From 5f6cc8daa83700d1c949d968a5cf0d935be337b7 Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter@wuille.net>
Date: Sun, 23 May 2021 18:32:30 -0700
Subject: [PATCH] Add XOnlyPubKey::CreateTapTweak

---
 src/pubkey.cpp | 18 ++++++++++++++++++
 src/pubkey.h   |  4 ++++
 2 files changed, 22 insertions(+)

diff --git a/src/pubkey.cpp b/src/pubkey.cpp
index f78779c182..8382897f95 100644
--- a/src/pubkey.cpp
+++ b/src/pubkey.cpp
@@ -209,6 +209,24 @@ bool XOnlyPubKey::CheckTapTweak(const XOnlyPubKey& internal, const uint256& merk
     return secp256k1_xonly_pubkey_tweak_add_check(secp256k1_context_verify, m_keydata.begin(), parity, &internal_key, tweak.begin());
 }
 
+std::optional<std::pair<XOnlyPubKey, bool>> XOnlyPubKey::CreateTapTweak(const uint256* merkle_root) const
+{
+    secp256k1_xonly_pubkey base_point;
+    if (!secp256k1_xonly_pubkey_parse(secp256k1_context_verify, &base_point, data())) return std::nullopt;
+    secp256k1_pubkey out;
+    uint256 tweak = ComputeTapTweakHash(merkle_root);
+    if (!secp256k1_xonly_pubkey_tweak_add(secp256k1_context_verify, &out, &base_point, tweak.data())) return std::nullopt;
+    int parity = -1;
+    std::pair<XOnlyPubKey, bool> ret;
+    secp256k1_xonly_pubkey out_xonly;
+    if (!secp256k1_xonly_pubkey_from_pubkey(secp256k1_context_verify, &out_xonly, &parity, &out)) return std::nullopt;
+    secp256k1_xonly_pubkey_serialize(secp256k1_context_verify, ret.first.begin(), &out_xonly);
+    assert(parity == 0 || parity == 1);
+    ret.second = parity;
+    return ret;
+}
+
+
 bool CPubKey::Verify(const uint256 &hash, const std::vector<unsigned char>& vchSig) const {
     if (!IsValid())
         return false;
diff --git a/src/pubkey.h b/src/pubkey.h
index e5d9d08b52..b9d5f5d622 100644
--- a/src/pubkey.h
+++ b/src/pubkey.h
@@ -13,6 +13,7 @@
 #include <uint256.h>
 
 #include <cstring>
+#include <optional>
 #include <vector>
 
 const unsigned int BIP32_EXTKEY_SIZE = 74;
@@ -251,6 +252,9 @@ public:
      *  Merkle root, and parity. */
     bool CheckTapTweak(const XOnlyPubKey& internal, const uint256& merkle_root, bool parity) const;
 
+    /** Construct a Taproot tweaked output point with this point as internal key. */
+    std::optional<std::pair<XOnlyPubKey, bool>> CreateTapTweak(const uint256* merkle_root) 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(); }