Hi,

On Thurday, March 30, 2023 at 3:43:06 PM UTC+3Kaustubha Govind wrote:

Would you be able to expand your Privacy Considerations section to address 
whether the API is likely to expose any new fingerprinting surfaces?


I have added Fingerprinting 
<https://github.com/riju/backgroundBlur/blob/main/explainer.md#fingerprinting> 
subsection to our  Privacy Considerations section.

Particularly; I am interested in understanding:

   1. Whether there is additional information about the platform (e.g. OS 
   version) that can be gleaned by querying whether the platform supports 
   background blurring. Note that with the User Agent Reduction 
   <https://developer.chrome.com/en/docs/privacy-sandbox/user-agent/> work, 
   we are attempting to limit default access to some of this platform 
   identifying information.

In short, there isn't but see the linked Fingerprinting subsection for 
detailed analysis. 


   1. How stable do we expect the blur to be across hardware 
   configurations? It would be unfortunate if this turned into a problem 
   similar to Canvas Fingerprinting.

In Canvas Fingerprinting, sites can draw to a canvas (the source) as they 
like and fingerprint the result which may be a bit different on different 
platforms and platform versions. I assume that you refer to this (lack of) 
stable result here.
This is really not an issue with background blur. In the case of the 
getUserMedia tracks, the sites has access to tracks which may have 
background blur in effect and may allow background blur to be 
disabled/enabled and to the resulting frames. However, sites have no 
control on the source (what's in the field-of-view of a camera) so it is 
not possible to reprocess the same frames on different platforms and 
platform versions and to compare the results.
 

Perhaps one mitigation here is that the capability is gated behind the 
getUserMedia() permission; which limits drive-by fingerprinting?


That is the main gate but see the linked Fingerprinting subsection for 
detailed analysis. 

BR,
 Eero

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