🌩️Solana -> Bitcoin Lightning

Requirements

• lightning invoice's payment hash P needs to be known upfront

• lightning invoice has to have a fixed amount

• payee needs to be online at the time of payment

Parties

• payer - the one paying in solana or spl token and using intermediary to do the swap

• LP node - handling the swap receives solana or spl token and sends lightning network payment

• payee - recipient of the lightning network payment

Process

1. Payee creates a regular bitcoin lightning invoice with fixed amount and sends it to the payer (this invoice contains the payment hash P)

2. Payer queries the LP node off-chain, sending the lightning invoice and desired locktime T, LP node tries to probe for the route of the payment and returns its confidence score (how likely LP node thinks that the payment will succeed) along with its fee and details the payer needs to construct a HTLC on-chain

3. Payer reviews the returned confidence score + fee and sends a transaction to construct an HTLC on Solana:

   • paying the funds to LP node if he can supply a valid secret S, such that hash of secret H(S) is equal to payment hash P, but only until a specific time in the future - locktime T

   • refunding the payer, but only after locktime T

   • refunding the payer, but only with a specific message Mr (refund) signed by LP node (for co-operative close, when payment fails)

   NOTE: locktime T is determined by the payer and is a trade-off between likelihood of payment being successful and locking the funds for shorter periods in case of LP node's non-cooperativeness.

   • Larger locktime means more payment paths can be considered (more hops on the lightning network), increases the likelihood of payment being successfully routed, but will also lead to longer lock periods when intermediary is non-cooperative.

   • Smaller locktime means less payment paths can be considered (less hops on the lightning network) and increases the likelihood of payment routing failures, but will also lead to shorter lock periods when intermediary is non-cooperative

4. LP node observes the creation of HTLC on Solana and proceeds to attempt a payment of the lightning invoice.

Successful payment

1. Payee reveals a secret S to intermediary in order to accept the payment of the lighting invoice.

2. LP node uses the knowledge of secret S to obtain the funds from the HTLC on Solana and swap is finished.

Failed payment

1. The payment was unsuccessful, so payee did not reveal a secret S to the LP node.

2. Upon request by payer, LP node creates a specific signed message Mr (refund), allowing the payer to refund his funds from the HTLC

LP node went offline

1. Payer waits till the expiry of locktime T and then refunds his funds back from the HTLC

Diagram

Solana -> Bitcoin Lightning swap process (Intermediary = LP Node, Smart chain = Solana)