[tor-commits] [tor-browser-spec/master] Finish fingerprinting updates.
mikeperry at torproject.org
mikeperry at torproject.org
Thu Oct 30 04:15:14 UTC 2014
Author: Mike Perry <mikeperry-git at torproject.org>
Date: Wed Oct 29 20:53:37 2014 -0700
Finish fingerprinting updates.
design-doc/design.xml | 114 ++++++++++++++++++++++++++++++++-----------------
1 file changed, 76 insertions(+), 38 deletions(-)
diff --git a/design-doc/design.xml b/design-doc/design.xml
index b469f2a..a49de34 100644
@@ -1531,7 +1531,11 @@ image can be used almost identically to a tracking cookie by the web server.
-To reduce the threat from this vector, we have patched Firefox to <ulink
+In some sense, the canvas can be seen as the union of many other
+fingerprinting vectors. If WebGL were normalized through software rendering,
+and the browser shipped a fixed collection of fonts, it might not be necessary
+to create a canvas permission. However, until then, to reduce the threat from
+this vector, we have patched Firefox to <ulink
before returning valid image data</ulink> to the Canvas APIs, and for <ulink
@@ -1540,6 +1544,8 @@ allowed the site in the URL bar to access Canvas image data, pure white image
<listitem>Open Local Port Fingerprinting
@@ -1563,13 +1569,17 @@ addresses by default.
<listitem>USB Device ID enumeration
-The GamePad API <ulink
-web pages with the USB device id, product id, and driver name</ulink> of all
-connected game controllers, as well as detailed information about their
-capabilities. This API should be behind a site permission in Private Browsing
-Modes. We simply disable it via the pref
+API</ulink> provides web pages with the <ulink
+device id, product id, and driver name</ulink> of all connected game
+controllers, as well as detailed information about their capabilities. This API
+should be behind a site permission in Private Browsing Modes, or should present a generic
+controller type (perhaps a two button controller that can be mapped to the keyboard) in all cases.
+We simply disable it via the pref <command>dom.gamepad.enabled</command>.
<listitem>Invasive Authentication Mechanisms (NTLM and SPNEGO)
@@ -1603,6 +1613,12 @@ provided by the following WebGL API calls: <command>getParameter()</command>,
+Another alternative for WebGL might be to fall back to software rendering only
+for private motes.
@@ -1615,19 +1631,25 @@ pre-built list to query, a large amount of fingerprintable information may
still be available.
-The sure-fire way to address font linkability is to ship the browser with a
-font for every language, typeface, and style in use in the world, and to only
-use those fonts at the exclusion of system fonts. However, this set may be
-impractically large. It is possible that a smaller <ulink
-subset</ulink> may be found that provides total coverage. Right now, it
-appears that the major languages on Wikipedia can be supported for about 3MB
-of additional distribution size, using the DejaVu font set.
+ <para><command>Design Goal:</command> The sure-fire way to address font
+linkability is to ship the browser with a font for every language, typeface,
+and style, and to only use those fonts at the exclusion of system fonts. We are
+investigating</ulink> this approach, and our current favorite font sets for
+this purpose are the <ulink url="http://www.droidfonts.com/droidfonts/">Droid
+fonts</ulink>, the <ulink url="http://hangeul.naver.com/">Nanum fonts</ulink>,
+and <ulink url="https://fedorahosted.org/lohit/">Lohit fonts</ulink>. The Droid
+font set is fairly complete by itself, but Nanum and Lohit have smaller
+versions of many South Asian languages. When combined in a way that chooses the
+smallest font implementations for each locale, these three font sets provide
+which provide coverage for the all languages used on Wikipedia with more than
+10,000 articles, and several others as well, in approximately 3MB of compressed
+overhead. The <ulink url="https://www.google.com/get/noto/">Noto font
+set</ulink> is another font set that aims for complete coverage, but is
+considerably larger than the combination of the Droid, Nanum, and Lohit fonts.
