Delta checksum approach

bin/checksum_from_scapy.py

@@ -2,6 +2,8 @@
 
 # Code copied from /usr/lib/python3/dist-packages/scapy/utils.py
 
+import struct
+import array
 
 if struct.pack("H",1) == b"\x00\x01": # big endian
     def checksum(pkt):
@@ -14,10 +16,52 @@ if struct.pack("H",1) == b"\x00\x01": # big endian
         return s & 0xffff
 else:
     def checksum(pkt):
+        # padding to even length
         if len(pkt) % 2 == 1:
             pkt += b"\0"
+
+        # create array with unsigned shorts,
+        # sum it up -> results in 16 bit uint (?)
         s = sum(array.array("H", pkt))
+
+        # (s & 0xffff) -> mask for 16 bit, truncate rest
+        # (s >> 16) right shift by 16 bits
+        # >>> 2**17 >> 16
+        # 2
+        # Basically:
+        # add higher 16 bits to lower 16 bits
+
         s = (s >> 16) + (s & 0xffff)
+
+        # Now we add the remaining possible 16 bits
+        # i.e. above we take bits 0..15 + 16..31
+        # now we add [0..15 + 16..31] + "32..47"
+        # I assume this is for catching the carry
+        # over bits from the previous calculation
         s += s >> 16
+
+
+        # according to https://stackoverflow.com/questions/8305199/the-tilde-operator-in-python
+        # ~ is the bitwise complement operator in python
+        # which essentially calculates -x - 1
+
+        # 2 complement
         s = ~s
+
+        # (s>>8)&0xff) -> filter bits 8..15
+        # | = bitwise OR
+        # s<<8 : left shift everything in original s
+        # | s<<8: ?
+        # & 0xffff: filter / allow only 16 bits
+        # Seems like the idea is to filter correctly for
+        # 16 bit - unclear why "s & 0xffff" does not do the job
+
         return (((s>>8)&0xff)|s<<8) & 0xffff
+
+
+if __name__ == '__main__':
+    import sys
+    pkt = sys.argv[1]
+    pkt_bytes = pkt.encode('utf-8')
+    print(pkt_bytes)
+    print(checksum(pkt_bytes))