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Light Blue Touchpaper Hot or Not: Revealing Hidden Services by their Clock Skew
Not sure whether this is using NTP, but the author tries to identify machines by changing their CPU load,Mon Sep 11 19:32:54 2006 - permalink -- http://www.lightbluetouchpaper.org/2006/09/04/hot-or-not-revealing-hidden-services-by-their-clock-skew/