Merge bitcoin/bitcoin#23199: refactor: use {Read,Write}BE32 helpers for BIP32 nChild (de)serialization

7fc487a refactor: use `{Read,Write}BE32` helpers for BIP32 nChild (de)serialization (Sebastian Falbesoner)

Pull request description:

  This small refactoring PR replaces manual bit-fiddling (de)serialization of the BIP32 child number (nChild) by the helpers `ReadBE32`/`WriteBE32`. Note that those were first introduced in #4100, almost one year _after_ the BIP32 derivation implementation has been merged (#2829, eb2c999).

ACKs for top commit:
  sipa:
    utACK 7fc487a
  laanwj:
    Code review ACK 7fc487a

Tree-SHA512: bbe3e411fb0429fa74c8a5705a91f4d6ed704dac9d6623ecb633563f22acf8e21f3189a16f1d0cf1aeedfc56a5b695df54ae51e9577e34eb6d7dc335de2da6de

Author: fanquake

src/hash.cpp

@@ -75,10 +75,7 @@ unsigned int MurmurHash3(unsigned int nHashSeed, Span<const unsigned char> vData
 void BIP32Hash(const ChainCode &chainCode, unsigned int nChild, unsigned char header, const unsigned char data[32], unsigned char output[64])
 {
     unsigned char num[4];
-    num[0] = (nChild >> 24) & 0xFF;
-    num[1] = (nChild >> 16) & 0xFF;
-    num[2] = (nChild >>  8) & 0xFF;
-    num[3] = (nChild >>  0) & 0xFF;
+    WriteBE32(num, nChild);
     CHMAC_SHA512(chainCode.begin(), chainCode.size()).Write(&header, 1).Write(data, 32).Write(num, 4).Finalize(output);
 }
 

src/key.cpp

@@ -343,8 +343,7 @@ CExtPubKey CExtKey::Neuter() const {
 void CExtKey::Encode(unsigned char code[BIP32_EXTKEY_SIZE]) const {
     code[0] = nDepth;
     memcpy(code+1, vchFingerprint, 4);
-    code[5] = (nChild >> 24) & 0xFF; code[6] = (nChild >> 16) & 0xFF;
-    code[7] = (nChild >>  8) & 0xFF; code[8] = (nChild >>  0) & 0xFF;
+    WriteBE32(code+5, nChild);
     memcpy(code+9, chaincode.begin(), 32);
     code[41] = 0;
     assert(key.size() == 32);
@@ -354,7 +353,7 @@ void CExtKey::Encode(unsigned char code[BIP32_EXTKEY_SIZE]) const {
 void CExtKey::Decode(const unsigned char code[BIP32_EXTKEY_SIZE]) {
     nDepth = code[0];
     memcpy(vchFingerprint, code+1, 4);
-    nChild = (code[5] << 24) | (code[6] << 16) | (code[7] << 8) | code[8];
+    nChild = ReadBE32(code+5);
     memcpy(chaincode.begin(), code+9, 32);
     key.Set(code+42, code+BIP32_EXTKEY_SIZE, true);
     if ((nDepth == 0 && (nChild != 0 || ReadLE32(vchFingerprint) != 0)) || code[41] != 0) key = CKey();

src/pubkey.cpp

@@ -337,8 +337,7 @@ bool CPubKey::Derive(CPubKey& pubkeyChild, ChainCode &ccChild, unsigned int nChi
 void CExtPubKey::Encode(unsigned char code[BIP32_EXTKEY_SIZE]) const {
     code[0] = nDepth;
     memcpy(code+1, vchFingerprint, 4);
-    code[5] = (nChild >> 24) & 0xFF; code[6] = (nChild >> 16) & 0xFF;
-    code[7] = (nChild >>  8) & 0xFF; code[8] = (nChild >>  0) & 0xFF;
+    WriteBE32(code+5, nChild);
     memcpy(code+9, chaincode.begin(), 32);
     assert(pubkey.size() == CPubKey::COMPRESSED_SIZE);
     memcpy(code+41, pubkey.begin(), CPubKey::COMPRESSED_SIZE);
@@ -347,7 +346,7 @@ void CExtPubKey::Encode(unsigned char code[BIP32_EXTKEY_SIZE]) const {
 void CExtPubKey::Decode(const unsigned char code[BIP32_EXTKEY_SIZE]) {
     nDepth = code[0];
     memcpy(vchFingerprint, code+1, 4);
-    nChild = (code[5] << 24) | (code[6] << 16) | (code[7] << 8) | code[8];
+    nChild = ReadBE32(code+5);
     memcpy(chaincode.begin(), code+9, 32);
     pubkey.Set(code+41, code+BIP32_EXTKEY_SIZE);
     if ((nDepth == 0 && (nChild != 0 || ReadLE32(vchFingerprint) != 0)) || !pubkey.IsFullyValid()) pubkey = CPubKey();