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.

4 files changed, 11 insertions, 12 deletions

diff --git a/src/main.cpp b/src/main.cpp
index c33b41ac4e..ab67219714 100644
--- a/src/main.cpp
+++ b/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;
         }
 
diff --git a/src/net.cpp b/src/net.cpp
index cce06f2d64..770e2d2959 100644
--- a/src/net.cpp
+++ b/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);
 
diff --git a/src/protocol.cpp b/src/protocol.cpp
index 247c6c2120..54ad62b1a2 100644
--- a/src/protocol.cpp
+++ b/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
diff --git a/src/protocol.h b/src/protocol.h
index 9b474ec79c..177d745308 100644
--- a/src/protocol.h
+++ b/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];
 };
 
 /**