# manipulating hex values

Stephen Cattaneo stephen.cattaneo at u4eatech.com
Tue Apr 1 19:11:31 CEST 2008

```Hi all,

I am relatively new to socket programming.  I am attempting to use raw
sockets to spoof my IP address.  From what I can tell I will have to
build from the Ethernet layer on up.   This is fine, but I am having
some trouble with manipulating my hex values.

Seems to me that there are two ways to store hex values:
1. as literal hex - 0x55aa
2. as a string - "\x55aa"

If I want to convert hex to decimal I can use:
int("\x55aa", 16)  # note that plain 0x55aa, instead of "\x55aa", will
raise an exception

The Question:
If I want to do any kind of calculation I have found its best to just
convert my values to decimal, do the math, then convert back to hex.  In
my bellow code I get
"""byteList.append(int(value,16))
ValueError: invalid literal for int()"""
when attempting to run.   I do not understand why this exception is
being raised?  It is a for loop iterating over a list of hex strings.
Sorry for the long-ish question.  Any help or comments would be appreciated.

----my checksum---
byteList = []
# convert groups from hex to dec
byteList.append(int(value,16))  # Exception raised here!

# 1's compliment
for index in range(len(byteList)):
byteList[index] = abs(byteList[index] - 65535)

# add bytes together shifting the extra byte
total = 0
for value in byteList:
total = total + value
if total > 65535:
total = total - 65535

return hex(total)

checksum = calcIPChecksum(["\x45" + "\x00", "\x01\x4a", "\x00\x00",
"\x40"+"\x00", "\x20"+"\x11"])
-------------------------

Cheers,

Steve

```