policy: Add OP_1 <0x4e73> as a standard output type

These outputs are called anchors, and allow
key-less anchor spends which are vsize-minimized
versus keyed anchors which require larger outputs
when creating and inputs when spending.

src/addresstype.cpp

@@ -87,6 +87,10 @@ bool ExtractDestination(const CScript& scriptPubKey, CTxDestination& addressRet)
         addressRet = tap;
         return true;
     }
+    case TxoutType::ANCHOR: {
+        addressRet = PayToAnchor();
+        return true;
+    }
     case TxoutType::WITNESS_UNKNOWN: {
         addressRet = WitnessUnknown{vSolutions[0][0], vSolutions[1]};
         return true;

src/addresstype.h

@@ -9,6 +9,7 @@
 #include <pubkey.h>
 #include <script/script.h>
 #include <uint256.h>
+#include <util/check.h>
 #include <util/hash_type.h>
 
 #include <algorithm>
@@ -116,6 +117,13 @@ public:
     }
 };
 
+struct PayToAnchor : public WitnessUnknown
+{
+    PayToAnchor() : WitnessUnknown(1, {0x4e, 0x73}) {
+        Assume(CScript::IsPayToAnchor(1, {0x4e, 0x73}));
+    };
+};
+
 /**
  * A txout script categorized into standard templates.
  *  * CNoDestination: Optionally a script, no corresponding address.
@@ -125,10 +133,11 @@ public:
  *  * WitnessV0ScriptHash: TxoutType::WITNESS_V0_SCRIPTHASH destination (P2WSH address)
  *  * WitnessV0KeyHash: TxoutType::WITNESS_V0_KEYHASH destination (P2WPKH address)
  *  * WitnessV1Taproot: TxoutType::WITNESS_V1_TAPROOT destination (P2TR address)
+ *  * PayToAnchor: TxoutType::ANCHOR destination (P2A address)
  *  * WitnessUnknown: TxoutType::WITNESS_UNKNOWN destination (P2W??? address)
  *  A CTxDestination is the internal data type encoded in a bitcoin address
  */
-using CTxDestination = std::variant<CNoDestination, PubKeyDestination, PKHash, ScriptHash, WitnessV0ScriptHash, WitnessV0KeyHash, WitnessV1Taproot, WitnessUnknown>;
+using CTxDestination = std::variant<CNoDestination, PubKeyDestination, PKHash, ScriptHash, WitnessV0ScriptHash, WitnessV0KeyHash, WitnessV1Taproot, PayToAnchor, WitnessUnknown>;
 
 /** Check whether a CTxDestination corresponds to one with an address. */
 bool IsValidDestination(const CTxDestination& dest);

src/key_io.cpp

@@ -181,6 +181,10 @@ CTxDestination DecodeDestination(const std::string& str, const CChainParams& par
                 return tap;
             }
 
+            if (CScript::IsPayToAnchor(version, data)) {
+                return PayToAnchor();
+            }
+
             if (version > 16) {
                 error_str = "Invalid Bech32 address witness version";
                 return CNoDestination();

src/rpc/rawtransaction.cpp

@@ -557,6 +557,7 @@ static RPCHelpMan decodescript()
         case TxoutType::SCRIPTHASH:
         case TxoutType::WITNESS_UNKNOWN:
         case TxoutType::WITNESS_V1_TAPROOT:
+        case TxoutType::ANCHOR:
             // Should not be wrapped
             return false;
         } // no default case, so the compiler can warn about missing cases
@@ -599,6 +600,7 @@ static RPCHelpMan decodescript()
             case TxoutType::WITNESS_V0_KEYHASH:
             case TxoutType::WITNESS_V0_SCRIPTHASH:
             case TxoutType::WITNESS_V1_TAPROOT:
+            case TxoutType::ANCHOR:
                 // Should not be wrapped
                 return false;
             } // no default case, so the compiler can warn about missing cases

src/rpc/util.cpp

@@ -332,6 +332,14 @@ public:
         return obj;
     }
 
+    UniValue operator()(const PayToAnchor& anchor) const
+    {
+        UniValue obj(UniValue::VOBJ);
+        obj.pushKV("isscript", true);
+        obj.pushKV("iswitness", true);
+        return obj;
+    }
+
     UniValue operator()(const WitnessUnknown& id) const
     {
         UniValue obj(UniValue::VOBJ);

src/script/script.cpp

@@ -204,6 +204,23 @@ unsigned int CScript::GetSigOpCount(const CScript& scriptSig) const
     return subscript.GetSigOpCount(true);
 }
 
