Improve Bitcoin TxBuilder API #157

bmancini55 · 2021-02-11T18:40:03Z

bmancini55
Feb 11, 2021
Maintainer

The existing TxBuilder was built to be functional first to ensure all the required pieces are covered. Because scriptSig and witness data must be constructed after the inputs and outputs are defined (for many SIGHASH_ALL at least), we need defer construction of the scriptSig for inputs until after the inputs and outputs are defined. The existing API is clunky and requires spelunking into the inputs via index.

const txb = new TxBuilder();
txb.addInput("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:0", Sequence.blockDelay(10)); 
txb.addInput("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb:1");
txb.inputs[0].scriptSig = Script.p2pkhUnlock(
     txb.sign(0, Script.p2pkhLock(pubA), privA),
     pubA,
 );

One nuance here is that the signatures must match the final ordering of inputs. The .inputs property goes through the InputSorter and thus matches the final ordering. This means that the index used to assign scriptSig MAY not be in the same index that the input was added under.

So without further ado, here are some attempts to improve this API, added here to a discussion to be maintained in posterity!

scriptSig setter function

This version is a mild improvement over the original and hides txb.inputs[0].scriptSig = <something> behind a function. The benefit here is that inputs and outputs can now be private which would simplify the API a bit.

The downside is that it still has the same input sorting issue as described above.

const b3 = new TxBuilder();
b3.addInput("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:0", Sequence.blockDelay(10)); // prettier-ignore
b3.addInput("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb:1");
b3.setScriptSig(0, Script.p2pkhUnlock(
    b1.sign(0, Script.p2pkhLock(pubA), privA),
    pubA,
));

scriptSig as Lambda

This version adds a lambda that takes an index and returns a Script, (index: number) => Script.

This method would allow us to couple the logic of the scriptSig with the rest of the input logic. Because it's a lambda, it also allows deferment of scriptSig building until some finalization function is called, .serialize() or .toTx() and pass in the final sorted index to the signature creation.

const txb = new TxBuilder();
txb.addInput(
    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:0",
    Sequence.blockDelay(10),
    idx => Script.p2pkhUnlock(
        sut.sign(idx, Script.p2pkhLock(pubA), privA),
        pubA
    ),
);
txb.addInput(
    "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb:1",
    undefined,
    idx => Script.p2pkhUnlock(
        sut.sign(idx, Script.p2pkhLock(pubB), privB),
        pubB
    ),
);

Fluent builder using lamba for scoping

This uses a fluent builder pattern. The nested builders uses lambda's to control scoping of the object being built. This can internally resolve the input sorting/signature issue.

new TxBuilder()
    .withInput(i => i
        .withOutPoint("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:0")
        .withBlockSequence(10)
        .withScriptSig(s => s
            .withSig(privA, Script.p2pkhLock(pubA))
            .withData(pubA)),
    )
    .withInput(i => i
        .withOutPoint("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb:1")
        .withScriptSig(s => s
            .withSig(privB, Script.p2pkhLock(pubB))
            .withData(pubB)),
    );

Fluent with scope finalizer

This example also uses a fluent builder. Instead of using lambdas to control scope we use a scope finalizer that kicks us back to the parent. This can internally resolve the input sorting/signature issue.

new TxBuilder()
    .buildInput()
        .withOutPoint("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:0")
        .withBlockSequence(10)
        .buildScriptSig()
            .withSig(privA, Script.p2pkhLock(pubA))
            .withData(pubA)
            .done()
        .done()
    .buildInput()
        .withOutPoint("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb:1")
        .buildScriptSig()
            .withSig(privB, Script.p2pkhLock(pubB))
            .withData(pubB)
            .done()
        .done();

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Improve Bitcoin TxBuilder API #157

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Improve Bitcoin TxBuilder API #157

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bmancini55 Feb 11, 2021 Maintainer

scriptSig setter function

scriptSig as Lambda

Fluent builder using lamba for scoping

Fluent with scope finalizer

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bmancini55
Feb 11, 2021
Maintainer