Add (sorted)multi_a descriptor and script derivation

Author: sipa

src/script/descriptor.cpp

@@ -794,6 +794,30 @@ class MultisigDescriptor final : public DescriptorImpl
     bool IsSingleType() const final { return true; }
 };
 
+/** A parsed (sorted)multi_a(...) descriptor. Always uses x-only pubkeys. */
+class MultiADescriptor final : public DescriptorImpl
+{
+    const int m_threshold;
+    const bool m_sorted;
+protected:
+    std::string ToStringExtra() const override { return strprintf("%i", m_threshold); }
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, Span<const CScript>, FlatSigningProvider&) const override {
+        CScript ret;
+        std::vector<XOnlyPubKey> xkeys;
+        for (const auto& key : keys) xkeys.emplace_back(key);
+        if (m_sorted) std::sort(xkeys.begin(), xkeys.end());
+        ret << ToByteVector(xkeys[0]) << OP_CHECKSIG;
+        for (size_t i = 1; i < keys.size(); ++i) {
+            ret << ToByteVector(xkeys[i]) << OP_CHECKSIGADD;
+        }
+        ret << m_threshold << OP_NUMEQUAL;
+        return Vector(std::move(ret));
+    }
+public:
+    MultiADescriptor(int threshold, std::vector<std::unique_ptr<PubkeyProvider>> providers, bool sorted = false) : DescriptorImpl(std::move(providers), sorted ? "sortedmulti_a" : "multi_a"), m_threshold(threshold), m_sorted(sorted) {}
+    bool IsSingleType() const final { return true; }
+};
+
 /** A parsed sh(...) descriptor. */
 class SHDescriptor final : public DescriptorImpl
 {
@@ -1032,7 +1056,6 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
     using namespace spanparsing;
 
     auto expr = Expr(sp);
-    bool sorted_multi = false;
     if (Func("pk", expr)) {
         auto pubkey = ParsePubkey(key_exp_index, expr, ctx, out, error);
         if (!pubkey) return nullptr;
@@ -1057,7 +1080,12 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
         error = "Can only have combo() at top level";
         return nullptr;
     }
-    if ((ctx == ParseScriptContext::TOP || ctx == ParseScriptContext::P2SH || ctx == ParseScriptContext::P2WSH) && ((sorted_multi = Func("sortedmulti", expr)) || Func("multi", expr))) {
+    const bool multi = Func("multi", expr);
+    const bool sortedmulti = !multi && Func("sortedmulti", expr);
+    const bool multi_a = !(multi || sortedmulti) && Func("multi_a", expr);
+    const bool sortedmulti_a = !(multi || sortedmulti || multi_a) && Func("sortedmulti_a", expr);
+    if (((ctx == ParseScriptContext::TOP || ctx == ParseScriptContext::P2SH || ctx == ParseScriptContext::P2WSH) && (multi || sortedmulti)) ||
+        (ctx == ParseScriptContext::P2TR && (multi_a || sortedmulti_a))) {
         auto threshold = Expr(expr);
         uint32_t thres;
         std::vector<std::unique_ptr<PubkeyProvider>> providers;
@@ -1078,9 +1106,12 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
             providers.emplace_back(std::move(pk));
             key_exp_index++;
         }
-        if (providers.empty() || providers.size() > MAX_PUBKEYS_PER_MULTISIG) {
+        if ((multi || sortedmulti) && (providers.empty() || providers.size() > MAX_PUBKEYS_PER_MULTISIG)) {
             error = strprintf("Cannot have %u keys in multisig; must have between 1 and %d keys, inclusive", providers.size(), MAX_PUBKEYS_PER_MULTISIG);
             return nullptr;
+        } else if ((multi_a || sortedmulti_a) && (providers.empty() || providers.size() > MAX_PUBKEYS_PER_MULTI_A)) {
+            error = strprintf("Cannot have %u keys in multi_a; must have between 1 and %d keys, inclusive", providers.size(), MAX_PUBKEYS_PER_MULTI_A);
+            return nullptr;
         } else if (thres < 1) {
             error = strprintf("Multisig threshold cannot be %d, must be at least 1", thres);
             return nullptr;
@@ -1101,10 +1132,17 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
                 return nullptr;
             }
         }
-        return std::make_unique<MultisigDescriptor>(thres, std::move(providers), sorted_multi);
-    } else if (Func("sortedmulti", expr) || Func("multi", expr)) {
+        if (multi || sortedmulti) {
+            return std::make_unique<MultisigDescriptor>(thres, std::move(providers), sortedmulti);
+        } else {
+            return std::make_unique<MultiADescriptor>(thres, std::move(providers), sortedmulti_a);
+        }
+    } else if (multi || sortedmulti) {
         error = "Can only have multi/sortedmulti at top level, in sh(), or in wsh()";
         return nullptr;
+    } else if (multi_a || sortedmulti_a) {
+        error = "Can only have multi_a/sortedmulti_a inside tr()";
+        return nullptr;
     }
     if ((ctx == ParseScriptContext::TOP || ctx == ParseScriptContext::P2SH) && Func("wpkh", expr)) {
         auto pubkey = ParsePubkey(key_exp_index, expr, ParseScriptContext::P2WPKH, out, error);

