policy: introduce a helper to detect whether a transaction spends Segwit outputs

We will use this helper in later commits to detect witness stripping without having to execute every input Script three times in a row.
2025-08-27 19:39:02 +02:00 · 2025-08-04 13:11:33 -04:00
parent eb073209db
commit 2907b58834
3 changed files with 196 additions and 0 deletions
--- a/src/policy/policy.cpp
+++ b/src/policy/policy.cpp
@@ -337,6 +337,42 @@ bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
    return true;
 }

+bool SpendsNonAnchorWitnessProg(const CTransaction& tx, const CCoinsViewCache& prevouts)
+{
+    if (tx.IsCoinBase()) {
+        return false;
+    }
+
+    int version;
+    std::vector<uint8_t> program;
+    for (const auto& txin: tx.vin) {
+        const auto& prev_spk{prevouts.AccessCoin(txin.prevout).out.scriptPubKey};
+
+        // Note this includes not-yet-defined witness programs.
+        if (prev_spk.IsWitnessProgram(version, program) && !prev_spk.IsPayToAnchor(version, program)) {
+            return true;
+        }
+
+        // For P2SH extract the redeem script and check if it spends a non-Taproot witness program. Note
+        // this is fine to call EvalScript (as done in AreInputsStandard/IsWitnessStandard) because this
+        // function is only ever called after IsStandardTx, which checks the scriptsig is pushonly.
+        if (prev_spk.IsPayToScriptHash()) {
+            // If EvalScript fails or results in an empty stack, the transaction is invalid by consensus.
+            std::vector <std::vector<uint8_t>> stack;
+            if (!EvalScript(stack, txin.scriptSig, SCRIPT_VERIFY_NONE, BaseSignatureChecker{}, SigVersion::BASE)
+                || stack.empty()) {
+                continue;
+            }
+            const CScript redeem_script{stack.back().begin(), stack.back().end()};
+            if (redeem_script.IsWitnessProgram(version, program)) {
+                return true;
+            }
+        }
+    }
+
+    return false;
+}
+
 int64_t GetVirtualTransactionSize(int64_t nWeight, int64_t nSigOpCost, unsigned int bytes_per_sigop)
 {
    return (std::max(nWeight, nSigOpCost * bytes_per_sigop) + WITNESS_SCALE_FACTOR - 1) / WITNESS_SCALE_FACTOR;
--- a/src/policy/policy.h
+++ b/src/policy/policy.h
@@ -166,6 +166,11 @@ bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs)
 * Also enforce a maximum stack item size limit and no annexes for tapscript spends.
 */
 bool IsWitnessStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs);
+/**
+ * Check whether this transaction spends any witness program but P2A, including not-yet-defined ones.
+ * May return `false` early for consensus-invalid transactions.
+ */
+bool SpendsNonAnchorWitnessProg(const CTransaction& tx, const CCoinsViewCache& prevouts);

 /** Compute the virtual transaction size (weight reinterpreted as bytes). */
 int64_t GetVirtualTransactionSize(int64_t nWeight, int64_t nSigOpCost, unsigned int bytes_per_sigop);
--- a/src/test/transaction_tests.cpp
+++ b/src/test/transaction_tests.cpp
@@ -1149,4 +1149,159 @@ BOOST_AUTO_TEST_CASE(max_standard_legacy_sigops)
    BOOST_CHECK(!::AreInputsStandard(CTransaction(tx_max_sigops), coins));
 }

