How to prove that a lightning invoice was paid?

In the realm of the Lightning Network (LN), a Proof of Payment (PoP) serves as a verifiable acknowledgment that a specific transaction has successfully occurred. Lightning payments are atomic meaning that if there is a pre-image received by the payer the payment was surely received by the destination node.

How Does Proof of Payment Work in the Lightning Network?

1. Invoice Generation by the Payee's Node:

   • The process begins with the creation of a Lightning Network invoice by the payee's node. This invoice is not just a simple payment request but a complex structure comprising various elements crucial for the transaction. One of the critical components of this invoice is a cryptographic hash of a secret, known as the 'payment pre-image'.

2. Invoice Signing:

   • To ensure authenticity and prevent tampering, the payee's node digitally signs the invoice. This signature is a cryptographic proof that the invoice was indeed generated by the actual owner of the node and not an imposter.

3. Payment and Disclosure of Pre-Image:

   • Once the payer decides to fulfill the invoice, the payment is routed through the Lightning Network to reach the payee. Upon successful receipt of the payment, the payee's node releases the 'payment pre-image' to the payer. This pre-image is essentially the secret whose hash was included in the invoice.

4. Combination of Invoice and Pre-Image as Proof of Payment:

   • The final step in establishing a Proof of Payment is the combination of the original LN invoice and the payment pre-image. The payer can use these two pieces to prove that a payment was made to the payee. Essentially, with this two pieces of information anyone can verify that the hash in the invoice corresponds to the pre-image provided upon payment, proving that the transaction indeed occurred as claimed.

Why is Proof of Payment Important?

1. Verification of Payment:

   • PoP is crucial for parties to confirm that the transaction was processed correctly without needing to relying on a third party.

2. Non-repudiation:

   • With the cryptographic evidence provided by the PoP, the payee cannot deny receiving the payment, and similarly, the payer cannot deny making the payment. This is crucial for dispute resolutions and trust-building in digital transactions.

3. Security:

   • The cryptographic nature of the invoice and the pre-image ensures that the transaction is secure and resistant to tampering or fraud.

In summary to prove a lightning invoice was paid there are two pieces of information needed:

1.The original Lightning Invoice provided by the node 2.The payment pre-image received by the payer on the successful payment

Please if you are contacting the Blink support team or operators of other lightning nodes have this information at hand.