Piggybacking information within payment_preimage is a suggestion by developer Jose Femenias, it is used to enable sharing a common secret seed between an offline device and an online LN node, both the devices calculate the same per_transaction_secret, the node encrypts the PIN within the payment_preimage and then when the LN wallet receives the payment_preimage it decrypts it showing the PIN, this is provided as proof of payment to the offline device.

An implementation of this scheme would give several popular user experiences a chance to be applied on Lightning. For example, the scheme could work in ordering something from a vending machine, you select a product, scan a QR code and upon payment your wallet shows you a PIN that you type into the vending machine and if the PIN is correct, the product is yours.

Another usage here is what developer ZmnSCPxj called a “pay to pay”, by piggybacking an address for the payer, the payer can use tiny payments to ensure the correctness of an address, thus making a larger payment more secure and preventing spamming to get countless payment addresses that won’t be used.

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