Calculating CRC16 in Python

python python-2.7 crc16
Qrlet picture Qrlet · Feb 4, 2016 · Viewed 40.1k times · Source

I'm trying to evaluate appropriate checksum based on CRC-16 algorithm using crcmod Python module and 2.7 version of Python interpreter. The checksum parameters are:

  • CRC order: 16
  • CRC polynomial: 0x8005
  • Inital value: 0xFFFF
  • Final value: 0x0000
  • Direct: True

Code:

crc16 = crcmod.mkCrcFun(0x18005, rev=False, initCrc=0xFFFF, xorOut=0x0000)
print hex(crc16(str(int(0x5A0001))))

and for the input 0x5A0001 it prints 0x7E16 while I should get something like 0xCE0A.

I checked on http://www.lokker.net/Java/crc/CRCcalculation2.htm and the computed value is 0xACE which is correct (with respect to the order).

Answer

Mark Adler picture Mark Adler · Feb 5, 2016

crcmod is working fine. You are not giving it the three bytes you think you are giving it. Your str(int(0x5A0001)) is providing seven bytes, which are the ASCII characters 5898241 — the conversion of 0x5a0001 to decimal.

To feed it the bytes 0x5a 0x00 0x01, you would instead (as one approach):

print hex(crc16("5a0001".decode("hex")))

That prints 0xace.

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