src/script/script.h

@@ -29,6 +29,9 @@ static const int MAX_OPS_PER_SCRIPT = 201;
 // Maximum number of public keys per multisig
 static const int MAX_PUBKEYS_PER_MULTISIG = 20;
 
+/** The limit of keys in OP_CHECKSIGADD-based scripts. It is due to the stack limit in BIP342. */
+static constexpr unsigned int MAX_PUBKEYS_PER_MULTI_A = 999;
+
 // Maximum script length in bytes
 static const int MAX_SCRIPT_SIZE = 10000;
 

src/script/standard.cpp

@@ -139,6 +139,40 @@ static bool MatchMultisig(const CScript& script, int& required_sigs, std::vector
     return (it + 1 == script.end());
 }
 
+std::optional<std::pair<int, std::vector<Span<const unsigned char>>>> MatchMultiA(const CScript& script)
+{
+    std::vector<Span<const unsigned char>> keyspans;
+
+    // Redundant, but very fast and selective test.
+    if (script.size() == 0 || script[0] != 32 || script.back() != OP_NUMEQUAL) return {};
+
+    // Parse keys
+    auto it = script.begin();
+    while (script.end() - it >= 34) {
+        if (*it != 32) return {};
+        ++it;
+        keyspans.emplace_back(&*it, 32);
+        it += 32;
+        if (*it != (keyspans.size() == 1 ? OP_CHECKSIG : OP_CHECKSIGADD)) return {};
+        ++it;
+    }
+    if (keyspans.size() == 0 || keyspans.size() > MAX_PUBKEYS_PER_MULTI_A) return {};
+
+    // Parse threshold.
+    opcodetype opcode;
+    std::vector<unsigned char> data;
+    if (!script.GetOp(it, opcode, data)) return {};
+    if (it == script.end()) return {};
+    if (*it != OP_NUMEQUAL) return {};
+    ++it;
+    if (it != script.end()) return {};
+    auto threshold = GetScriptNumber(opcode, data, 1, (int)keyspans.size());
+    if (!threshold) return {};
+
+    // Construct result.
+    return std::pair{*threshold, std::move(keyspans)};
+}
+
 TxoutType Solver(const CScript& scriptPubKey, std::vector<std::vector<unsigned char>>& vSolutionsRet)
 {
     vSolutionsRet.clear();

src/script/standard.h

@@ -191,6 +191,10 @@ CScript GetScriptForDestination(const CTxDestination& dest);
 /** Generate a P2PK script for the given pubkey. */
 CScript GetScriptForRawPubKey(const CPubKey& pubkey);
 
+/** Determine if script is a "multi_a" script. Returns (threshold, keyspans) if so, and nullopt otherwise.
+ *  The keyspans refer to bytes in the passed script. */
+std::optional<std::pair<int, std::vector<Span<const unsigned char>>>> MatchMultiA(const CScript& script LIFETIMEBOUND);
+
 /** Generate a multisig script. */
 CScript GetScriptForMultisig(int nRequired, const std::vector<CPubKey>& keys);