miniscript: account for keys as being 32 bytes under Taproot context

Author: darosior

src/script/miniscript.cpp

@@ -258,11 +258,12 @@ Type ComputeType(Fragment fragment, Type x, Type y, Type z, const std::vector<Ty
     assert(false);
 }
 
-size_t ComputeScriptLen(Fragment fragment, Type sub0typ, size_t subsize, uint32_t k, size_t n_subs, size_t n_keys) {
+size_t ComputeScriptLen(Fragment fragment, Type sub0typ, size_t subsize, uint32_t k, size_t n_subs,
+                        size_t n_keys, MiniscriptContext ms_ctx) {
     switch (fragment) {
         case Fragment::JUST_1:
         case Fragment::JUST_0: return 1;
-        case Fragment::PK_K: return 34;
+        case Fragment::PK_K: return IsTapscript(ms_ctx) ? 33 : 34;
         case Fragment::PK_H: return 3 + 21;
         case Fragment::OLDER:
         case Fragment::AFTER: return 1 + BuildScript(k).size();

src/script/miniscript.h

@@ -280,7 +280,7 @@ constexpr uint32_t MaxScriptSize(MiniscriptContext ms_ctx)
 Type ComputeType(Fragment fragment, Type x, Type y, Type z, const std::vector<Type>& sub_types, uint32_t k, size_t data_size, size_t n_subs, size_t n_keys, MiniscriptContext ms_ctx);
 
 //! Helper function for Node::CalcScriptLen.
-size_t ComputeScriptLen(Fragment fragment, Type sub0typ, size_t subsize, uint32_t k, size_t n_subs, size_t n_keys);
+size_t ComputeScriptLen(Fragment fragment, Type sub0typ, size_t subsize, uint32_t k, size_t n_subs, size_t n_keys, MiniscriptContext ms_ctx);
 
 //! A helper sanitizer/checker for the output of CalcType.
 Type SanitizeType(Type x);
@@ -437,7 +437,7 @@ struct Node {
             subsize += sub->ScriptSize();
         }
         Type sub0type = subs.size() > 0 ? subs[0]->GetType() : ""_mst;
-        return internal::ComputeScriptLen(fragment, sub0type, subsize, k, subs.size(), keys.size());
+        return internal::ComputeScriptLen(fragment, sub0type, subsize, k, subs.size(), keys.size(), m_script_ctx);
     }
 
     /* Apply a recursive algorithm to a Miniscript tree, without actual recursive calls.
@@ -1698,7 +1698,7 @@ inline NodeRef<Key> Parse(Span<const char> in, const Ctx& ctx)
                 auto& [key, key_size] = *res;
                 constructed.push_back(MakeNodeRef<Key>(internal::NoDupCheck{}, ctx.MsContext(), Fragment::WRAP_C, Vector(MakeNodeRef<Key>(internal::NoDupCheck{}, ctx.MsContext(), Fragment::PK_K, Vector(std::move(key))))));
                 in = in.subspan(key_size + 1);
-                script_size += 34;
+                script_size += IsTapscript(ctx.MsContext()) ? 33 : 34;
             } else if (Const("pkh(", in)) {
                 auto res = ParseKeyEnd<Key>(in, ctx);
                 if (!res) return {};
@@ -1712,7 +1712,7 @@ inline NodeRef<Key> Parse(Span<const char> in, const Ctx& ctx)
                 auto& [key, key_size] = *res;
                 constructed.push_back(MakeNodeRef<Key>(internal::NoDupCheck{}, ctx.MsContext(), Fragment::PK_K, Vector(std::move(key))));
                 in = in.subspan(key_size + 1);
-                script_size += 33;
+                script_size += IsTapscript(ctx.MsContext()) ? 32 : 33;
             } else if (Const("pk_h(", in)) {
                 auto res = ParseKeyEnd<Key>(in, ctx);
                 if (!res) return {};
@@ -2058,7 +2058,7 @@ inline NodeRef<Key> DecodeScript(I& in, I last, const Ctx& ctx)
                 break;
             }
             // Public keys
-            if (in[0].second.size() == 33) {
+            if (in[0].second.size() == 33 || in[0].second.size() == 32) {
                 auto key = ctx.FromPKBytes(in[0].second.begin(), in[0].second.end());
                 if (!key) return {};
                 ++in;

src/test/fuzz/miniscript.cpp

@@ -796,7 +796,8 @@ NodeRef GenNode(F ConsumeNode, Type root_type, bool strict_valid = false) {
             // Update predicted resource limits. Since every leaf Miniscript node is at least one
             // byte long, we move one byte from each child to their parent. A similar technique is
             // used in the miniscript::internal::Parse function to prevent runaway string parsing.
-            scriptsize += miniscript::internal::ComputeScriptLen(node_info->fragment, ""_mst, node_info->subtypes.size(), node_info->k, node_info->subtypes.size(), node_info->keys.size()) - 1;
+            scriptsize += miniscript::internal::ComputeScriptLen(node_info->fragment, ""_mst, node_info->subtypes.size(), node_info->k, node_info->subtypes.size(),
+                                                                 node_info->keys.size(), miniscript::MiniscriptContext::P2WSH) - 1;
             if (scriptsize > MAX_STANDARD_P2WSH_SCRIPT_SIZE) return {};
             switch (node_info->fragment) {
             case Fragment::MULTI_A: