Merge bitcoin/bitcoin#24043: Add (sorted)multi_a descriptor for k-of-n multisig inside tr

4828d53eccd52a67631c64cef0ba7df90dff138d Add (sorted)multi_a descriptors to doc/descriptors.md (Pieter Wuille)
b5f33ac1f82aea290b4653af36ac2ad1bf1cce7b Simplify wallet_taproot.py functional test (Pieter Wuille)
eb0667ea96d52db9135514a5e95ab943f6abd8a6 Add tests for (sorted)multi_a derivation/signing (Pieter Wuille)
c17c6aa08df81aa0086d80b50187c8cd60ecc222 Add signing support for (sorted)multi_a scripts (Pieter Wuille)
3eed6fca57d1fa7544f372e6e7de0a9ae1b5715a Add multi_a descriptor inference (Pieter Wuille)
79728c4a3d8a74f276daf1e72abbdecdab85a5d8 Add (sorted)multi_a descriptor and script derivation (Pieter Wuille)
25e95f9ff89a97b87ce218f28274c3c821b2d54d Merge/generalize IsValidMultisigKeyCount/GetMultisigKeyCount (Pieter Wuille)

Pull request description:

  This adds a new `multi_a(k,key_1,key_2,...,key_n)` (and corresponding `sortedmulti_a`) descriptor for k-of-n policies inside `tr()`. Semantically it is very similar to the existing `multi()` descriptor, but with the following changes:
  * The corresponding script is `<key1> OP_CHECKSIG <key2> OP_CHECKSIGADD <key3> OP_CHECKSIGADD ... <key_n> OP_CHECKSIGADD <k> OP_NUMEQUAL`, rather than the traditional `OP_CHECKMULTISIG`-based script, making it usable inside the `tr()` descriptor.
  * The keys can optionally be specified in x-only notation.
  * Both the number of keys and the threshold can be as high as 999; this is the limit due to the consensus stacksize=1000 limit

  I expect that this functionality will later be replaced with a miniscript-based implementation, but I don't think it's necessary to wait for that.

  Limitations:
  * The wallet code will for not estimate witness size incorrectly for script path spends, which may result in a (dramatic) fee underpayment with large multi_a scripts.
  * The multi_a script construction is (slightly) suboptimal for n-of-n (where a `<key1> OP_CHECKSIGVERIFY ... <key_n-1> OP_CHECKSIGVERIFY <key_n> OP_CHECKSIG` would be better). Such a construction is not included here.

ACKs for top commit:
  achow101:
    ACK 4828d53eccd52a67631c64cef0ba7df90dff138d
  gruve-p:
    ACK https://github.com/bitcoin/bitcoin/pull/24043/commits/4828d53eccd52a67631c64cef0ba7df90dff138d
  sanket1729:
    code review ACK 4828d53eccd52a67631c64cef0ba7df90dff138d
  darosior:
    Code review ACK 4828d53eccd52a67631c64cef0ba7df90dff138d

Tree-SHA512: 5dcd434b79585f0ff830f7d501d27df5e346f5749f47a3109ec309ebf2cbbad0e1da541eec654026d911ab67fd7cf7793fab0f765628d68d81b96ef2a4d234ce

5 files changed, 145 insertions, 25 deletions

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index 798c4b3ea0..23540f6aef 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -802,6 +802,30 @@ public:
     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
 {
@@ -1040,7 +1064,6 @@ 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;
@@ -1065,7 +1088,12 @@ 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;
@@ -1086,9 +1114,12 @@ 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;
@@ -1109,10 +1140,17 @@ 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);
@@ -1257,6 +1295,21 @@ std::unique_ptr<PubkeyProvider> InferXOnlyPubkey(const XOnlyPubKey& xkey, ParseS
     return key_provider;
 }
 
+std::unique_ptr<DescriptorImpl> InferMultiA(const CScript& script, ParseScriptContext ctx, const SigningProvider& provider)
+{
+    auto match = MatchMultiA(script);
+    if (!match) return {};
+    std::vector<std::unique_ptr<PubkeyProvider>> keys;
+    keys.reserve(match->second.size());
+    for (const auto keyspan : match->second) {
+        if (keyspan.size() != 32) return {};
+        auto key = InferXOnlyPubkey(XOnlyPubKey{keyspan}, ctx, provider);
+        if (!key) return {};
+        keys.push_back(std::move(key));
+    }
+    return std::make_unique<MultiADescriptor>(match->first, std::move(keys));
+}
+
 std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptContext ctx, const SigningProvider& provider)
 {
     if (ctx == ParseScriptContext::P2TR && script.size() == 34 && script[0] == 32 && script[33] == OP_CHECKSIG) {
@@ -1264,6 +1317,11 @@ std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptCo
         return std::make_unique<PKDescriptor>(InferXOnlyPubkey(key, ctx, provider), true);
     }
 
