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Lightning Loop

Lightning Loop is a non-custodial service offered by Lightning Labs to bridge on-chain and off-chain Bitcoin using submarine swaps. This repository is home to the Loop client and depends on the Lightning Network daemon lnd. All of lnd’s supported chain backends are fully supported when using the Loop client: Neutrino, Bitcoin Core, and btcd.

In the current iteration of the Loop software, only off-chain to on-chain exchanges are supported, where the Loop client sends funds off-chain in exchange for the funds back on-chain.

The service can be used in various situations:

  • Acquiring inbound channel liquidity from arbitrary nodes on the Lightning network
  • Depositing funds to a Bitcoin on-chain address without closing active channels
  • Paying to on-chain fallback addresses in the case of insufficient route liquidity

Future iterations of the Loop software will also allow on-chain to off-chain swaps. These swaps can be useful for additional use-cases:

  • Refilling depleted channels with funds from cold-wallets or exchange withdrawals
  • Servicing off-chain Lightning withdrawals using on-chain payments, with no funds in channels required
  • As a failsafe payment method that can be used when channel liquidity along a route is insufficient

Development and Support

The Loop client is current in an early beta state, and offers a simple command line application. Future APIs will be added to support implementation or use of the Loop service.

The GitHub issue tracker can be used to request specific improvements or register and get help with any problems. Community support is also available in the LND Slack .

Loop Out Swap (off -> on-chain)

       loop out 500   
                  |
                  |
                  v
   .-----------------------------.
   | Loop CLI                    |
   | ./cmd/loop                  |
   |                             |
   |                             |
   |       .-------------------. |            .--------------.                   .---------------.
   |       | Loop Client (lib) | |            | LND node     |                   | Bitcoin node  |
   |       | ./                |<-------------|              |-------------------|               |
   |       |                   | |            |              |   on-chain        |               |
   |       |                   |------------->|              |   htlc            |               |
   |       |                   | | off-chain  |              |                   |               |
   |       '-------------------' | htlc       '--------------'                   '---------------'
   '-----------------|-----------'                    |                                  ^
                     |                                |                                  |
                     |                                v                                  |
                     |                              .--.                               .--.               
                     |                          _ -(    )- _                       _ -(    )- _           
                     |                     .--,(            ),--.             .--,(            ),--.      
             initiate|                 _.-(                       )-._    _.-(                       )-._ 
             swap    |                (       LIGHTNING NETWORK       )  (        BITCOIN NETWORK        )
                     |                 '-._(                     )_.-'    '-._(                     )_.-' 
                     |                      '__,(            ),__'             '__,(            ),__'     
                     |                           - ._(__)_. -                       - ._(__)_. -          
                     |                                |                                  ^
                     |                                |                                  |
                     v                                v                                  |
           .--------------------.  off-chain   .--------------.                  .---------------.
           | Loop Server        |  htlc        | LND node     |                  | Bitcoin node  |
           |                    |<-------------|              |                  |               |
           |                    |              |              |   on-chain       |               |
           |                    |              |              |   htlc           |               |
           |                    |--------------|              |----------------->|               |
           |                    |              |              |                  |               |
           '--------------------'              '--------------'                  '---------------'

Setup

LND and the swaplet are using go modules. Make sure that the GO111MODULE env variable is set to on.

In order to execute a swap, LND needs to be rebuilt with sub servers enabled.

LND

  • Checkout branch master

  • make install tags="signrpc walletrpc chainrpc" to build and install lnd with required sub-servers enabled.

  • Make sure there are no macaroons in the lnd dir ~/.lnd/data/chain/bitcoin/mainnet. If there are, lnd has been started before and in that case, it could be that admin.macaroon doesn't contain signer permission. Delete macaroons.db and *.macaroon.

    DO NOT DELETE wallet.db !

  • Start lnd

Loopd

Execute a swap

  • Swaps are executed by a client daemon process. Run:

    loopd

    By default loopd attempts to connect to an lnd instance running on localhost:10009 and reads the macaroon and tls certificate from ~/.lnd. This can be altered using command line flags. See loopd --help.

    loopd only listens on localhost and uses an unencrypted and unauthenticated connection.

  • To initiate a swap, run:

    loop uncharge <amt_msat>

    When the swap is initiated successfully, loopd will see the process through.

  • To query and track the swap status, run loop without arguments.

Resume

When loopd is terminated (or killed) for whatever reason, it will pickup pending swaps after a restart.

Information about pending swaps is stored persistently in the swap database. Its location is ~/.swaplet/<network>/loopent.db.

Multiple simultaneous swaps

It is possible to execute multiple swaps simultaneously.