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In a more limited sense, note that if a censor or law enforcement agency has the ability to obtain specific observation of parts of the network, it is possible for them to verify a suspicion that you talk regularly to your friend by observing traffic at both ends and correlating the timing of only that traffic.
Again, this is only useful to verify that parties already suspected of communicating with one another are doing so.
In most countries, the suspicion required to obtain a warrant...