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### XOR'ing to calculate a hex checksum

by c4onastick (Friar)
 on Jan 28, 2008 at 17:13 UTC ( #664712=perlquestion: print w/ replies, xml ) Need Help??
c4onastick has asked for the wisdom of the Perl Monks concerning the following question:

Greetings Monks!

I need to calculate the hex checksum of a bunch of digits to talk to a piece of equipment at work. According to the equipment manufacturers documented serial protocol, the checksum is the "bitwise inversion (XOR) of the one byte sum with FF hex".

Here's what I have so far, it seems to work fine except for the XOR operation, so my question is two-fold:

1. Can this be done all in hex (so I don't have to keep jumping back and forth between hex, decimal and binary)?
2. How can I modify this to correctly XOR the checksum and FF hex?

Code:

```#! C:\perl\bin\perl.exe

use strict;
use warnings;

print checksum('000181080002020202020202');

sub checksum {
my \$region = shift;
\$region =~ s/(\w{2})(?!\$)/\$1,/g;
my @region_array = split(/,/, \$region);
my \$total = 0;
foreach(@region_array){
\$total += hex(\$_);
}
print "Total decimal: \$total\n";
my \$total_hex = sprintf('%X', \$total);
print "Total Hex: \$total_hex\n";
my (\$lsb) = \$total_hex =~ /([a-zA-Z0-9]{2}\$)/;
print "LSB: \$lsb\n";
my \$cs_bin = \$lsb ^ 0xff;
my \$cs_int = sprintf('%d', \$cs_bin);
print "CS Decimal: \$cs_int\n";
my \$cs_hex = sprintf('%X', \$cs_int);
return \$cs_hex;
}

Which returns:

```Total decimal: 152
Total Hex: 98
LSB: 98
CS Decimal: 157
9D

From my hand calculations (and I could be mistaken here, it's been a while since I've used anything besides base-10), the check sum for this command should be 67 hex (103 decimal) but it's comming up as 157 decimal. Any insights on this problem would be greatly appreciated! Also, any comments on cleaning up this script would be great too (it's pretty messy currently). Thanks in advance!

Comment on XOR'ing to calculate a hex checksum
Replies are listed 'Best First'.
Re: XOR'ing calculate a hex checksum
by halley (Prior) on Jan 28, 2008 at 17:32 UTC
Don't do math in terms of digits or strings. Do math in terms of numbers. Digits are just a way of viewing numbers, they aren't numbers themselves.

You appear to be converting all your incoming bytes into hex before we even get to look at it in your checksum() routine, and then going through some painful conversion work to turn the pairs of hex digits back into bytes. I'm going to ignore that, and assume you have an array of @bytes instead.

```my \$total = 0;
\$total += \$_ for @bytes;
\$check = (\$total & 0xFF) ^ 0xFF;

Once you have your check value, then you can decide how to display it (if it needs displaying at all).

--
[ e d @ h a l l e y . c c ]

Re: XOR'ing calculate a hex checksum
by BrowserUk (Pope) on Jan 28, 2008 at 17:54 UTC
Re: XOR'ing to calculate a hex checksum
by thospel (Hermit) on Jan 28, 2008 at 23:28 UTC
Unpack has the little known property to be able to calculate sums in several bit widths. In this case first convert the hexadecimal representation to a character string, sum over that in 8 bits and invert:
```perl -wle 'print 0xff ^ unpack "%8C*", pack"H*", "00018108000202020202
+0202"'
103
Re: XOR'ing calculate a hex checksum
by kyle (Abbot) on Jan 28, 2008 at 17:33 UTC
```use List::Util qw( sum );
my \$input = '000181080002020202020202';
my \$sum = sum map { hex } split /([a-f0-9]{2})/, \$input;
my \$xored_sum = 0xff ^ \$sum;
printf "hex of %d: %X\n", \$xored_sum, \$xored_sum;
__END__
hex of 103: 67

I think what's going on in your code is that \$lsb is a string, and it gets treated that way when you try to XOR it.

If you want the least significant byte of your sum (as it appears you do), you could just say \$sum %= 256 without having to do a hex conversion.

Re: XOR'ing calculate a hex checksum
by moritz (Cardinal) on Jan 28, 2008 at 17:26 UTC
I'd suggest to use pack to create a binary string, work with that, and then use unpack to return to the format you want.

See perlpacktut for a gentler introduction.

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