Make consensus checking of tweaks in pubkey.* Taproot-specific

That results in a much safer interface (making the tweak commit
to the key implicitly using a fixed tag means it can't be used for
unrelated tweaking).

src/pubkey.cpp

@@ -188,11 +188,25 @@ bool XOnlyPubKey::VerifySchnorr(const uint256& msg, Span<const unsigned char> si
     return secp256k1_schnorrsig_verify(secp256k1_context_verify, sigbytes.data(), msg.begin(), &pubkey);
 }
 
-bool XOnlyPubKey::CheckPayToContract(const XOnlyPubKey& base, const uint256& hash, bool parity) const
+static const CHashWriter HASHER_TAPTWEAK = TaggedHash("TapTweak");
+
+uint256 XOnlyPubKey::ComputeTapTweakHash(const uint256* merkle_root) const
+{
+    if (merkle_root == nullptr) {
+        // We have no scripts. The actual tweak does not matter, but follow BIP341 here to
+        // allow for reproducible tweaking.
+        return (CHashWriter(HASHER_TAPTWEAK) << m_keydata).GetSHA256();
+    } else {
+        return (CHashWriter(HASHER_TAPTWEAK) << m_keydata << *merkle_root).GetSHA256();
+    }
+}
+
+bool XOnlyPubKey::CheckTapTweak(const XOnlyPubKey& internal, const uint256& merkle_root, bool parity) const
 {
-    secp256k1_xonly_pubkey base_point;
-    if (!secp256k1_xonly_pubkey_parse(secp256k1_context_verify, &base_point, base.data())) return false;
-    return secp256k1_xonly_pubkey_tweak_add_check(secp256k1_context_verify, m_keydata.begin(), parity, &base_point, hash.begin());
+    secp256k1_xonly_pubkey internal_key;
+    if (!secp256k1_xonly_pubkey_parse(secp256k1_context_verify, &internal_key, internal.data())) return false;
+    uint256 tweak = internal.ComputeTapTweakHash(&merkle_root);
+    return secp256k1_xonly_pubkey_tweak_add_check(secp256k1_context_verify, m_keydata.begin(), parity, &internal_key, tweak.begin());
 }
 
 bool CPubKey::Verify(const uint256 &hash, const std::vector<unsigned char>& vchSig) const {

src/pubkey.h

@@ -236,7 +236,20 @@ public:
      * sigbytes must be exactly 64 bytes.
      */
     bool VerifySchnorr(const uint256& msg, Span<const unsigned char> sigbytes) const;
-    bool CheckPayToContract(const XOnlyPubKey& base, const uint256& hash, bool parity) const;
+
+    /** Compute the Taproot tweak as specified in BIP341, with *this as internal
+     * key:
+     *  - if merkle_root == nullptr: H_TapTweak(xonly_pubkey)
+     *  - otherwise:                 H_TapTweak(xonly_pubkey || *merkle_root)
+     *
+     * Note that the behavior of this function with merkle_root != nullptr is
+     * consensus critical.
+     */
+    uint256 ComputeTapTweakHash(const uint256* merkle_root) const;
+
+    /** Verify that this is a Taproot tweaked output point, against a specified internal key,
+     *  Merkle root, and parity. */
+    bool CheckTapTweak(const XOnlyPubKey& internal, const uint256& merkle_root, bool parity) const;
 
     const unsigned char& operator[](int pos) const { return *(m_keydata.begin() + pos); }
     const unsigned char* data() const { return m_keydata.begin(); }

src/script/interpreter.cpp

@@ -1486,7 +1486,6 @@ template PrecomputedTransactionData::PrecomputedTransactionData(const CMutableTr
 static const CHashWriter HASHER_TAPSIGHASH = TaggedHash("TapSighash");
 static const CHashWriter HASHER_TAPLEAF = TaggedHash("TapLeaf");
 static const CHashWriter HASHER_TAPBRANCH = TaggedHash("TapBranch");
-static const CHashWriter HASHER_TAPTWEAK = TaggedHash("TapTweak");
 
 static bool HandleMissingData(MissingDataBehavior mdb)
 {
@@ -1869,10 +1868,8 @@ static bool VerifyTaprootCommitment(const std::vector<unsigned char>& control, c
         }
         k = ss_branch.GetSHA256();
     }
-    // Compute the tweak from the Merkle root and the internal pubkey.
-    k = (CHashWriter(HASHER_TAPTWEAK) << MakeSpan(p) << k).GetSHA256();
     // Verify that the output pubkey matches the tweaked internal pubkey, after correcting for parity.
-    return q.CheckPayToContract(p, k, control[0] & 1);
+    return q.CheckTapTweak(p, k, control[0] & 1);
 }
 
 static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion, const std::vector<unsigned char>& program, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror, bool is_p2sh)