script: allow up to 20 keys in wsh() descriptors

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

src/script/descriptor.cpp

@@ -998,8 +998,8 @@ 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() > 16) {
-            error = strprintf("Cannot have %u keys in multisig; must have between 1 and 16 keys, inclusive", providers.size());
+        if (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 (thres < 1) {
             error = strprintf("Multisig threshold cannot be %d, must be at least 1", thres);
@@ -1015,6 +1015,7 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
             }
         }
         if (ctx == ParseScriptContext::P2SH) {
+            // This limits the maximum number of compressed pubkeys to 15.
             if (script_size + 3 > MAX_SCRIPT_ELEMENT_SIZE) {
                 error = strprintf("P2SH script is too large, %d bytes is larger than %d bytes", script_size + 3, MAX_SCRIPT_ELEMENT_SIZE);
                 return nullptr;

src/script/standard.cpp

@@ -198,9 +198,9 @@ TxoutType Solver(const CScript& scriptPubKey, std::vector<std::vector<unsigned c
     int required;
     std::vector<std::vector<unsigned char>> keys;
     if (MatchMultisig(scriptPubKey, required, keys)) {
-        vSolutionsRet.push_back({static_cast<unsigned char>(required)}); // safe as required is in range 1..16
+        vSolutionsRet.push_back({static_cast<unsigned char>(required)}); // safe as required is in range 1..20
         vSolutionsRet.insert(vSolutionsRet.end(), keys.begin(), keys.end());
-        vSolutionsRet.push_back({static_cast<unsigned char>(keys.size())}); // safe as size is in range 1..16
+        vSolutionsRet.push_back({static_cast<unsigned char>(keys.size())}); // safe as size is in range 1..20
         return TxoutType::MULTISIG;
     }
 
@@ -350,10 +350,11 @@ CScript GetScriptForMultisig(int nRequired, const std::vector<CPubKey>& keys)
 {
     CScript script;
 
-    script << CScript::EncodeOP_N(nRequired);
+    script << nRequired;
     for (const CPubKey& key : keys)
         script << ToByteVector(key);
-    script << CScript::EncodeOP_N(keys.size()) << OP_CHECKMULTISIG;
+    script << keys.size() << OP_CHECKMULTISIG;
+
     return script;
 }