[tbb-dev] Canvas Breakage Ideas

[Sanketh Menda]

Hello,

> On 5/19/20 6:02 AM, Matthew Finkel wrote:
> > On Wed, Apr 29, 2020 at 01:07:40PM +0200, Alex Catarineu wrote:
> > Are any of the conversions passed onto the GPU? Do we know if format
> > conversation is deterministic?
>
> True, I did not consider that the extraction (e.g. `toDataURL('image/jpeg')`)
> might add some entropy by itself. Good questions, we would need to
> investigate if this approach is going to be pursued. And I agree with tom, it
> would be good first to investigate what these sites are doing exactly with
> the canvas to evaluate what would be the best approach.

I did a little bit of digging and it seems like `toDataURL('image/xxx')`
calls one of the encoders listed here
<https://searchfox.org/mozilla-central/search?q=symbol:_ZN11imgIEncoder12InitFromDataEPKhjjjjjRK12nsTSubstringIDsE&redirect=false>.
In particular, the JPEG Encoder
<https://searchfox.org/mozilla-central/source/image/encoders/jpeg/nsJPEGEncoder.cpp#94>
seems to be doing numerical math to compress the image. Indeed, it seems like
this observation was already made by

Hiding in the Crowd: an Analysis of the Effectiveness of Browser Fingerprinting at Large Scale
Gómez-Boix, Laperdrix, and Baudry
WWW '18 https://doi.org/10.1145/3178876.3186097

where on page 314, they say

> We also tested the impact of compressing a canvas rendering to the JPEG
> format. It should be noted that the JPEG compression comes directly from the
> Canvas API and is not applied after collection. Due to the lossy compression,
> it should come as no surprise that the entropy from JPEG images is lower than
> the PNG one usually used by canvas fingerprinting tests (from 0.407 to 0.391)

Is there an easy way for us to do a study on this? Specifically, fix a random
image, and then on a bunch of different computers, read the image and then do
a `toDataURL('image/xxx')` for each of the formats.

Best,
Sanketh