In the meantime while we investigate shipping our own fonts, we disable
@@ -1639,8 +1661,7 @@ a Firefox patch</ulink>. We create two prefs,
<command>browser.display.max_font_count</command> for this purpose. Once these
limits are reached, the browser behaves as if
-<command>browser.display.use_document_fonts</command> was set. We are still
-working to determine optimal values for these prefs.
+<command>browser.display.use_document_fonts</command> was set.
@@ -1670,13 +1691,14 @@ properties of the content window, but report an effective size of 0 for all
border material, and also report that the desktop is only as big as the inner
content window. Additionally, new browser windows are sized such that their
content windows are one of a few fixed sizes based on the user's desktop
-resolution. The user should also be informed that maximizing their windows can
-lead to fingerprintability under this scheme. To further reduce
-resolution-based fingerprinting, we are <ulink
+resolution. In addition, to further reduce resolution-based fingerprinting, we
-zoom/viewport-based mechanisms</ulink> that might allow us to always report
-the same desktop resolution regardless of the actual size of the content
-window, and simply scale to make up the difference.
+zoom/viewport-based mechanisms</ulink> that might allow us to always report the
+same desktop resolution regardless of the actual size of the content window,
+and simply scale to make up the difference. Until then, the user should also
+be informed that maximizing their windows can lead to fingerprintability under
@@ -1702,15 +1724,15 @@ to privacy in this mode.
<listitem>CSS Media Queries
resolution, usable desktop size, OS widget size, toolbar size, title bar size,
system theme colors, and other desktop features that are not at all relevant
-to rendering and serve only to provide information for fingerprinting.
+to rendering and serve only to provide information for fingerprinting. Most of this information comes from
+<ulink url="https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Media_queries">CSS Media Queries</ulink>, but
+Mozilla has exposed <ulink url="https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#System_Colors">several user and OS theme defined color values</ulink> to CSS as well.
-<!-- XXX: Link to CSS spec for this stuff -->
In Private Browsing Mode, CSS should not be able infer anything that the user
has configured about their computer. Additionally, it should not be able to
infer machine-specific details such as screen orientation or type.
@@ -1751,11 +1773,28 @@ url="http://pseudo-flaw.net/tor/torbutton/fingerprint-firefox.html">can be
used</ulink> to fingerprint OS, platform, and Firefox minor version. </para>
-XXX: 2. bug 10703: force the default charset to avoid locale fingerprinting
+In Tor Browser, we provide non-English users the option of concealing their OS
+and browser locale from websites. It is debatable if this should be as high of
+a priority as information specific to the user's computer, but for
+completeness, we attempt to maintain this property.
+ <para><command>Implementation Status:</command>
+We set the fallback character set to set to windows-1252 for all locales, via
+<command>intl.charset.default</command>. We also patch Firefox to allow us to
+the JS engine</ulink> to use en-US as its internal C locale for all Date, Math,
+and exception handling.
<listitem>Timezone and clock offset
@@ -1786,10 +1825,11 @@ several seconds, it is still possible for the remote site to detect large
differences between the user's clock and an official reference timesource.
- <para><command>Design Goal:</command> Ideally, the browser would be
-able to correct the source of this clock drift using an external time source,
-either through something like tlsdate, or directly through the Tor protocol.
-Additionally, the timezone should be set to UTC.
+ <para><command>Design Goal:</command> Ideally, the browser would be able
+to correct the source of this clock drift using an external time source,
+either through something like <ulink
+linkend="https://github.com/ioerror/tlsdate">tlsdate</ulink>, or directly
+through the Tor protocol. Additionally, the timezone should be set to UTC.
@@ -1829,8 +1869,6 @@ optimum trade-off between quantization+jitter and amortization time.
-<!-- XXX: Disabled network performance timers too -->
Currently, the only mitigation against performance fingerprinting is to
disable <ulink url="http://www.w3.org/TR/navigation-timing/">Navigation
Timing</ulink> through the Firefox preference
More information about the tor-commits