blip-0042: Bolt 12 Contacts

This is a bLIP ("Bitcoin Lightning Improvement Proposal"), the Lightning analog of a BIP, authored by Bastien Teinturier of ACINQ. It builds on BOLT12 offers — reusable, static payment codes — to add a contacts concept. The proposal was created in 2024 and was merged into the bLIPs repository with Active status in May 2026, which is why it surfaced on the BitDevs 51 agenda.

The core idea is optional, mutual identification between people who pay each other. When paying a contact, the sender can include a contact_secret and payment information in the invoice_request, which lets the recipient recognize the payment as coming from a known contact, add the payer as a new contact, or pay them back without further setup. The goal is to make peer-to-peer Lightning payments feel closer to mainstream fiat payment apps.

The design works without modifying offers themselves: both nodes independently derive a matching contact_secret, so contact information only appears inside actual payments to trusted parties rather than being exposed in a public offer. The proposal is candid about a trade-off — two people who add each other as contacts independently may initially derive different secrets until a first payment reconciles them.

For a non-specialist, the value here is seeing how Lightning's UX frontier is moving beyond "send to an invoice" toward persistent identities and contact lists, and that this work happens in the open as a licensed, reviewable specification (CC0).