net: Consistent checksum handling

In principle, the checksums of P2P packets are simply 4-byte blobs which
are the first four bytes of SHA256(SHA256(payload)).

Currently they are handled as little-endian 32-bit integers half of the
time, as blobs the other half, sometimes copying the one to the other,
resulting in somewhat confused code.

This PR changes the handling to be consistent both at packet creation
and receiving, making it (I think) easier to understand.

src/main.cpp

@@ -6253,11 +6253,12 @@ bool ProcessMessages(CNode* pfrom, CConnman& connman)
         // Checksum
         CDataStream& vRecv = msg.vRecv;
         uint256 hash = Hash(vRecv.begin(), vRecv.begin() + nMessageSize);
-        unsigned int nChecksum = ReadLE32((unsigned char*)&hash);
-        if (nChecksum != hdr.nChecksum)
+        if (memcmp(hash.begin(), hdr.pchChecksum, CMessageHeader::CHECKSUM_SIZE) != 0)
         {
-            LogPrintf("%s(%s, %u bytes): CHECKSUM ERROR nChecksum=%08x hdr.nChecksum=%08x\n", __func__,
-               SanitizeString(strCommand), nMessageSize, nChecksum, hdr.nChecksum);
+            LogPrintf("%s(%s, %u bytes): CHECKSUM ERROR expected %s was %s\n", __func__,
+               SanitizeString(strCommand), nMessageSize,
+               HexStr(hash.begin(), hash.begin()+CMessageHeader::CHECKSUM_SIZE),
+               HexStr(hdr.pchChecksum, hdr.pchChecksum+CMessageHeader::CHECKSUM_SIZE));
             continue;
         }
 

src/net.cpp

@@ -2661,10 +2661,8 @@ void CNode::EndMessage(const char* pszCommand) UNLOCK_FUNCTION(cs_vSend)
 
     // Set the checksum
     uint256 hash = Hash(ssSend.begin() + CMessageHeader::HEADER_SIZE, ssSend.end());
-    unsigned int nChecksum = 0;
-    memcpy(&nChecksum, &hash, sizeof(nChecksum));
-    assert(ssSend.size () >= CMessageHeader::CHECKSUM_OFFSET + sizeof(nChecksum));
-    memcpy((char*)&ssSend[CMessageHeader::CHECKSUM_OFFSET], &nChecksum, sizeof(nChecksum));
+    assert(ssSend.size () >= CMessageHeader::CHECKSUM_OFFSET + CMessageHeader::CHECKSUM_SIZE);
+    memcpy((char*)&ssSend[CMessageHeader::CHECKSUM_OFFSET], hash.begin(), CMessageHeader::CHECKSUM_SIZE);
 
     LogPrint("net", "(%d bytes) peer=%d\n", nSize, id);
 

src/protocol.cpp

@@ -79,7 +79,7 @@ CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn)
     memcpy(pchMessageStart, pchMessageStartIn, MESSAGE_START_SIZE);
     memset(pchCommand, 0, sizeof(pchCommand));
     nMessageSize = -1;
-    nChecksum = 0;
+    memset(pchChecksum, 0, CHECKSUM_SIZE);
 }
 
 CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn, const char* pszCommand, unsigned int nMessageSizeIn)
@@ -88,7 +88,7 @@ CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn, const
     memset(pchCommand, 0, sizeof(pchCommand));
     strncpy(pchCommand, pszCommand, COMMAND_SIZE);
     nMessageSize = nMessageSizeIn;
-    nChecksum = 0;
+    memset(pchChecksum, 0, CHECKSUM_SIZE);
 }
 
 std::string CMessageHeader::GetCommand() const

src/protocol.h

@@ -45,7 +45,7 @@ public:
         READWRITE(FLATDATA(pchMessageStart));
         READWRITE(FLATDATA(pchCommand));
         READWRITE(nMessageSize);
-        READWRITE(nChecksum);
+        READWRITE(FLATDATA(pchChecksum));
     }
 
     // TODO: make private (improves encapsulation)
@@ -62,7 +62,7 @@ public:
     char pchMessageStart[MESSAGE_START_SIZE];
     char pchCommand[COMMAND_SIZE];
     unsigned int nMessageSize;
-    unsigned int nChecksum;
+    uint8_t pchChecksum[CHECKSUM_SIZE];
 };
 
 /**