[tbb-bugs] #13400 [Tor Browser]: Canvas Fingerprinting: fonts

Tor Bug Tracker & Wiki blackhole at torproject.org
Wed Oct 15 16:06:15 UTC 2014

#13400: Canvas Fingerprinting: fonts
     Reporter:  cypherpunks  |      Owner:  tbb-team
         Type:  defect       |     Status:  new
     Priority:  major        |  Milestone:
    Component:  Tor Browser  |    Version:
   Resolution:               |   Keywords:
Actual Points:               |  Parent ID:
       Points:               |

Comment (by mcs):

 There are several things going on here.  The fingerprinter.html page uses
 page reload / iteration and a cookie to defeat TBB's max_font preferences.
 It also uses canvas measureText() to generate its fingerprints, which has
 the advantage that it returns a floating point value for width.  This may
 be better than using something like offsetWidth of a <span> (which as far
 as we can tell only provides integer width values), but we were able to
 get similar fingerprinting results without a canvas.

 We could change canvas measureText() to return a rounded number or disable
 it entirely, but it may not be worth it.  A real solution will require
 fixing #13313 or another approach.

Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13400#comment:4>
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