Offline Bitcoin Lightning payments will soon be possible

One of Bitcoin’s popular Lightning Network implementations, Éclair, is actively testing support for asynchronous payments. The new feature will make it possible to send money over Bitcoin’s most popular Layer 2, even if a node is offline, such as a Lightning node on an offline phone.

Currently, most Lightning users seek third-party custodian services to deposit payments to offline nodes. Éclair’s upgrade is one of several proposals to reduce reliance on third parties to hold funds until the node comes back online.

Éclair #2435 suggested a so-called trampoline relay it may temporarily hold funds to a node restores the Internet connection. These nodes can include tablets, devices that automatically sleep, or any Lightning node with an unreliable Internet connection.

Developers bundled Éclair #2435 as an optional update for Lightning users. The developer-oriented Bitcoin Optech Newsletter describes it as a foundational technology for asynchronous payments that enables trampoline relays. This basic implementation will allow experimentation.

Blockchain developer Richard Myers described Éclair #2435 as the first step in addressing issue #2424, which outlines seven tasks developers must complete before asynchronous payments are fully enabled.

Offline Lightning payments are still ongoing

Other developers like Matt Corallo believe Point Time Locked Contracts (PTLCs) are a better solution for offline Lightning payments than trampoline relays.

PTLCs provide an alternative method of securing contingent payments by locking them with a public key and unlocking them with a corresponding signature when the node regains an Internet connection. Developers say PTLCs will make contingent payments more private and takes up less block space than an earlier hash-based proposal, HTLCs, which use hash digests and preview images to lock and unlock payments.

While brainstorming the idea of ​​asynchronous payments in October 2021, Matt Corallo called a PTLC-based solution his best suggestion, although he suggested he might be open to other ideas.

In contrast, Éclair #2435 does not use PTLCs. Instead, Éclair will require a third party to do so delay the forwarding of the funds until the disconnected node can re-establish the connection. Developers call it a partial implementation of asynchronous payments that serves as a first step to get it fully up and running.

Advantages of asynchronous payments

Regular Layer 1 bitcoin transactions are always possible to any address regardless of internet uptime. However, Lightning requires synchronous internet connection to sign the contract needed to move bitcoin between Lightning users.

The idea for asynchronous payments on the Lightning Network arose from a brainstorming thread started by Bitcoin Core developer Matt Corallo. He cited a shortcoming of the Lightning Network: people want to use it for tips or receive payments on their phone without having their Lightning app online and running all the time.

Corallo mentioned some solutions that are sometimes effective but add extra steps or expense to the process.

Whether with trampoline relays, hashed or point timelock contracts, or other forms of cryptography, asynchronous payments will soon become possible over Bitcoin’s low-cost Lightning Network.

The first Éclair test implementation of asynchronous payments prioritizes offline Lightning payments. Éclair bounces funds from a trampoline relay that can temporarily hold funds until the node comes back online.

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