+    if (ctx == ParseScriptContext::P2TR) {
+        auto ret = InferMultiA(script, ctx, provider);
+        if (ret) return ret;
+    }
+
     std::vector<std::vector<unsigned char>> data;
     TxoutType txntype = Solver(script, data);
 
diff --git a/src/script/script.h b/src/script/script.h
index 8b7a7bb7b3..a89c987306 100644
--- a/src/script/script.h
+++ b/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;
 
diff --git a/src/script/sign.cpp b/src/script/sign.cpp
index 371a937bc8..2e5c49e0b6 100644
--- a/src/script/sign.cpp
+++ b/src/script/sign.cpp
@@ -174,6 +174,29 @@ static bool SignTaprootScript(const SigningProvider& provider, const BaseSignatu
             result = Vector(std::move(sig));
             return true;
         }
+        return false;
+    }
+
+    // multi_a scripts (<key> OP_CHECKSIG <key> OP_CHECKSIGADD <key> OP_CHECKSIGADD <k> OP_NUMEQUAL)
+    if (auto match = MatchMultiA(script)) {
+        std::vector<std::vector<unsigned char>> sigs;
+        int good_sigs = 0;
+        for (size_t i = 0; i < match->second.size(); ++i) {
+            XOnlyPubKey pubkey{*(match->second.rbegin() + i)};
+            std::vector<unsigned char> sig;
+            bool good_sig = CreateTaprootScriptSig(creator, sigdata, provider, sig, pubkey, leaf_hash, sigversion);
+            if (good_sig && good_sigs < match->first) {
+                ++good_sigs;
+                sigs.push_back(std::move(sig));
+            } else {
+                sigs.emplace_back();
+            }
+        }
+        if (good_sigs == match->first) {
+            result = std::move(sigs);
+            return true;
+        }
+        return false;
     }
 
     return false;
diff --git a/src/script/standard.cpp b/src/script/standard.cpp
index 5fb98cc307..806b3169cd 100644
--- a/src/script/standard.cpp
+++ b/src/script/standard.cpp
@@ -96,51 +96,83 @@ static constexpr bool IsPushdataOp(opcodetype opcode)
     return opcode > OP_FALSE && opcode <= OP_PUSHDATA4;
 }
 
-static constexpr bool IsValidMultisigKeyCount(int n_keys)
-{
-    return n_keys > 0 && n_keys <= MAX_PUBKEYS_PER_MULTISIG;
-}
-
-static bool GetMultisigKeyCount(opcodetype opcode, valtype data, int& count)
+/** Retrieve a minimally-encoded number in range [min,max] from an (opcode, data) pair,
+ *  whether it's OP_n or through a push. */
+static std::optional<int> GetScriptNumber(opcodetype opcode, valtype data, int min, int max)
 {
+    int count;
     if (IsSmallInteger(opcode)) {
         count = CScript::DecodeOP_N(opcode);
-        return IsValidMultisigKeyCount(count);
-    }
-
-    if (IsPushdataOp(opcode)) {
-        if (!CheckMinimalPush(data, opcode)) return false;
+    } else if (IsPushdataOp(opcode)) {
+        if (!CheckMinimalPush(data, opcode)) return {};
         try {
             count = CScriptNum(data, /* fRequireMinimal = */ true).getint();
-            return IsValidMultisigKeyCount(count);
         } catch (const scriptnum_error&) {
-            return false;
+            return {};
         }
+    } else {
+        return {};
     }
-
-    return false;
+    if (count < min || count > max) return {};
+    return count;
 }
 
 static bool MatchMultisig(const CScript& script, int& required_sigs, std::vector<valtype>& pubkeys)
 {
     opcodetype opcode;
     valtype data;
-    int num_keys;
 
     CScript::const_iterator it = script.begin();
     if (script.size() < 1 || script.back() != OP_CHECKMULTISIG) return false;
 
-    if (!script.GetOp(it, opcode, data) || !GetMultisigKeyCount(opcode, data, required_sigs)) return false;
+    if (!script.GetOp(it, opcode, data)) return false;
+    auto req_sigs = GetScriptNumber(opcode, data, 1, MAX_PUBKEYS_PER_MULTISIG);
+    if (!req_sigs) return false;
+    required_sigs = *req_sigs;
     while (script.GetOp(it, opcode, data) && CPubKey::ValidSize(data)) {
         pubkeys.emplace_back(std::move(data));
     }
-    if (!GetMultisigKeyCount(opcode, data, num_keys)) return false;
-
-    if (pubkeys.size() != static_cast<unsigned long>(num_keys) || num_keys < required_sigs) return false;
+    auto num_keys = GetScriptNumber(opcode, data, required_sigs, MAX_PUBKEYS_PER_MULTISIG);
+    if (!num_keys) return false;
+    if (pubkeys.size() != static_cast<unsigned long>(*num_keys)) return false;
 
     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();
diff --git a/src/script/standard.h b/src/script/standard.h
index eb50421768..75bfe2db38 100644
--- a/src/script/standard.h
+++ b/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);