Hexadecimal Computations in Perl - perl

Question

I have a string of ASCII characters. I convert this to hex string using the unpack function.
#! /usr/bin/perl
use strict;
use warnings;
my $str="hello";
my $value=unpack("H*",$str);
print $value,"\n";
**output:** 68656c6c6f
Now, lets say, I want to use this output as a string of hex bytes, read one byte at a time and perform some computation on it and store the output in another variable.
For instance,
#! /usr/bin/perl
use strict;
use warnings;
my $str="hello";
my $value=unpack("H*",$str);
my $num=0x12;
my $i=0;
while($i<length($value))
{
my $result.=(substr($value,$i,2)^$num);
$i+=2;
}
print $result,"\n";
**output:**
Argument "6c" isn't numeric in bitwise xor (^) at test.pl line 13.
Argument "6c" isn't numeric in bitwise xor (^) at test.pl line 13.
Argument "6f" isn't numeric in bitwise xor (^) at test.pl line 13.
8683202020
The output is incorrect and also there are several warnings.
If we take the first hex byte of the string, "hello" as an example:
68 xor 12 = 7A
However, the output shows it as 86. The output is incorrect and also I am not sure how
it got an output of 86.
What is the right way to do it?

Answer 1

If something is in hex, it is necessarily a string, since hex is a human-readable representation of a number. You don't want a string; you want a series of numbers, where each of those numbers is the numerical value of the char. You could use ord to get the number character by character, but unpack also provides the means:
my #bytes = unpack 'C*', $str;
Do the processing you want:
$_ ^= $num for #bytes;
And reconstitute the string:
$str = pack 'C*', #bytes;
The above three combined:
$str = pack 'C*', map $_ ^ $num, unpack 'C*', $str;
You can also do it as follows:
my $mask = chr($num) x length($str);
$str ^= $mask;