CODEWITHSUNDEEP
pythoncrc32
SangminKim
SangminKim

Reputation: 9166

Python binascii.crc32 seems not working properly

I am trying to CRC16/32 checksum with binascii.crc32, binacsii.crc_hqx

This is the values I used as an example(I got this example values from some network protocol specification I am reading).

  1. CRC 16

    input(as ASCII string):123456789

    seed:0x0000

    hash result:0x31c3

  2. CRC 32

    input(as ASCII string):123456789

    seed:0x00000000

    hash result:0xd202d277

Below is my test code with above test values.

>>> import binascii
>>> hex(binascii.crc_hqx("123456789", 0x0000))
'0x31c3'   #corret result, same as test value result

>>> hex(binascii.crc32("123456789", 0x00000000) & 0xFFFFFFFF)
'0xcbf43926L'#Wrong result, different value from test value result, It Must be 0xd202d277

What is a problem...?

Actually I found this sentence in Python doc Since the algorithm is designed for use as a checksum algorithm, it is not suitable for use as a general hash algorithm.

Does it mean I can't use it for CRC32 checksum?

If so, Is there any recommendation?

----------------------Edit----------------------

I didn't make the test values myself.

I Just got the values from a document explaining CRC32.

Below is the table I referenced

enter image description here

----------------------Edit----------------------

In the document containing the figure table, It describes G2S_crc like below.

The CRC-32 polynomial defined for G2S can be described as follows:
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1
It is also represented as 0x04C11DB7. This polynomial is also used by IEEE 802.3.

.

function crc(bit array bitString[1..len], int polynomial)
{
shiftRegister := initial value // either the seed or all 1 bits
for i from 1 to len
{
if most significant bit of shiftRegister = 1
{
// Shift left, place data bit as LSB, then divide
shiftRegister := shiftRegister left shift 1
shiftRegister := shiftRegister or bitString[i]
shiftRegister := shiftRegister xor polynomial
}
else
{
// shiftRegister is not divisible by polynomial yet.
// Just shift left and bring current data bit onto LSB of shiftRegister
shiftRegister := shiftRegister left shift 1
shiftRegister := shiftRegister or bitString[i]
}
}
return shiftRegister
}

Is it different from binascii?

Upvotes: 1

Views: 6834

Answers (2)

foudfou
foudfou

Reputation: 1076

From this CRC catalog, you seem to be using CRC-32/MPEG-2. Whereas python uses plain CRC32, with polynom 0xEDB88320.

BTW, the CRC returned by python is an int. And python's int are signed. If you need the unsigned representation, you need to invert the result, like hex(~0x76e943d9 & 0xffffffff).

Upvotes: 2

John La Rooy
John La Rooy

Reputation: 304463

This online calculator

http://www.lammertbies.nl/comm/info/crc-calculation.html

agrees with Python

enter image description here

Can you quote the source you got 0xd202d277 from? There are variations regarding bit order etc.

Upvotes: 4

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