[messaging] Masking contact addresses with ECDH

[Jeff Burdges]

On Tue, 2016-03-01 at 13:37 +0100, Jeff Burdges wrote:
> I donno if that buys much though since if a user's device
> can handle O(|contact|^2) cycles or storage then an adversary is
> likely
> to posses O(|interesting_people|^2) cycles or storage.

In fact, one could make the adversary much more work by building a kind
of "contact exchange ratchet" that bases the blinding for new bloom
filter exchanges on the current shared contact state, as opposed to
merely the public keys of Alice and Bob.  

I'm not 100% sure this can be done without leaking information over
time, but if so it'd give the adversary some nasty subexponential
algorithm, which sounds good enough.

Jeff
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