feat(wasm-utxo): expose requiresPrevTxForP2sh prevTx-inclusion policy

Add a pure-JS prevTx-inclusion predicate to @bitgo/wasm-utxo so all
callers share one source of truth for whether a p2sh PSBT input needs
non_witness_utxo (full prevTx) or can be signed from witness_utxo-only.

requiresPrevTxForP2sh(coinName) answers only the coin-level question
for an input the caller has already determined is p2sh (non-segwit) and
whose tx format includes prevTx (e.g. "psbt", not "psbt-lite"). It
returns false for value-committing coins whose sighash commits the
input amount, making prevTx cryptographically pointless for signing
p2sh inputs:

- Zcash (zec/tzec): ZIP-243 transparent sighash commits the amount.
  Including prevTx also crashes wasm-utxo, whose consensus::deserialize
  rejects Zcash overwintered transactions.
- BCH family (bch/bcha/eCash, bsv, btg + testnets): replay-protected
  BIP-143 sighash (SIGHASH_FORKID, the default for the whole family)
  commits the 8-byte value as preimage item #6.

The value-committing mainnets {zec, bch, bcha, bsv, btg} are matched
via a switch on getMainnet(coinName), so testnets are covered. The
module is pure JS — no WASM initialization — so the predicate is cheap
to evaluate without loading wasm-utxo.

Exported both namespaced (fixedScriptWallet.requiresPrevTxForP2sh) and
as a top-level named export (alongside CoinName/getMainnet).

Refs: T1-3654

Author: OttoAllmendinger

packages/wasm-utxo/js/fixedScriptWallet/index.ts

@@ -14,6 +14,9 @@ export {
 } from "./scriptType.js";
 export { ChainCode, chainCodes, assertChainCode, type Scope } from "./chains.js";
 
+// Previous-transaction inclusion policy (pure JS, no WASM init)
+export { requiresPrevTxForP2sh } from "./prevTx.js";
+
 // Bitcoin-like PSBT (for all non-Zcash networks)
 export {
   BitGoPsbt,

packages/wasm-utxo/js/fixedScriptWallet/prevTx.ts

@@ -0,0 +1,48 @@
+/**
+ * Previous-transaction inclusion policy for fixed-script wallet PSBT inputs.
+ *
+ * Decides whether a p2sh input requires the full previous transaction
+ * (PSBT_IN_NON_WITNESS_UTXO) or can be signed from witness_utxo-only.
+ *
+ * This is a pure-JS module (no WASM initialization) so callers can evaluate
+ * the policy cheaply without loading the wasm-utxo module.
+ */
+import { type CoinName, getMainnet } from "../coinName.js";
+
+/**
+ * Whether a p2sh input requires the full previous transaction
+ * (PSBT_IN_NON_WITNESS_UTXO). Callers are expected to have already
+ * confirmed the input is p2sh (non-segwit) and that the tx format
+ * includes prevTx (e.g. "psbt", not "psbt-lite"); this predicate only
+ * answers the coin-level question.
+ *
+ * Returns false for value-committing coins whose sighash commits the
+ * input amount, making `non_witness_utxo` (full prevTx) cryptographically
+ * pointless for signing p2sh inputs — `witness_utxo` (value +
+ * scriptPubKey) suffices:
+ *
+ * - Zcash (`zec`/`tzec`): ZIP-243 transparent sighash commits the amount.
+ *   Including prevTx also crashes wasm-utxo, whose consensus::deserialize
+ *   rejects Zcash overwintered transactions.
+ * - BCH family (`bch`/`bcha`/`bsv`/`btg` + testnets): replay-protected
+ *   BIP-143 sighash (SIGHASH_FORKID, the default for the whole family)
+ *   commits the 8-byte value as preimage item #6. eCash is `bcha`/`tbcha`.
+ *   For the BCH family, skipping prevTx is an optimization (no DB fetch)
+ *   plus defense-in-depth, with the same fee-validation risk that the
+ *   existing `psbt-lite` path already accepts for all coins.
+ *
+ * Testnets are normalized via `getMainnet` before the switch. True
+ * otherwise.
+ */
+export function requiresPrevTxForP2sh(coinName: CoinName): boolean {
+  switch (getMainnet(coinName)) {
+    case "zec": // Zcash (ZIP-243)
+    case "bch": // Bitcoin Cash (BIP-143/FORKID)
+    case "bcha": // eCash (BIP-143/FORKID)
+    case "bsv": // Bitcoin SV (BIP-143/FORKID)
+    case "btg": // Bitcoin Gold (BIP-143/FORKID)
+      return false;
+    default:
+      return true;
+  }
+}

