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Several users have suggested that Briar should relay encrypted messages between contacts, so that users who can't communicate directly can pass messages through mutual contacts.

This would have implications for battery and bandwidth consumption. If message propagation was restricted then it would also have privacy implications (Alice could see that Bob and Carol were both sending messages at 3am, when nobody else was sending anything). If message propagation was unrestricted then the battery and bandwidth impact would be hard to control - this would affect scalability and enable flooding attacks. Fair queueing might help to mitigate flooding attacks (#511).

One user suggested that the privacy impact could be mitigated through fine-grained controls (e.g. Bob and Carol would only choose Alice to relay their messages if they didn't mind her knowing when they were communicating). I'm skeptical about whether most people can predict and manage their privacy needs at that granularity in advance, but I could be wrong.