Bitcoin Miniscript

This project is a JavaScript implementation of Bitcoin Miniscript, a high-level language for describing Bitcoin spending conditions.

It includes a novel Miniscript Satisfier for generating explicit script witnesses that are decoupled from the tx signer, as well as a transpilation of Peter Wuille's C++ code for compiling spending policies into Miniscript and Bitcoin scripts.

Features

• Compile Policies into Miniscript and Bitcoin scripts.

• A Miniscript Satisfier that discards malleable solutions.

• The Miniscript Satisfier is able to generate explicit script witnesses from Miniscript expressions using variables, such as pk(key).

  For example, Miniscript and_v(v:pk(key),after(10)) can be satisfied with [{ asm: '<sig(key)>', nLockTime: 10 }].

• The ability to generate different satisfactions depending on the presence of unknowns (or complimentary knowns).

  For example, Miniscript c:and_v(or_c(pk(key1),v:ripemd160(H)),pk_k(key2)) can be satisfied with: [{ asm: '<sig(key2)> <ripemd160_preimage(H)> 0' }].

  However, if unknowns: ['<ripemd160_preimage(H)>'] is set, then the Miniscript can be satisfied with: [{ asm: '<sig(key2)> <sig(key1)>' }] because this solution can no longer be considered malleable, given then assumption that an attacker does not have access to the preimage.

• Thoroughly tested.

Installation

To install the package, use npm:

npm install @bitcoinerlab/miniscript

Usage

You can test the examples in this section using the online playground demo available at https://bitcoinerlab.com/modules/miniscript.

Compiling Policies into Miniscript and Bitcoin script

To compile a Policy into a Miniscript and Bitcoin ASM, you can use the compilePolicy function:

const { compilePolicy } = require('@bitcoinerlab/miniscript');

const policy = 'or(and(pk(A),older(8640)),pk(B))';

const { miniscript, asm, issane } = compilePolicy(policy);

issane is a boolean that indicates whether the Miniscript is valid and follows the consensus and standardness rules for Bitcoin scripts. A sane Miniscript should have non-malleable solutions, not mix different timelock units on a single branch of the script, and not contain duplicate keys. In other words, it should be a well-formed and standards-compliant script that can be safely used in transactions.

Compiling Miniscript into Bitcoin script

To compile a Miniscript into Bitcoin ASM you can use the compileMiniscript function:

const { compileMiniscript } = require('@bitcoinerlab/miniscript');

const miniscript = 'and_v(v:pk(key),or_b(l:after(100),al:after(200)))';

const { asm, issane } = compileMiniscript(miniscript);

Generating explicit script witnesses

To generate explicit script witnesses from a Miniscript, you can use the satisfier function:

const { satisfier } = require('@bitcoinerlab/miniscript');

const miniscript =
  'c:or_i(andor(c:pk_h(key1),pk_h(key2),pk_h(key3)),pk_k(key4))';

const { nonMalleableSats, malleableSats } = satisfier(miniscript);

satisfier makes sure that output satisfactions are non-malleable and that the miniscript is sane by itself and it returns an object with keys:

• nonMalleableSats: an array of objects representing good, non-malleable witnesses.
• malleableSats: an array of objects representing malleable witnesses that should not be used.

In the example above nonMalleableSats is:

nonMalleableSats: [
  {asm: "<sig(key4)> 0"}
  {asm: "<sig(key3)> <key3> 0 <key1> 1"}
  {asm: "<sig(key2)> <key2> <sig(key1)> <key1> 1"}
]

Where satisfactions are ordered in ascending Weight Unit size.

In addition, unknowns can be set with the pieces of information the user does not have, f.ex., <sig(key)> or <ripemd160_preimage(H)>:

const { satisfier } = require('@bitcoinerlab/miniscript');

const miniscript =
  'c:or_i(andor(c:pk_h(key1),pk_h(key2),pk_h(key3)),pk_k(key4))';
const unknowns = ['<sig(key1)>', '<sig(key2)>'];

const { nonMalleableSats, malleableSats, unknownSats } = satisfier(
  miniscript,
  { unknowns }
);

When passing unknowns, satisfier returns an additional object: { unknownSats } with an array of objects representing satisfactions that containt some of the unknown pieces of information:

nonMalleableSats: [
  {asm: "<sig(key4)> 0"}
  {asm: "<sig(key3)> <key3> 0 <key1> 1"}
]
unknownSats: [ {asm: "<sig(key2)> <key2> <sig(key1)> <key1> 1"} ]

Instead of unknowns, the user has the option to provide the complementary argument knowns: satisfier( miniscript, { knowns }). This argument corresponds to the only pieces of information that are known. For instance, in the example above, knowns would be ['<sig(key3)>', '<sig(key4)>']. It's important to note that either knowns or unknowns must be provided, but not both. If neither argument is provided, it's assumed that all signatures and preimages are known.

The objects returned in the nonMalleableSats, malleableSats and unknownSats arrays consist of the following properties:

• asm: a string with the script witness.
• nSequence: an integer representing the nSequence value, if needed.
• nLockTime: an integer representing the nLockTime value, if needed.

Authors and Contributors

The project was initially developed and is currently maintained by Jose-Luis Landabaso. Contributions and help from other developers are welcome.

Here are some resources to help you get started with contributing:

Building from source

To download the source code and build the project, follow these steps:

1. Clone the repository:

git clone https://github.com/bitcoinerlab/miniscript.git

2. Install the dependencies:

npm install

3. Make sure you have the em++ compiler in your PATH.

4. Run the Makefile:

make

This will download and build Wuille's sources and generate the necessary Javascript files for the compilers.

5. Build the project:

npm run build

This will build the project and generate the necessary files in the dist directory.

Documentation

To generate the programmers's documentation, which describes the library's programming interface, use the following command:

npm run docs

This will generate the documentation in the docs directory.

Testing

Before committing any code, make sure it passes all tests by running:

npm run tests

License

This project is licensed under the MIT License.