CRC implementation in python

Question

Here is my code for the implementation of a CRC in python:

import math

divisors  = [0b1100000001111, 0b11000000000000101, 0b10001000000100001, 0b1011, 0b10011,0b00000111, 0b11001]

def get_Length(arg):
    return math.floor(math.log2(arg)) +1

def CRC(message, type):
    print("Message ",bin(message)[2:], hex(message))
    # int message_length = get_Length(message);
    divisor_length = get_Length(divisors[type])
    divisor = divisors[type]
    print("Divisor: ",bin(divisor)[2:], hex(divisor))
    message = message << (divisor_length-1)

    old_message = message
    while( (message >> (divisor_length-1)) !=0 ):
        ml = get_Length(message)

        divisor_copy = divisor << (ml-divisor_length)

        message = message ^ divisor_copy

    print(bin(message)[2:], hex(message))
    print(bin(old_message| message)[2:], hex(old_message|message), end="\n\n")

def main():
   CRC(0b1101011011, 4)
   CRC(0x34ec, 1)
main()

The first message is from this Wikipedia example and gives the correct result. However the second one (x34ec), which demonstrates a CRC-16 is not giving the correct result (correct result). I'm attaching the output snapshot as well:

.

It would be appreciative if somebody can shed some light on it.

Thanks in advance.

Your result matches the CRC-16/BUYPASS result in that table. — Tim Roberts, Mar 09 '21 at 21:20
@TimRoberts What about the ARC? Also, another question what is the significance of the Check column in the link given above? — Saptarshi Sahoo, Mar 09 '21 at 21:32
I don't know what "Check" means. You can look at the web page. They send each one to a web service for the computation. — Tim Roberts, Mar 09 '21 at 21:43
Does this help: https://stackoverflow.com/questions/55179867/crc16-modbus-computing-algorithm — rfkortekaas, Mar 09 '21 at 21:48
Yes, thanks -- the "Check" column is the result of running the checksum on the ASCII characters "123456789". — Tim Roberts, Mar 09 '21 at 21:51
As for ARC, you have implemented the CRC-16/BUYPASS algorithm. If you wanted CRC-16/ARC, then you did it wrong. — Tim Roberts, Mar 09 '21 at 21:52
@Tim could please tell what are the extra steps do I need to do, to go from CRC-16/BUYPASS to ARC? — Saptarshi Sahoo, Mar 09 '21 at 22:07
You can use Google just as easily as I can. ARC has the same polynomial as BUYPASS, but the bits are reversed on the way in, and on the way out. — Tim Roberts, Mar 09 '21 at 22:16

score 1 · Accepted Answer · answered Mar 10 '21 at 04:06

There are many CRC-16's. I count 30 here alone, and I'm sure there are more in use that are not in that catalog.

The one you have implemented is the CRC-16/UMTS in that catalog, also known as CRC-16/BUYPASS and CRC-16/VERIFONE. It is the most straightforward of definitions, with the bits in and bits out not reflected, with an initial value of zero, and with no final exclusive-or.

The result CRC you implemented on the message 34 ec can in fact be found directly in your linked "correct result", on the fourth row of the table which is labeled "CRC-16/BUYPASS".

If you want to implement a different CRC-16, the first thing you need to do is specify which one. That specification is the polynomial, reflection of input and output, initial value, and final exclusive-or value.

CRC implementation in python

1 Answers1