Minimint is similar to Liquid. But instead of 1 monolithic codebase, it is very modular. So it's more like a "framework for building federations". One module implements "Chaumian ecash", another integrates with Lightning, another is an on-chain wallet.
The on-chain wallet module currently has a public descriptor like this: wsh(sortedmulti(3,02b5b951e6426bc30ba6e7e1cb9cfae06c2658c043614c0fa37a7c4a649a4a3056,026ad269307053e5a11f7ab73209e89791ae31cabfa00b5fa2811bb25633eec6d6,034c71c3f3c2a8ecf07477158be4d9f37b44694b2987f6487c3884857002348fd1,033d94f61ac8bd8fc460a91f7b61e45a16b51d10097200f17fb0998a7483cc8f19))#cc9uhqy3. Each federation server controls one of these keys. When a user deposits into the federation, they
(1) Tweak each pubkey in this descriptor with some random number X.
(2) Deposit to this address associated with this tweaked
(3) Wait for N confirmations
(4a) prove they deposited to the federation by supplying a merkle proof that their transaction was included a block.
(4b) give the tweak X to each federation member so they can spend the deposit.
(5) fetch their ecash tokens - blinded IOUs from the federation.
The primary reason we use tweaking instead of HD derivation is to not require a round of consensus for all federation servers to agree on the address. The primary downside is that backups are harder. Since Minimint is so modular, we will eventually add an HD on-chain wallet module as well. Then users could decide between both options.
Regarding the code:
- Clients generate a tweaked address here.
- Servers spend from tweaked UTXOs here
- This is how we actually apply the tweak
BDK can't do key tweaking!
Here is an EXTREMELY HACKY BDK BRANCH I made at a hackathon to add key tweaking to BDK.
The idea was to pass options to any calls that might derive scripts. These options could tell BDK that we want to do tweaking and include the tweak number itself.
- Address caching
- Syncing
- We'd probably need another on for PSBT signing and other places
This is our progress so far.