Calculating CRC16 in Python

Question

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 1

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.

Answer 2

Here is a python implementation of CRC-16/CCITT-FALSE

def crc16(data : bytearray, offset , length):
    if data is None or offset < 0 or offset > len(data)- 1 and offset+length > len(data):
        return 0
    crc = 0xFFFF
    for i in range(0, length):
        crc ^= data[offset + i] << 8
        for j in range(0,8):
            if (crc & 0x8000) > 0:
                crc =(crc << 1) ^ 0x1021
            else:
                crc = crc << 1
    return crc & 0xFFFF

• data : bytearray of the data you want to calculate CRC for
• offset : from which offset you want to start calculating CRC
• length : to which offset you want to calculate CRC