Python network convert byte

Question

I'm trying to implement a CRC verification on a network based protocol. CRC calculation is done via the PyCRC lib.

PyCRC will generate a checksum for the given packet and return a result like this: CB3D9FD1

When I try to send it on the wire, somehow python modify my string and convert it in ascii to Hex, example:

Crc = "CB3D9FD1"
...
buffer = "\x41\x42\x43\x44"+crc
...
s.send(buffer)

On the wire I will see that:

\x41\x42\x43\x44***\x43\x42\x33\x44\x39\x46\x44\x31***

Instead of:

 \x41\x42\x43\x44***\xCB\x3D\x9F\xD1***

Any ways to solve this issue easily ?

Thanks !

Answer 1

Actually, don't convert your CRC to ASCII. Keep it in hex and then pack it into a byte string using struct.pack::

buffer += struct.pack("L", crc)

Answer 2

If you have converted the CRC to a string containing "CB3D9FD1", then the correct byte sequence is 0x43 0x42 0x33 0x44 0x39 0x46 0x44 0x31. Don't convert the cry into a string or parse the string as a hex (two characters to each byte).

Answer 3

By assigning it this way: crc = "CB3D9FD1" you are saying make var crc an 8 byte string. As @Bashwork says you need to convert it from hex string to numeric or keep it numeric. Assign it with 0x like this to get a numeric value: crc=0xCB3D9FD1.

pack("L",x) means I am giving you x which is(must be) an integer and I want you to pack it as an unsigned long.
You get "struct.error: cannot convert argument to integer" because you are passing pack("L",...) a string "CB3D9FD1".

RTFM what "L" and "I" and other args mean: https://docs.python.org/2/library/struct.html

Pack the CRC as long integer works: and you get 0s padding out to 64bits (on my PC. YMMV)

>>> from struct import pack
>>> crc=0xcb3d9fd1
>>> repr(crc)
'3409813457'
>>> pack("L", crc)
'\xd1\x9f=\xcb\x00\x00\x00\x00'
>>> print crc
3409813457

Maybe you want to use pack("I",...): to get 32bits

>>> pack("I",crc)
'\xd1\x9f=\xcb'

Fail if try to pack a CRC string with pack("L",...):

>>> crc = "CB3D9FD1"
>>> repr(crc)
"'CB3D9FD1'"
>>> pack("L", crc)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: cannot convert argument to integer