feat(psbt): add signWithXprv method

Add PSBT signing functionality using private extended keys (xprv).
Supports individual key signing and returns a map of signed pubkeys.

Issue: BTC-1845

Author: OttoAllmendinger

packages/wasm-miniscript/js/index.ts

@@ -8,6 +8,10 @@ export type DescriptorPkType = "derivable" | "definite" | "string";
 
 export type ScriptContext = "tap" | "segwitv0" | "legacy";
 
+export type SignPsbtResult = {
+  [inputIndex: number]: [pubkey: string][];
+};
+
 declare module "./wasm/wasm_miniscript" {
   interface WrapDescriptor {
     /** These are not the same types of nodes as in the ast module */
@@ -27,6 +31,10 @@ declare module "./wasm/wasm_miniscript" {
     function fromString(miniscript: string, ctx: ScriptContext): WrapMiniscript;
     function fromBitcoinScript(script: Uint8Array, ctx: ScriptContext): WrapMiniscript;
   }
+
+  interface WrapPsbt {
+    signWithXprv(this: WrapPsbt, xprv: string): SignPsbtResult;
+  }
 }
 
 export { WrapDescriptor as Descriptor } from "./wasm/wasm_miniscript";

packages/wasm-miniscript/src/psbt.rs

@@ -1,10 +1,12 @@
+use std::str::FromStr;
 use crate::descriptor::WrapDescriptorEnum;
 use crate::WrapDescriptor;
 use miniscript::bitcoin::secp256k1::Secp256k1;
 use miniscript::bitcoin::Psbt;
 use miniscript::psbt::PsbtExt;
 use wasm_bindgen::prelude::wasm_bindgen;
-use wasm_bindgen::JsError;
+use wasm_bindgen::{JsError, JsValue};
+use crate::try_into_js_value::TryIntoJsValue;
 
 #[wasm_bindgen]
 pub struct WrapPsbt(Psbt);
@@ -63,6 +65,16 @@ impl WrapPsbt {
         }
     }
 
+    #[wasm_bindgen(js_name = signWithXprv)]
+    pub fn sign_with_xprv(&mut self, xprv: String) -> Result<JsValue, JsError> {
+        let key = miniscript::bitcoin::bip32::Xpriv::from_str(&xprv)
+            .map_err(|_| JsError::new("Invalid xprv"))?;
+        self.0
+            .sign(&key, &Secp256k1::new())
+            .map_err(|(_, errors)| JsError::new(&format!("{} errors: {:?}", errors.len(), errors)))
+            .and_then(|r| r.try_to_js_value())
+    }
+
     #[wasm_bindgen(js_name = finalize)]
     pub fn finalize_mut(&mut self) -> Result<(), JsError> {
         self.0

packages/wasm-miniscript/src/try_into_js_value.rs

@@ -7,6 +7,7 @@ use miniscript::{
     MiniscriptKey, RelLockTime, ScriptContext, Terminal, Threshold,
 };
 use std::sync::Arc;
+use miniscript::bitcoin::psbt::{SigningKeys, SigningKeysMap};
 use wasm_bindgen::{JsError, JsValue};
 
 pub(crate) trait TryIntoJsValue {
@@ -256,3 +257,28 @@ impl TryIntoJsValue for DescriptorType {
         Ok(JsValue::from_str(&str_from_enum))
     }
 }
+
+
+impl TryIntoJsValue for SigningKeys {
+    fn try_to_js_value(&self) -> Result<JsValue, JsError> {
+        match self {
+            SigningKeys::Ecdsa(v) => {
+                js_obj!("Ecdsa" => v)
+            }
+            SigningKeys::Schnorr(v) => {
+                js_obj!("Schnorr" => v)
+            }
+        }
+    }
+}
+
+impl TryIntoJsValue for SigningKeysMap {
+    fn try_to_js_value(&self) -> Result<JsValue, JsError> {
+        let obj = js_sys::Object::new();
+        for (key, value) in self.iter() {
+            js_sys::Reflect::set(&obj, &key.to_string().into(), &value.try_to_js_value()?.into())
+                .map_err(|_| JsError::new("Failed to set object property"))?;
+        }
+        Ok(obj.into())
+    }
+}

packages/wasm-miniscript/test/psbt.ts

@@ -5,6 +5,7 @@ import { Descriptor, Psbt } from "../js";
 
 import { getDescriptorForScriptType } from "./descriptorUtil";
 import { assertEqualPsbt, toUtxoPsbt, toWrappedPsbt, updateInputWithDescriptor } from "./psbt.util";
+import { getKey } from "@bitgo/utxo-lib/dist/src/testutil";
 
 const rootWalletKeys = new utxolib.bitgo.RootWalletKeys(utxolib.testutil.getKeyTriple("wasm"));
 
