Spark, Bitcoin's L2, Collides with the Open Source Culture of Its Own Ecosystem

• In the silence of Lightspark, developer benthecarman built an open alternative.
• Calle added this tool to his own testing environment and invited other wallets to use it.

In the Bitcoin ecosystem, a problem that had been unresolved for days was solved in a handful of hours. On September 4, developer benthecarman built and published the code for a piece of software that Lightspark, the company behind the Spark protocol, had not publicly shared with other teams.

Once that open piece was built, Calle, creator of the Cashu protocol, immediately used it, having been asking for that same tool for several days without receiving a response from Lightspark.

Calle maintains cashu-regtest, a regression testing environment (regtest), which is a local replica of a Bitcoin network where developers test their software without risking real funds. The repository allows simulating Lightning nodes and, recently, also Spark.

Spark is the second layer (L2) network of Bitcoin created by the Lightspark team, based on a statechains design, a scheme that moves the ownership of bitcoins (BTC) off-chain through cryptographic key turns between users and a group of nodes called Spark Operators, which operate under a threshold signature scheme of two out of three.

The code for those Operators, along with the rest of the protocol, is public, in the Spark repository. Lightspark presents that core as open source.

However, to connect to Spark, a wallet does not communicate directly with the Operators, but with an intermediary called Spark Service Provider (SSP), which manages liquidity and connections with Lightning.

According to the frequently asked questions published by Lightspark, anyone can operate their own SSP, but in practice, Lightspark is the only company that operates one, and the code for that service, unlike that of the Operators, is closed and not publicly available.

Calle specifically requested a public testing network, similar to a signet or a testnet, for the SSP, which would allow him to write integration tests with Lightning without relying on a private invitation-only regtest or testing directly on the main Bitcoin network.

For several days, Calle posted direct requests on social media X to Lightspark's account asking for explanations on how to write those tests, without receiving a response that resolved the issue.

The only alternative offered to him, as Calle recounted, was to write an email to get an invitation to that private regtest. After that, Calle highlighted that the company has funding of USD 175 million but did not provide public integration tests.

Later, with the tool rebuilt by benthecarman, he posted a new message directed at Lightspark's account: <>.

Developer benthecarman, who is already collaborating on the development of the Lightning Development Kit (LDK), did not rebuild Spark. He took the already public protocol and built, on ldk-server, an open-source Lightning node, a version of the SSP that he called open-ssp (that is, an open version of the Spark Service Provider).

He also created a small patch that exposed an existing function in the public software of the Operators, necessary for the two pieces to communicate with each other.

benthecarman announced the result on social media X and described it as open source. According to Calle, the work took about twelve hours and the license used was MIT. The new piece was deployed on mutinynet, a public Bitcoin testing network that Lightning developers commonly use to test software without using the main network.

Calle integrated open-ssp into his cashu-regtest environment and invited wallets like Wallet of Satoshi, Cake Wallet, Blink, Breez Technology, and Coinbase to use the environment.

Finally, Calle also announced that he will soon add Bark and Arkade, two other second-layer protocols that were indeed born open source.

The episode raises a question within the Bitcoin ecosystem. Publishing the code of a protocol is not the same as opening the infrastructure with which that protocol is operated and tested in practice, and Spark stands as an example of that difference.