Add rawtr() descriptor for P2TR with unknown tweak

Author: sipa

doc/descriptors.md

@@ -77,6 +77,7 @@ Descriptors consist of several types of expressions. The top level expression is
 - `tr(KEY)` or `tr(KEY,TREE)` (top level only): P2TR output with the specified key as internal key, and optionally a tree of script paths.
 - `addr(ADDR)` (top level only): the script which ADDR expands to.
 - `raw(HEX)` (top level only): the script whose hex encoding is HEX.
+- `rawtr(KEY)` (top level only): P2TR output with the specified key as output key. NOTE: while it's possible to use this to construct wallets, it has several downsides, like being unable to prove no hidden script path exists. Use at your own risk.
 
 `KEY` expressions:
 - Optionally, key origin information, consisting of:
@@ -87,7 +88,7 @@ Descriptors consist of several types of expressions. The top level expression is
 - Followed by the actual key, which is either:
   - Hex encoded public keys (either 66 characters starting with `02` or `03` for a compressed pubkey, or 130 characters starting with `04` for an uncompressed pubkey).
     - Inside `wpkh` and `wsh`, only compressed public keys are permitted.
-    - Inside `tr`, x-only pubkeys are also permitted (64 hex characters).
+    - Inside `tr` and `rawtr`, x-only pubkeys are also permitted (64 hex characters).
   - [WIF](https://en.bitcoin.it/wiki/Wallet_import_format) encoded private keys may be specified instead of the corresponding public key, with the same meaning.
   - `xpub` encoded extended public key or `xprv` encoded extended private key (as defined in [BIP 32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki)).
     - Followed by zero or more `/NUM` unhardened and `/NUM'` hardened BIP32 derivation steps.

src/script/descriptor.cpp

@@ -1015,6 +1015,24 @@ class MiniscriptDescriptor final : public DescriptorImpl
     bool IsSingleType() const final { return true; }
 };
 
+/** A parsed rawtr(...) descriptor. */
+class RawTRDescriptor final : public DescriptorImpl
+{
+protected:
+    std::vector<CScript> MakeScripts(const std::vector<CPubKey>& keys, Span<const CScript> scripts, FlatSigningProvider& out) const override
+    {
+        assert(keys.size() == 1);
+        XOnlyPubKey xpk(keys[0]);
+        if (!xpk.IsFullyValid()) return {};
+        WitnessV1Taproot output{xpk};
+        return Vector(GetScriptForDestination(output));
+    }
+public:
+    RawTRDescriptor(std::unique_ptr<PubkeyProvider> output_key) : DescriptorImpl(Vector(std::move(output_key)), "rawtr") {}
+    std::optional<OutputType> GetOutputType() const override { return OutputType::BECH32M; }
+    bool IsSingleType() const final { return true; }
+};
+
 ////////////////////////////////////////////////////////////////////////////
 // Parser                                                                 //
 ////////////////////////////////////////////////////////////////////////////
@@ -1453,6 +1471,16 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
         error = "Can only have tr at top level";
         return nullptr;
     }
+    if (ctx == ParseScriptContext::TOP && Func("rawtr", expr)) {
+        auto arg = Expr(expr);
+        auto output_key = ParsePubkey(key_exp_index, arg, ParseScriptContext::P2TR, out, error);
+        if (!output_key) return nullptr;
+        ++key_exp_index;
+        return std::make_unique<RawTRDescriptor>(std::move(output_key));
+    } else if (Func("rawtr", expr)) {
+        error = "Can only have rawtr at top level";
+        return nullptr;
+    }
     if (ctx == ParseScriptContext::TOP && Func("raw", expr)) {
         std::string str(expr.begin(), expr.end());
         if (!IsHex(str)) {
@@ -1626,6 +1654,13 @@ std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptCo
                 }
             }
         }
+        // If the above doesn't work, construct a rawtr() descriptor with just the encoded x-only pubkey.
+        if (pubkey.IsFullyValid()) {
+            auto key = InferXOnlyPubkey(pubkey, ParseScriptContext::P2TR, provider);
+            if (key) {
+                return std::make_unique<RawTRDescriptor>(std::move(key));
+            }
+        }
     }
 
     if (ctx == ParseScriptContext::P2WSH) {

test/functional/data/rpc_decodescript.json

@@ -4,7 +4,7 @@
         {
             "asm": "1 eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee",
             "address": "bcrt1pamhwamhwamhwamhwamhwamhwamhwamhwamhwamhwamhwamhwamhqz6nvlh",
-            "desc": "addr(bcrt1pamhwamhwamhwamhwamhwamhwamhwamhwamhwamhwamhwamhwamhqz6nvlh)#v52jnujz",
+            "desc": "rawtr(eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee)#jk7c6kys",
             "type": "witness_v1_taproot"
         }
     ],