@@ -14,16 +15,6 @@ function assertEqualBuffer(a: Buffer | Uint8Array, b: Buffer | Uint8Array, messa
 
 const fixtures = getPsbtFixtures(rootWalletKeys);
 
-function getWasmDescriptor(
-  scriptType: utxolib.bitgo.outputScripts.ScriptType2Of3,
-  scope: "internal" | "external",
-) {
-  return Descriptor.fromString(
-    getDescriptorForScriptType(rootWalletKeys, scriptType, scope),
-    "derivable",
-  );
-}
-
 function describeUpdateInputWithDescriptor(
   psbt: utxolib.bitgo.UtxoPsbt,
   scriptType: utxolib.bitgo.outputScripts.ScriptType2Of3,
@@ -40,12 +31,21 @@ function describeUpdateInputWithDescriptor(
   const index = 0;
   const descriptor = Descriptor.fromString(descriptorStr, "derivable");
 
+  function getWrappedPsbt() {
+    return toWrappedPsbt(psbt);
+  }
+
+  function getWrappedPsbtWithDescriptorInfo(): Psbt {
+    const wrappedPsbt = getWrappedPsbt();
+    const descriptorAtDerivation = descriptor.atDerivationIndex(index);
+    wrappedPsbt.updateInputWithDescriptor(0, descriptorAtDerivation);
+    wrappedPsbt.updateOutputWithDescriptor(0, descriptorAtDerivation);
+    return wrappedPsbt;
+  }
+
   describe("Wrapped PSBT updateInputWithDescriptor", function () {
     it("should update the input with the descriptor", function () {
-      const wrappedPsbt = toWrappedPsbt(psbt);
-      const descriptorAtDerivation = descriptor.atDerivationIndex(index);
-      wrappedPsbt.updateInputWithDescriptor(0, descriptorAtDerivation);
-      wrappedPsbt.updateOutputWithDescriptor(0, descriptorAtDerivation);
+      const wrappedPsbt = getWrappedPsbtWithDescriptorInfo();
       const updatedPsbt = toUtxoPsbt(wrappedPsbt);
       assertEqualPsbt(updatedPsbt, getFixtureAtStage("unsigned").psbt);
       updatedPsbt.signAllInputsHD(rootWalletKeys.triple[0]);
@@ -85,6 +85,44 @@ function describeUpdateInputWithDescriptor(
       );
     });
   });
+
+  describe("psbt signWithXprv", function () {
+    type KeyName = utxolib.bitgo.KeyName | "unrelated";
+    function signWithKey(keys: KeyName[], { checkFinalized = false } = {}) {
+      it(`signs the input with keys ${keys}`, function () {
+        const psbt = getWrappedPsbtWithDescriptorInfo();
+        keys.forEach((keyName) => {
+          const key = keyName === "unrelated" ? getKey(keyName) : rootWalletKeys[keyName];
+          const derivationPaths = toUtxoPsbt(psbt).data.inputs[0].bip32Derivation.map(
+            (d) => d.path,
+          );
+          assert.ok(derivationPaths.every((p) => p === derivationPaths[0]));
+          const derived = key.derivePath(derivationPaths[0]);
+          assert.deepStrictEqual(psbt.signWithXprv(key.toBase58()), {
+            // map: input index -> pubkey array
+            0: { Ecdsa: keyName === "unrelated" ? [] : [derived.publicKey.toString("hex")] },
+          });
+        });
+
+        if (checkFinalized) {
+          psbt.finalize();
+          assertEqualBuffer(
+            toUtxoPsbt(psbt).extractTransaction().toBuffer(),
+            getFixtureAtStage("fullsigned")
+              .psbt.finalizeAllInputs()
+              .extractTransaction()
+              .toBuffer(),
+          );
+        }
+      });
+    }
+
+    signWithKey(["user"]);
+    signWithKey(["backup"]);
+    signWithKey(["bitgo"]);
+    signWithKey(["unrelated"]);
+    signWithKey(["user", "bitgo"], { checkFinalized: true });
+  });
 }
 
 fixtures.forEach(({ psbt, scriptType, stage }) => {