blip-0042: Bolt 12 Contacts

[t-bast]

Bolt 12 introduces offers, which are static lightning "addresses". An offer can be stored and reused to pay the same node many times. It then becomes natural to associate Bolt 12 offers to your friends and contacts.

When sending payments to contacts, you may want them to know that the payment came from you. We propose a scheme to optionally include contact information in outgoing payments to allow the recipient to:

• detect that the payment is coming from one of their known contacts
• otherwise, be able to add the payer to their contacts list
• send funds back to the payer without additional interaction

This feature provides a better UX for lightning wallets, by making payments between contacts look very similar to fiat payment applications.