+/** Sanity check the return value of SpendsNonAnchorWitnessProg for various output types. */
+BOOST_AUTO_TEST_CASE(spends_witness_prog)
+{
+    CCoinsView coins_dummy;
+    CCoinsViewCache coins(&coins_dummy);
+    CKey key;
+    key.MakeNewKey(true);
+    const CPubKey pubkey{key.GetPubKey()};
+    CMutableTransaction tx_create{}, tx_spend{};
+    tx_create.vout.emplace_back(0, CScript{});
+    tx_spend.vin.emplace_back(Txid{}, 0);
+    std::vector<std::vector<uint8_t>> sol_dummy;
+
+    // CNoDestination, PubKeyDestination, PKHash, ScriptHash, WitnessV0ScriptHash, WitnessV0KeyHash,
+    // WitnessV1Taproot, PayToAnchor, WitnessUnknown.
+    static_assert(std::variant_size_v<CTxDestination> == 9);
+
+    // Go through all defined output types and sanity check SpendsNonAnchorWitnessProg.
+
+    // P2PK
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(PubKeyDestination{pubkey});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::PUBKEY);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2PKH
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(PKHash{pubkey});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::PUBKEYHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2SH
+    auto redeem_script{CScript{} << OP_1 << OP_CHECKSIG};
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(ScriptHash{redeem_script});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::SCRIPTHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    tx_spend.vin[0].scriptSig = CScript{} << OP_0 << ToByteVector(redeem_script);
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+    tx_spend.vin[0].scriptSig.clear();
+
+    // native P2WSH
+    const auto witness_script{CScript{} << OP_12 << OP_HASH160 << OP_DUP << OP_EQUAL};
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(WitnessV0ScriptHash{witness_script});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::WITNESS_V0_SCRIPTHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2SH-wrapped P2WSH
+    redeem_script = tx_create.vout[0].scriptPubKey;
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(ScriptHash(redeem_script));
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::SCRIPTHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    tx_spend.vin[0].scriptSig = CScript{} << ToByteVector(redeem_script);
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+    tx_spend.vin[0].scriptSig.clear();
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // native P2WPKH
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(WitnessV0KeyHash{pubkey});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::WITNESS_V0_KEYHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2SH-wrapped P2WPKH
+    redeem_script = tx_create.vout[0].scriptPubKey;
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(ScriptHash(redeem_script));
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::SCRIPTHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    tx_spend.vin[0].scriptSig = CScript{} << ToByteVector(redeem_script);
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+    tx_spend.vin[0].scriptSig.clear();
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2TR
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(WitnessV1Taproot{XOnlyPubKey{pubkey}});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::WITNESS_V1_TAPROOT);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2SH-wrapped P2TR (undefined, non-standard)
+    redeem_script = tx_create.vout[0].scriptPubKey;
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(ScriptHash(redeem_script));
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::SCRIPTHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    tx_spend.vin[0].scriptSig = CScript{} << ToByteVector(redeem_script);
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+    tx_spend.vin[0].scriptSig.clear();
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2A
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(PayToAnchor{});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::ANCHOR);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2SH-wrapped P2A (undefined, non-standard)
+    redeem_script = tx_create.vout[0].scriptPubKey;
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(ScriptHash(redeem_script));
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::SCRIPTHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    tx_spend.vin[0].scriptSig = CScript{} << ToByteVector(redeem_script);
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+    tx_spend.vin[0].scriptSig.clear();
+
+    // Undefined version 1 witness program
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(WitnessUnknown{1, {0x42, 0x42}});
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::WITNESS_UNKNOWN);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // P2SH-wrapped undefined version 1 witness program
+    redeem_script = tx_create.vout[0].scriptPubKey;
+    tx_create.vout[0].scriptPubKey = GetScriptForDestination(ScriptHash(redeem_script));
+    BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::SCRIPTHASH);
+    tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+    tx_spend.vin[0].scriptSig = CScript{} << ToByteVector(redeem_script);
+    AddCoins(coins, CTransaction{tx_create}, 0, false);
+    BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+    tx_spend.vin[0].scriptSig.clear();
+    BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+    // Various undefined version >1 32-byte witness programs.
+    const auto program{ToByteVector(XOnlyPubKey{pubkey})};
+    for (int i{2}; i <= 16; ++i) {
+        tx_create.vout[0].scriptPubKey = GetScriptForDestination(WitnessUnknown{i, program});
+        BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::WITNESS_UNKNOWN);
+        tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+        AddCoins(coins, CTransaction{tx_create}, 0, false);
+        BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+
+        // It's also detected within P2SH.
+        redeem_script = tx_create.vout[0].scriptPubKey;
+        tx_create.vout[0].scriptPubKey = GetScriptForDestination(ScriptHash(redeem_script));
+        BOOST_CHECK_EQUAL(Solver(tx_create.vout[0].scriptPubKey, sol_dummy), TxoutType::SCRIPTHASH);
+        tx_spend.vin[0].prevout.hash = tx_create.GetHash();
+        tx_spend.vin[0].scriptSig = CScript{} << ToByteVector(redeem_script);
+        AddCoins(coins, CTransaction{tx_create}, 0, false);
+        BOOST_CHECK(::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+        tx_spend.vin[0].scriptSig.clear();
+        BOOST_CHECK(!::SpendsNonAnchorWitnessProg(CTransaction{tx_spend}, coins));
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()