html.en 825) that may only be used by a limited number of domains. In some cases projects/en/torbrowser/design/index.html.en 826) effectively recording on disk the fact that a website owned by a certain projects/en/torbrowser/design/index.html.en 827) organization was viewed. projects/en/torbrowser/design/index.html.en 828) projects/en/torbrowser/design/index.html.en 829) </p><p><span class="command"><strong>Design Goal:</strong></span> projects/en/torbrowser/design/index.html.en 830) projects/en/torbrowser/design/index.html.en 831) As an additional design goal, we would like to later alter this patch to allow this projects/en/torbrowser/design/index.html.en 832) information to be cleared from memory. The implementation does not currently projects/en/torbrowser/design/index.html.en 833) allow this. projects/en/torbrowser/design/index.html.en 834) projects/en/torbrowser/design/index.html.en 835) </p></li><li class="listitem">Add HTTP auth headers before on-modify-request fires projects/en/torbrowser/design/index.html.en 836) <p> projects/en/torbrowser/design/index.html.en 837) projects/en/torbrowser/design/index.html.en 838) This patch provides a trivial modification to allow us to properly remove HTTP projects/en/torbrowser/design/index.html.en 839) auth for third parties. This patch allows us to defend against an adversary projects/en/torbrowser/design/index.html.en 840) attempting to use <a class="ulink" href="http://jeremiahgrossman.blogspot.com/2007/04/tracking-users-without-cookies.html" target="_top">HTTP projects/en/torbrowser/design/index.html.en 841) auth to silently track users between domains</a>. projects/en/torbrowser/design/index.html.en 842) projects/en/torbrowser/design/index.html.en 843) </p></li><li class="listitem">Add a string-based cacheKey property for domain isolation projects/en/torbrowser/design/index.html.en 844) <p> projects/en/torbrowser/design/index.html.en 845) projects/en/torbrowser/design/index.html.en 846) To <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3666" target="_top">increase the projects/en/torbrowser/design/index.html.en 847) security of cache isolation</a> and to <a class="ulink" href="https://trac.torproject.org/projects/tor/ticket/3754" target="_top">solve strange and projects/en/torbrowser/design/index.html.en 848) unknown conflicts with OCSP</a>, we had to <a class="ulink" href="https://gitweb.torproject.org/torbrowser.git/blob/refs/heads/maint-2.2:/src/current-patches/0005-Add-a-string-based-cacheKey.patch" target="_top">patch projects/en/torbrowser/design/index.html.en 849) Firefox to provide a cacheDomain cache attribute</a>. We use the full projects/en/torbrowser/design/index.html.en 850) url bar domain as input to this field. projects/en/torbrowser/design/index.html.en 851) projects/en/torbrowser/design/index.html.en 852) </p></li><li class="listitem">Randomize HTTP pipeline order and depth projects/en/torbrowser/design/index.html.en 853) <p> projects/en/torbrowser/design/index.html.en 854) As an projects/en/torbrowser/design/index.html.en 855) <a class="ulink" href="https://blog.torproject.org/blog/experimental-defense-website-traffic-fingerprinting" target="_top">experimental projects/en/torbrowser/design/index.html.en 856) defense against Website Traffic Fingerprinting</a>, we patch the standard projects/en/torbrowser/design/index.html.en 857) HTTP pipelining code to randomize the number of requests in a projects/en/torbrowser/design/index.html.en 858) pipeline, as well as their order. projects/en/torbrowser/design/index.html.en 859) </p></li><li class="listitem">Block all plugins except flash projects/en/torbrowser/design/index.html.en 860) <p> projects/en/torbrowser/design/index.html.en 861) 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"> projects/en/torbrowser/design/index.html.en 862) @mozilla.org/extensions/blocklist;1</a> service, because we projects/en/torbrowser/design/index.html.en 863) actually want to stop plugins from ever entering the browser's process space projects/en/torbrowser/design/index.html.en 864) and/or executing code (for example, AV plugins that collect statistics/analyze projects/en/torbrowser/design/index.html.en 865) URLs, magical toolbars that phone home or "help" the user, skype buttons that projects/en/torbrowser/design/index.html.en 866) ruin our day, and censorship filters). Hence we rolled our own. projects/en/torbrowser/design/index.html.en 867) </p></li><li class="listitem">Make content-prefs service memory only projects/en/torbrowser/design/index.html.en 868) <p> projects/en/torbrowser/design/index.html.en 869) This patch prevents random URLs from being inserted into content-prefs.sqllite in projects/en/torbrowser/design/index.html.en 870) the profile directory as content prefs change (includes site-zoom and perhaps projects/en/torbrowser/design/index.html.en 871) other site prefs?).
Add a couple extra sentence... Mike Perry authored 12 years ago	projects/torbrowser/design/index.html.en 872) </p></li></ol></div></div></div><div class="sect1" title="4. Packaging"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Packaging"></a>4. Packaging</h2></div></div></div><p> </p><div class="sect2" title="4.1. Build Process Security"><div class="titlepage"><div><div><h3 class="title"><a id="build-security"></a>4.1. Build Process Security</h3></div></div></div><p> </p></div><div class="sect2" title="4.2. External Addons"><div class="titlepage"><div><div><h3 class="title"><a id="addons"></a>4.2. External Addons</h3></div></div></div><p> </p><div class="sect3" title="Included Addons"><div class="titlepage"><div><div><h4 class="title"><a id="id2974027"></a>Included Addons</h4></div></div></div></div><div class="sect3" title="Excluded Addons"><div class="titlepage"><div><div><h4 class="title"><a id="id2999979"></a>Excluded Addons</h4></div></div></div></div><div class="sect3" title="Dangerous Addons"><div class="titlepage"><div><div><h4 class="title"><a id="id3006218"></a>Dangerous Addons</h4></div></div></div></div></div><div class="sect2" title="4.3. Pref Changes"><div class="titlepage"><div><div><h3 class="title"><a id="prefs"></a>4.3. Pref Changes</h3></div></div></div><p> </p></div><div class="sect2" title="4.4. Update Security"><div class="titlepage"><div><div><h3 class="title"><a id="update-mechanism"></a>4.4. Update Security</h3></div></div></div><p> </p></div></div><div class="sect1" title="5. Testing"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Testing"></a>5. Testing</h2></div></div></div><p>