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.

Author: instagibbs

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 @@ struct WitnessUnknown
     }
 };
 
+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 @@ struct WitnessUnknown
  *  * 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 @@ class DescribeAddressVisitor
         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 @@ class CScript : public CScriptBase
      */
     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,