Expand on fingerprinting specifically in the Privacy considerations section

[domenic]

A common question these days with web APIs is "how does this enable fingerprinting"? I think your existing explainer Privacy considerations section addresses this, but only indirectly. For readers laser-focused on this popular question, it might be worth spelling it out explicitly.

In particular, I believe the argument goes something like:

• Yes, when the user affirmatively consents to give access to a device, that gives you extra information about the user. Such as:
  • The fact that they are willing to give access to a device at all. (At least one bit.)
  • The specific device identifiers, which---among the set of people who grant Web Serial access---will further narrow down the population.
• If the user consents to the prompt repeatedly on multiple different sites, then this allows cross-site joining of those bits.
• However, this is very similar to existing APIs which grant a good number of bits behind a prompt. The strongest analogy is to <input type=file>, where giving the site access to a file gives it as many bits as compose the file, and if multiple sites all use <input type=file>, and the user gives them all access to the same file, the sites can can cross-site track the user based on those bits.

[domenic]

It seems some of this is discussed in https://github.com/WICG/serial/blob/gh-pages/security-privacy-questionnaire.md, although I find the <input type=file> analogy a good bit better than the native application analogy.

As a related issue, probably the Security/privacy considerations sections should cross-link to the nice, detailed answers in the questionnaire.