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).

bitcoin/bitcoin#22051 (5/9)
We actually preserve the "unrelated tweaking" method so we can
use it in OP_TWEAKVERIFY

Author: apoelstra

src/pubkey.cpp

@@ -181,13 +181,35 @@ bool XOnlyPubKey::VerifySchnorr(const Span<const unsigned char> msg, Span<const
     return secp256k1_schnorrsig_verify(secp256k1_context_verify, sigbytes.data(), msg.data(), msg.size(), &pubkey);
 }
 
+// ELEMENTS: this is preserved from an old version of the Taproot code for use in OP_TWEAKVERIFY
 bool XOnlyPubKey::CheckPayToContract(const XOnlyPubKey& base, const uint256& hash, 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());
 }
 
+static const CHashWriter HASHER_TAPTWEAK_ELEMENTS = TaggedHash("TapTweak/elements");
+
+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_ELEMENTS) << m_keydata).GetSHA256();
+    } else {
+        return (CHashWriter(HASHER_TAPTWEAK_ELEMENTS) << m_keydata << *merkle_root).GetSHA256();
+    }
+}
+
+bool XOnlyPubKey::CheckTapTweak(const XOnlyPubKey& internal, const uint256& merkle_root, bool parity) const
+{
+    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::TweakMulVerify(const CPubKey& untweaked, const uint256& tweak) const
 {
     assert(this->IsCompressed());

src/pubkey.h

@@ -231,8 +231,23 @@ class XOnlyPubKey
      * sigbytes must be exactly 64 bytes.
      */
     bool VerifySchnorr(const Span<const unsigned char> msg, Span<const unsigned char> sigbytes) const;
+    // ELEMENTS: this is preserved from an old version of the Taproot code for use in OP_TWEAKVERIFY
     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(); }
     static constexpr size_t size() { return decltype(m_keydata)::size(); }

src/script/interpreter.cpp

@@ -569,7 +569,6 @@ static bool EvalChecksig(const valtype& sig, const valtype& pubkey, CScript::con
 
 static const CHashWriter HASHER_TAPLEAF_ELEMENTS = TaggedHash("TapLeaf/elements");
 static const CHashWriter HASHER_TAPBRANCH_ELEMENTS = TaggedHash("TapBranch/elements");
-static const CHashWriter HASHER_TAPTWEAK_ELEMENTS = TaggedHash("TapTweak/elements");
 static const CHashWriter HASHER_TAPSIGHASH_ELEMENTS = TaggedHash("TapSighash/elements");
 
 bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptExecutionData& execdata, ScriptError* serror)
@@ -3074,8 +3073,8 @@ static bool VerifyTaprootCommitment(const std::vector<unsigned char>& control, c
         }
         k = ss_branch.GetSHA256();
     }
-    k = (CHashWriter(HASHER_TAPTWEAK_ELEMENTS) << MakeSpan(p) << k).GetSHA256();
-    return q.CheckPayToContract(p, k, control[0] & 1);
+    // Verify that the output pubkey matches the tweaked internal pubkey, after correcting for parity.
+    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)