+bool CScript::IsPayToAnchor() const
+{
+    return (this->size() == 4 &&
+        (*this)[0] == OP_1 &&
+        (*this)[1] == 0x02 &&
+        (*this)[2] == 0x4e &&
+        (*this)[3] == 0x73);
+}
+
+bool CScript::IsPayToAnchor(int version, const std::vector<unsigned char>& program)
+{
+    return version == 1 &&
+        program.size() == 2 &&
+        program[0] == 0x4e &&
+        program[1] == 0x73;
+}
+
 bool CScript::IsPayToScriptHash() const
 {
     // Extra-fast test for pay-to-script-hash CScripts:

src/script/script.h

@@ -533,6 +533,14 @@ public:
      */
     unsigned int GetSigOpCount(const CScript& scriptSig) const;
 
+    /*
+     * OP_1 <0x4e73>
+     */
+    bool IsPayToAnchor() const;
+    /** Checks if output of IsWitnessProgram comes from a P2A output script
+     */
+    static bool IsPayToAnchor(int version, const std::vector<unsigned char>& program);
+
     bool IsPayToScriptHash() const;
     bool IsPayToWitnessScriptHash() const;
     bool IsWitnessProgram(int& version, std::vector<unsigned char>& program) const;

src/script/sign.cpp

@@ -475,6 +475,9 @@ static bool SignStep(const SigningProvider& provider, const BaseSignatureCreator
 
     case TxoutType::WITNESS_V1_TAPROOT:
         return SignTaproot(provider, creator, WitnessV1Taproot(XOnlyPubKey{vSolutions[0]}), sigdata, ret);
+
+    case TxoutType::ANCHOR:
+        return true;
     } // no default case, so the compiler can warn about missing cases
     assert(false);
 }

src/script/solver.cpp

@@ -24,6 +24,7 @@ std::string GetTxnOutputType(TxoutType t)
     case TxoutType::SCRIPTHASH: return "scripthash";
     case TxoutType::MULTISIG: return "multisig";
     case TxoutType::NULL_DATA: return "nulldata";
+    case TxoutType::ANCHOR: return "anchor";
     case TxoutType::WITNESS_V0_KEYHASH: return "witness_v0_keyhash";
     case TxoutType::WITNESS_V0_SCRIPTHASH: return "witness_v0_scripthash";
     case TxoutType::WITNESS_V1_TAPROOT: return "witness_v1_taproot";
@@ -165,6 +166,9 @@ TxoutType Solver(const CScript& scriptPubKey, std::vector<std::vector<unsigned c
             vSolutionsRet.push_back(std::move(witnessprogram));
             return TxoutType::WITNESS_V1_TAPROOT;
         }
+        if (scriptPubKey.IsPayToAnchor()) {
+            return TxoutType::ANCHOR;
+        }
         if (witnessversion != 0) {
             vSolutionsRet.push_back(std::vector<unsigned char>{(unsigned char)witnessversion});
             vSolutionsRet.push_back(std::move(witnessprogram));

src/script/solver.h

@@ -22,6 +22,7 @@ template <typename C> class Span;
 enum class TxoutType {
     NONSTANDARD,
     // 'standard' transaction types:
+    ANCHOR, //!< anyone can spend script
     PUBKEY,
     PUBKEYHASH,
     SCRIPTHASH,

src/test/fuzz/script.cpp

@@ -76,11 +76,13 @@ FUZZ_TARGET(script, .init = initialize_script)
         assert(which_type == TxoutType::PUBKEY ||
                which_type == TxoutType::NONSTANDARD ||
                which_type == TxoutType::NULL_DATA ||
-               which_type == TxoutType::MULTISIG);
+               which_type == TxoutType::MULTISIG ||
+               which_type == TxoutType::ANCHOR);
     }
     if (which_type == TxoutType::NONSTANDARD ||
         which_type == TxoutType::NULL_DATA ||
-        which_type == TxoutType::MULTISIG) {
+        which_type == TxoutType::MULTISIG ||
+        which_type == TxoutType::ANCHOR) {
         assert(!extract_destination_ret);
     }
 
@@ -94,6 +96,7 @@ FUZZ_TARGET(script, .init = initialize_script)
     (void)Solver(script, solutions);
 
