Cross-Chain HTLC

Fiber supports cross-chain payments between the CKB network and the Bitcoin Lightning Network using Hash Time-Locked Contracts (HTLC). This enables atomic swaps — transactions that either complete in full or not at all, eliminating the risk of partial transfers.

How It Works

Cross-chain HTLC involves three actors:

• Alice — in the CKB (Fiber) network, wants to send or receive funds
• Bob — in the Bitcoin (Lightning) network, wants to receive or send funds
• Ingrid — the Cross-Chain Hub (CCH) service provider who runs both a Fiber node and a Lightning node

CKB → Bitcoin (Send BTC)

1. Bob generates a Lightning invoice for X BTC
2. Alice creates a CCH order with send_btc, providing the Lightning invoice
3. Ingrid accepts the order and creates an invoice in the Fiber network for X + F (wrapped BTC + fee)
4. Alice pays the Fiber invoice
5. Ingrid pays the Lightning invoice to Bob
6. Both payments settle atomically

Bitcoin → CKB (Receive BTC)

1. Alice creates a CCH order with receive_btc
2. Ingrid creates a Lightning invoice
3. Bob pays the Lightning invoice
4. Ingrid pays the Fiber invoice to Alice
5. Both payments settle atomically

Requirements

• The assets in both networks must maintain a fixed swap ratio (e.g., 1:1 wrapped BTC on CKB)
• Both networks must use the same hash algorithm for HTLCs
• The CCH operator charges a fee for the swap service

Expiry Safety

Cross-chain payments require careful time lock management to prevent fund loss. The CCH validates that:

• The outgoing invoice's final expiry is less than half of the incoming invoice's expiry
• Route-level expiry is checked dynamically after the incoming payment is accepted

See CCH Expiry Dependency for the full specification.

RPC Methods

Future: PTLC

Fiber plans to migrate from HTLC to Point Time-Locked Contracts (PTLC), which offer:

• Improved privacy (hidden payment hash)
• Reduced on-chain footprint (adaptor signatures)
• Enhanced security (resistance to wormhole attacks)

Related Topics

• Invoice Guide — creating and managing invoices
• Hold Invoice — deferred settlement for conditional payments