packages/wasm-utxo/js/index.ts

@@ -28,6 +28,7 @@ export function getWasmUtxoVersion(): WasmUtxoVersionInfo {
 }
 
 export { type CoinName, getMainnet, isMainnet, isTestnet, isCoinName } from "./coinName.js";
+export { requiresPrevTxForP2sh } from "./fixedScriptWallet/prevTx.js";
 export type { Triple } from "./triple.js";
 export type { AddressFormat } from "./address.js";
 export type { TapLeafScript, PreparedInscriptionRevealData } from "./inscriptions.js";

packages/wasm-utxo/test/fixedScript/prevTx.ts

@@ -0,0 +1,80 @@
+import assert from "node:assert";
+
+import { requiresPrevTxForP2sh, type CoinName } from "../../js/index.js";
+
+describe("prevTx policy", function () {
+  describe("requiresPrevTxForP2sh", function () {
+    // Callers gate on script type (p2sh) and tx format themselves; this
+    // predicate only answers the coin-level question for a p2sh input.
+    const cases: { coin: CoinName; expected: boolean; note: string }[] = [
+      // Value-committing coins: skip prevTx (sighash commits the amount)
+      {
+        coin: "zec",
+        expected: false,
+        note: "zec p2sh — skip prevTx (ZIP-243, the fix)",
+      },
+      {
+        coin: "tzec",
+        expected: false,
+        note: "tzec p2sh — skip prevTx (ZIP-243, the fix)",
+      },
+      {
+        coin: "bch",
+        expected: false,
+        note: "bch p2sh — skip prevTx (FORKID commits value)",
+      },
+      {
+        coin: "tbch",
+        expected: false,
+        note: "tbch p2sh — skip prevTx (FORKID commits value)",
+      },
+      {
+        coin: "bcha",
+        expected: false,
+        note: "bcha (eCash) p2sh — skip prevTx (FORKID commits value)",
+      },
+      {
+        coin: "bsv",
+        expected: false,
+        note: "bsv p2sh — skip prevTx (FORKID commits value)",
+      },
+      {
+        coin: "btg",
+        expected: false,
+        note: "btg p2sh — skip prevTx (FORKID commits value)",
+      },
+      // Non-value-committing coins: prevTx still required (unchanged)
+      {
+        coin: "btc",
+        expected: true,
+        note: "btc p2sh — unchanged (needs prevTx)",
+      },
+      {
+        coin: "tbtc",
+        expected: true,
+        note: "tbtc p2sh — unchanged (needs prevTx)",
+      },
+      {
+        coin: "ltc",
+        expected: true,
+        note: "ltc p2sh — unchanged (needs prevTx)",
+      },
+      {
+        coin: "doge",
+        expected: true,
+        note: "doge p2sh — unchanged (needs prevTx)",
+      },
+      {
+        coin: "dash",
+        expected: true,
+        note: "dash p2sh — unchanged (needs prevTx)",
+      },
+    ];
+
+    for (const c of cases) {
+      it(`returns ${c.expected} for ${c.note}`, function () {
+        assert.strictEqual(requiresPrevTxForP2sh(c.coin), c.expected);
+      });
+    }
+  });
+});