[rxaabygg] Contact info which cannot be shared

To prevent spam, telemarketing or other robo calls, you give a different contact information (email, number, etc.) to each person who wants to contact you.  You can then filter on receipt.  The canonical example is plus-addressing of email, like gmail.

But it doesn't completely solve the problem, because one of the senders may share his version of your address, resulting in you seeing several senders sending to the same address.

Create contact information which cannot be shared: it is locked to the sender's public key, so the sender must use their private key to send with it.  This will require a different protocol than the conventional ones.

When you provide your "uniquified" address to a potential sender, how do we induce them to use a public key that they care not to share?

Does this solve any problem that would be more straightforwardly solved by simply requiring senders to cryptographically sign their messages (as part of the protocol)?