[messaging] Zero knowledge proofs of passport

[Natanael]

This reminds me of both Namecoin, stealth addresses and that Distributed
Anonymous Credentials paper from the Zerocoin folks (consider it like an
anonymized Namecoin).

https://eprint.iacr.org/2013/622.pdf

My take on how to use Namecoin profiles:
http://roamingaroundatrandom.wordpress.com/2014/05/29/universal-p2p-address-book-software-using-namecoin/

Being able to tie a real world identity (vouched for by some entity like ID
card issuers or CA:s) and credentials to your profile in a way which you
can prove selectively in a non-replayable manner (I can prove my name to my
friend, he can't prove it to somebody else) would be awesome. You could
choose what would be public for all to see, and what only can be seen and
verified in-person.

(Stealth addresses comes into the mix for the anonymous messaging. It can
not only be applied to Bitcoin addresses, but also for mailing.)

You could actually use this practically everywhere. Proving you're a paying
subscriber of your VPN service (just switch IP and even they don't know
which of their customers you are), proving your citizenship as you
mentioned, proving your gym membership, etc...

And to make sure you can't be tracked across services but that you also
can't make multiple profiles, they can challenge you to hash your ID's
private key (or a similar challenge that can't be bruteforced on their side
to reveal your ID) with their service ID, and prove with ZKP that it was
created using your profile. You can only create one such hash per service,
and they still don't know who you are. (About the same thing as your pond
anti-spam idea.)
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