How to get a CCITT-CRC16 on Python's crcmod lybrary?

Question

I'm having problems to make a code in Python 3.4 using the CRCMOD library to get the CCITT CRC16 check.

Thats my string:

a731986b1500087f9206e82e3829fe8bcffed5555efd00a100980000010000000100000009010013bb1d001e287107009b3000000300000088330000f427500077026309

The spected crc value is 1d7f

My code:

import crcmod

crc16 = crcmod.mkCrcFun(0x11021, 0x1d0f, False, 0x0000)

hex(crc16(b'a731986b1500087f9206e82e3829fe8bcffed5555efd00a100980000010000000100000009010013bb1d001e287107009b3000000300000088330000f427500077026309'))

It returns: 7d67

What am I doing wrong?

Are you sure the initial string isn't just a hex representation of binary data ? (i.e. you'll have to convert the hex to binary before calculating the crc) — nos, Jun 16 '14 at 13:45
Yes, man. I saw it now... have you a tip for me with how can I convert it into HEX??? I try hex('ba7.....) but it doesn't work. — SnowBG, Jun 16 '14 at 14:19

score 7 · Accepted Answer · answered Jun 16 '14 at 15:14

7

You first need to convert the data from its hex representation to binary. You also need to use the correct CRC algorithm, which I think is "xmodem" - crcmod.mkCrcFun(0x11021, 0x0000, False, 0x0000)

import crcmod.predefined
from binascii import unhexlify

s = unhexlify('a731986b1500087f9206e82e3829fe8bcffed5555efd00a100980000010000000100000009010013bb1d001e287107009b3000000300000088330000f427500077026309')

crc16 = crcmod.predefined.Crc('xmodem')
crc16.update(s)
print crc16.hexdigest()

Outputs 7F1D (which is what you expected but with the bytes reversed)

answered Jun 16 '14 at 15:14

mhawke

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@Uri: you are correct for Python 2, however, the OP has tagged the question as Python 3 which does not support direct hex decoding of strings. – mhawke May 15 '16 at 22:45
why the ouput is reversed? how I get the real ouput 1d7f? – eduardosufan Jul 25 '17 at 19:07

How to get a CCITT-CRC16 on Python's crcmod lybrary?

1 Answers1