72 lines
2.0 KiB
Python
72 lines
2.0 KiB
Python
#!/usr/bin/env python3
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# Code copied from /usr/lib/python3/dist-packages/scapy/utils.py
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import struct
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import array
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if struct.pack("H",1) == b"\x00\x01": # big endian
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def checksum(pkt):
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if len(pkt) % 2 == 1:
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pkt += b"\0"
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s = sum(array.array("H", pkt))
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s = (s >> 16) + (s & 0xffff)
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s += s >> 16
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s = ~s
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return s & 0xffff
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else:
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def checksum(pkt):
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# padding to even length
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if len(pkt) % 2 == 1:
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pkt += b"\0"
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# create array with unsigned shorts,
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# sum it up -> results in 16 bit uint (?)
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s = sum(array.array("H", pkt))
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print("sum={}".format(s))
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# This step is understood
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# (s & 0xffff) -> mask for 16 bit, truncate rest
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# (s >> 16) right shift by 16 bits
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# >>> 2**17 >> 16
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# 2
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# Basically:
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# add higher 16 bits to lower 16 bits
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s = (s >> 16) + (s & 0xffff)
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# Now we add the remaining possible 16 bits
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# i.e. above we take bits 0..15 + 16..31
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# now we add [0..15 + 16..31] + "32..47"
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# I assume this is for catching the carry
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# over bits from the previous calculation
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s += s >> 16
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# according to https://stackoverflow.com/questions/8305199/the-tilde-operator-in-python
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# ~ is the bitwise complement operator in python
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# which essentially calculates -x - 1
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# 2 complement
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s = ~s
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# (s>>8)&0xff) -> filter bits 8..15
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# | = bitwise OR
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# s<<8 : left shift everything in original s
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# | s<<8: ?
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# & 0xffff: filter / allow only 16 bits
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# Seems like the idea is to filter correctly for
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# 16 bit - unclear why "s & 0xffff" does not do the job
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return (((s>>8)&0xff)|s<<8) & 0xffff
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if __name__ == '__main__':
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import sys
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pkt = sys.argv[1]
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pkt_bytes = pkt.encode('utf-8')
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print(pkt_bytes)
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print(checksum(pkt_bytes))
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ipv4 = "AAAA"
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ipv6 = "BBAA"
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