     (void)script.HasValidOps();
+    (void)script.IsPayToAnchor();
     (void)script.IsPayToScriptHash();
     (void)script.IsPayToWitnessScriptHash();
     (void)script.IsPushOnly();

src/test/fuzz/util.cpp

@@ -213,6 +213,9 @@ CTxDestination ConsumeTxDestination(FuzzedDataProvider& fuzzed_data_provider) no
         [&] {
             tx_destination = WitnessV1Taproot{XOnlyPubKey{ConsumeUInt256(fuzzed_data_provider)}};
         },
+        [&] {
+            tx_destination = PayToAnchor{};
+        },
         [&] {
             std::vector<unsigned char> program{ConsumeRandomLengthByteVector(fuzzed_data_provider, /*max_length=*/40)};
             if (program.size() < 2) {

src/test/script_standard_tests.cpp

@@ -128,6 +128,20 @@ BOOST_AUTO_TEST_CASE(script_standard_Solver_success)
     BOOST_CHECK(solutions[0] == std::vector<unsigned char>{16});
     BOOST_CHECK(solutions[1] == ToByteVector(uint256::ONE));
 
+    // TxoutType::ANCHOR
+    std::vector<unsigned char> anchor_bytes{0x4e, 0x73};
+    s.clear();
+    s << OP_1 << anchor_bytes;
+    BOOST_CHECK_EQUAL(Solver(s, solutions), TxoutType::ANCHOR);
+    BOOST_CHECK(solutions.empty());
+
+    // Sanity-check IsPayToAnchor
+    int version{-1};
+    std::vector<unsigned char> witness_program;
+    BOOST_CHECK(s.IsPayToAnchor());
+    BOOST_CHECK(s.IsWitnessProgram(version, witness_program));
+    BOOST_CHECK(CScript::IsPayToAnchor(version, witness_program));
+
     // TxoutType::NONSTANDARD
     s.clear();
     s << OP_9 << OP_ADD << OP_11 << OP_EQUAL;
@@ -186,6 +200,18 @@ BOOST_AUTO_TEST_CASE(script_standard_Solver_failure)
     s.clear();
     s << OP_0 << std::vector<unsigned char>(19, 0x01);
     BOOST_CHECK_EQUAL(Solver(s, solutions), TxoutType::NONSTANDARD);
+
+    // TxoutType::ANCHOR but wrong witness version
+    s.clear();
+    s << OP_2 << std::vector<unsigned char>{0x4e, 0x73};
+    BOOST_CHECK(!s.IsPayToAnchor());
+    BOOST_CHECK_EQUAL(Solver(s, solutions), TxoutType::WITNESS_UNKNOWN);
+
+    // TxoutType::ANCHOR but wrong 2-byte data push
+    s.clear();
+    s << OP_1 << std::vector<unsigned char>{0xff, 0xff};
+    BOOST_CHECK(!s.IsPayToAnchor());
+    BOOST_CHECK_EQUAL(Solver(s, solutions), TxoutType::WITNESS_UNKNOWN);
 }
 
 BOOST_AUTO_TEST_CASE(script_standard_ExtractDestination)

src/test/transaction_tests.cpp

@@ -1026,6 +1026,14 @@ BOOST_AUTO_TEST_CASE(test_IsStandard)
         t.vout[0].nValue = 239;
         CheckIsNotStandard(t, "dust");
     }
+
+    // Check anchor outputs
+    t.vout[0].scriptPubKey = CScript() << OP_1 << std::vector<unsigned char>{0x4e, 0x73};
+    BOOST_CHECK(t.vout[0].scriptPubKey.IsPayToAnchor());
+    t.vout[0].nValue = 240;
+    CheckIsStandard(t);
+    t.vout[0].nValue = 239;
+    CheckIsNotStandard(t, "dust");
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/wallet/rpc/addresses.cpp

@@ -501,6 +501,7 @@ public:
     }
 
     UniValue operator()(const WitnessV1Taproot& id) const { return UniValue(UniValue::VOBJ); }
+    UniValue operator()(const PayToAnchor& id) const { return UniValue(UniValue::VOBJ); }
     UniValue operator()(const WitnessUnknown& id) const { return UniValue(UniValue::VOBJ); }
 };
 

src/wallet/rpc/backup.cpp

@@ -910,6 +910,7 @@ static std::string RecurseImportData(const CScript& script, ImportData& import_d
     case TxoutType::NONSTANDARD:
     case TxoutType::WITNESS_UNKNOWN:
     case TxoutType::WITNESS_V1_TAPROOT:
+    case TxoutType::ANCHOR:
         return "unrecognized script";
     } // no default case, so the compiler can warn about missing cases
     NONFATAL_UNREACHABLE();

src/wallet/scriptpubkeyman.cpp

@@ -115,6 +115,7 @@ IsMineResult IsMineInner(const LegacyDataSPKM& keystore, const CScript& scriptPu
     case TxoutType::NULL_DATA:
     case TxoutType::WITNESS_UNKNOWN:
     case TxoutType::WITNESS_V1_TAPROOT:
+    case TxoutType::ANCHOR:
         break;
     case TxoutType::PUBKEY:
         keyID = CPubKey(vSolutions[0]).GetID();