From 4b24bfeab9d6732aae3e69efd33105792ef1198f Mon Sep 17 00:00:00 2001
From: Ava Chow <github@achow101.com>
Date: Mon, 4 Mar 2024 15:40:00 -0500
Subject: [PATCH] pubkey: Return tweaks from BIP32 derivation

MuSig2 needs the BIP32 derivation tweaks in order to sign with a key
derived from the aggregate pubkey.
---
 src/pubkey.cpp | 9 ++++++---
 src/pubkey.h   | 4 ++--
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/pubkey.cpp b/src/pubkey.cpp
index 6041c89e7f1..264f861bc7a 100644
--- a/src/pubkey.cpp
+++ b/src/pubkey.cpp
@@ -338,13 +338,16 @@ bool CPubKey::Decompress() {
     return true;
 }
 
-bool CPubKey::Derive(CPubKey& pubkeyChild, ChainCode &ccChild, unsigned int nChild, const ChainCode& cc) const {
+bool CPubKey::Derive(CPubKey& pubkeyChild, ChainCode &ccChild, unsigned int nChild, const ChainCode& cc, uint256* bip32_tweak_out) const {
     assert(IsValid());
     assert((nChild >> 31) == 0);
     assert(size() == COMPRESSED_SIZE);
     unsigned char out[64];
     BIP32Hash(cc, nChild, *begin(), begin()+1, out);
     memcpy(ccChild.begin(), out+32, 32);
+    if (bip32_tweak_out) {
+        memcpy(bip32_tweak_out->begin(), out, 32);
+    }
     secp256k1_pubkey pubkey;
     if (!secp256k1_ec_pubkey_parse(secp256k1_context_static, &pubkey, vch, size())) {
         return false;
@@ -409,13 +412,13 @@ void CExtPubKey::DecodeWithVersion(const unsigned char code[BIP32_EXTKEY_WITH_VE
     Decode(&code[4]);
 }
 
-bool CExtPubKey::Derive(CExtPubKey &out, unsigned int _nChild) const {
+bool CExtPubKey::Derive(CExtPubKey &out, unsigned int _nChild, uint256* bip32_tweak_out) const {
     if (nDepth == std::numeric_limits<unsigned char>::max()) return false;
     out.nDepth = nDepth + 1;
     CKeyID id = pubkey.GetID();
     memcpy(out.vchFingerprint, &id, 4);
     out.nChild = _nChild;
-    return pubkey.Derive(out.pubkey, out.chaincode, _nChild, chaincode);
+    return pubkey.Derive(out.pubkey, out.chaincode, _nChild, chaincode, bip32_tweak_out);
 }
 
 /* static */ bool CPubKey::CheckLowS(const std::vector<unsigned char>& vchSig) {
diff --git a/src/pubkey.h b/src/pubkey.h
index 442dc2d6431..5ae7f75dde9 100644
--- a/src/pubkey.h
+++ b/src/pubkey.h
@@ -224,7 +224,7 @@ public:
     bool Decompress();
 
     //! Derive BIP32 child pubkey.
-    [[nodiscard]] bool Derive(CPubKey& pubkeyChild, ChainCode &ccChild, unsigned int nChild, const ChainCode& cc) const;
+    [[nodiscard]] bool Derive(CPubKey& pubkeyChild, ChainCode &ccChild, unsigned int nChild, const ChainCode& cc, uint256* bip32_tweak_out = nullptr) const;
 };
 
 class XOnlyPubKey
@@ -379,7 +379,7 @@ struct CExtPubKey {
     void Decode(const unsigned char code[BIP32_EXTKEY_SIZE]);
     void EncodeWithVersion(unsigned char code[BIP32_EXTKEY_WITH_VERSION_SIZE]) const;
     void DecodeWithVersion(const unsigned char code[BIP32_EXTKEY_WITH_VERSION_SIZE]);
-    [[nodiscard]] bool Derive(CExtPubKey& out, unsigned int nChild) const;
+    [[nodiscard]] bool Derive(CExtPubKey& out, unsigned int nChild, uint256* bip32_tweak_out = nullptr) const;
 };
 
 #endif // BITCOIN_PUBKEY_H