Report low entropy source value in X-Client-Data
The low entropy source is reported as a variation ID offset from 3320978. For example, a low entropy source value of 10 is reported as variation ID 3320988. The low entropy source is a locally-generated value in the range of 0-7999 that's used by Chrome to derive the variation ids included in the X-Client-Data header. Since other variation ids have been selected based on this value, this does not expose any additional entropy in the header given enough variation ids. Note: 8000 possible values is under 2^13, therefore having 13+ A/B variations would carry the same amount of entropy. A typical Chrome client has more than 13 variation ids reported. Adding this value allows for retrospective A/A tests for validating that there's no existing bias between two randomized groups of clients for a later A/B study. Bug: 1134444 Change-Id: Ia1af76f469cb2e3e25b03884abc4974cec237977 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461906 Commit-Queue: Karthika Pai <karthikapai@google.com> Reviewed-by:Ilya Sherman <isherman@chromium.org> Reviewed-by:
Caitlin Fischer <caitlinfischer@google.com> Reviewed-by:
Alexei Svitkine <asvitkine@chromium.org> Cr-Commit-Position: refs/heads/master@{#825962}
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