Comment by jefftk

Are you talking about the x-client-data header (which isn't unique, but is relatively high entropy at <= 13-bits)? [1] that is used for evaluating the effect of experiments that Chrome is running on other Google services, which does include ads. But it is not used for personalization (I wish they would say that publicly).

For example, when I look at a Google Fonts request in Chrome developer tools I see:

    x-client-data: CKe1yQEIkrbJAQiitskBCMS2yQEIqZ3KAQiVocsBCOeEzAEIhKvMAQjys8wBCL+1zAE=
    Decoded:
    message ClientVariations {
      // Active client experiment variation IDs.
      repeated int32 variation_id = [3300007, 3300114, 3300130, 3300164, 3313321, 3330197, 3342951, 3347844, 3348978, 3349183];
    }

Each of those numbers represents an experimental treatment that is currently active for my Chrome instance. (It looks like more entropy because it's multiple values, but they're all derived from a single 13-bit per-instance seed.)

[1] https://www.google.com/chrome/privacy/whitepaper.html#variat...