🔗Reusable Payment Codes (BIP47)

The problem

Stealth addresses are great, but for an ongoing relationship — a vendor, a donation address, a paying client — the sender needs your stealth address each time, or has to maintain a contact channel. BIP47 fixes this with a one-time notification transaction that establishes a permanent shared secret.

How it works

1. Alice generates a single payment code she can publish anywhere
2. Bob sends a tiny notification TX to Alice's notification address — it carries Bob's payment code, blinded with an ECDH shared secret, in OP_RETURN
3. Now Bob (or anyone with the same payment code relationship) can derive infinite unique stealth addresses for Alice without further interaction
4. Each subsequent payment is a fresh stealth output. The notification TX itself reveals only that some notification happened — not the payment code relationship

Step-by-step

1. Get your payment code

   $ MaryJaneCoind getpaymentcode
   {
     "payment_code": "PM8TJJvP5XPvcn4oy75XBydTpZrZxtKSYBqRtj7WuVywEYaWJmWcnpPyCi2AZVhbtZudqu4Go2fGQeFdeyRFtqtwkjRkmFPPWFDkLYvtpGPBeipi8nG4",
     "notification_address": "MPNLrBVBYYWKpSNdKJ3ABipZEUC8aDvsYZ",
     "scan_pubkey": "0209683bcb222d6f8f8d0c9d13432a...",
     "spend_pubkey": "0328c613cd05ba604e049acfc5cdf1..."
   }

   Publish the payment_code string anywhere — Twitter bio, website, QR code, business card.

2. Sender: establish a payment channel

   To send Alice money the first time, Bob runs:

   $ MaryJaneCoind sendtonotify "PM8TJJvP5XP..."
   {
     "txid": "abc123...",
     "notification_address": "MPNLrBVBYY...",
     "status": "notification_sent"
   }

3. Receiver: scan for incoming notifications

   $ MaryJaneCoind scannotifications
   2   # found 2 inbound payment-code notifications

4. Send unlimited payments — no more interaction

   Once the channel exists, Bob can derive any address index for Alice:

   $ MaryJaneCoind deriveaddress "PM8TJJvP5XP..." 0
   {"index": 0, "address": "MQx7k9f..."}
   
   $ MaryJaneCoind deriveaddress "PM8TJJvP5XP..." 1
   {"index": 1, "address": "MRy8l0g..."}

   Each address is a fresh DKSAP destination — same privacy as standard stealth.

5. List all your payment relationships

   $ MaryJaneCoind listpaymentcodes
   [
     {
       "payment_code": "PM8TJKx...",
       "label": "Bob",
       "notification_sent": true,
       "notification_received": true,
       "next_send_index": 3,
       "next_recv_index": 1
     }
   ]

Notification blinding

The 80-byte payment-code payload is XOR-masked with ECDH(sender_scan_priv, receiver_scan_pub). Only the recipient can unblind it. Observers see a notification TX exists, but cannot identify which payment code it carries — preventing attackers from building a directory of "who knows whose code".