We initially showcased Revault using a fixed set of participants. We quickly ran into issues as we tried to generalize it to more participants in a production setting.
- Actually are we sure the script we used in the demo is secure? Is it possible to spend it in all the ways it’s advertized to be? Is there no other way of spending it than how it’s advertized?
- Even if it is, how can we generalize it to various number of participants and keep it secure? How can we apply some optimizations and make sure the resulting script has the same semantics?
- In addition, Revault is using pre-signed transactions (to enforce spending policies). How can we know in advance the budget to allocate for fee-bumping given the configuration of the script? How can we make sure any transaction spending those scripts will pass the most common standardness checks?
- Finally, even assuming our scripts correspond to the intended semantics and are always spendable, how can we concretely spend them? As in how can we produce a satisfying witness for (“sign for”) each and every possible configuration? How can we make hardware signing devices compatible with our scripts?
These questions would have been a show stopper if it was not for miniscript. Two guys in a garage are not going to write a software that creates some script on the fly, hope for the best and on top of that call it a security enhancing Bitcoin wallet. We wanted to start a company around the development of Revault, but we wouldn’t get funding without providing some reasonable assurance to an investor we could bring a safe product to market. And we wouldn’t be able to solve all these engineering problems without funding.
Enters miniscript, “a language for writing (a subset of) Bitcoin Scripts in a structured way, enabling analysis, composition, generic signing and more. […] It has a structure that allows composition. It is very easy to statically analyze for various properties (spending conditions, correctness, security properties, malleability, …).” This is exactly what we needed. Armed with this powerful tool, we could give better guarantees  to our investors, raise funds, and start the development of Revault.
At the time, miniscript was still far from being a turnkey solution for any Bitcoin application developer to use. (If you are a new Bitcoin dev reading this after the year 2023, yes, there was a time where we used to write Bitcoin scripts BY HAND.) We had to integrate miniscript in Bitcoin Core (see PRs #24147, #24148 and #24149), which we used as the backend of the Revault wallet, and convince signing devices manufacturers to implement it in their firmware. The latter part would turn out to be the most difficult.
It was a chicken-and-egg problem: incentives were low for manufacturers to implement miniscript with no demand from users. And we couldn’t release Revault without signing device support. Fortunately this cycle was eventually broken by Stepan Snigirev in March 2021 by bringing support for miniscript descriptors to the Specter DIY. The Specter DIY was however for a long time disclaimed as being merely a “functional prototype”, and Salvatore Ingala brought miniscript to a production-ready signing device for the first time in 2022 with the new Bitcoin app for the Ledger Nano S(+). The app was released in January 2023, allowing us to publish the Liana wallet with support for the most popular signing device.
One last development is left to wrap up our miniscript journey. Liana is a Bitcoin wallet focused on recovery options. It lets one specify some timelocked recovery conditions (for instance a third-party recovery key that cannot normally spend the funds, or a decaying/expanding multisig). Miniscript was initially only available for P2WSH scripts. Almost 2 years after taproot activated, it’s unfortunate that you would have to publish your recovery spending paths onchain every time you spend. To this end, we have been working to port miniscript to tapscript (see here and here).
The future is bright. With most signing devices either having implemented or in the process of implementing miniscript support (for instance recently Bitbox and Coldcard), along with taproot and miniscript native frameworks being polished, contracting on Bitcoin with safe primitives is more accessible than ever.
It’s interesting to note how the funding of Open Source tools and frameworks lower the barrier to entry for innovative companies to compete and, more generally, projects to be implemented. This trend accelerating in the past years can make us hopeful about the future of the space.
 There was still risk, of course. But at least we were confident we could get past the onchain part. The offchain one would prove to be (as expected) more challenging.