strings to number and XOR

[alager]

I would like to xor every two bytes in an arbitrary string with the same value, or key. Here is one things I've tried, but the result is not correct. For the first two characters (th) xor I expect it to be 0x3ba3, but I'm getting 0x52e7, I can't figure out what is going wrong.

#!/usr/bin/perl
  2
  3 $msg = "this is a test messagez";
  4 $key = 0x4fcb;
  5 print "$msg\n";
  6
  7 @array = split(/(..)/, $msg);
  8
  9 
 10 printf "key: %x\n", $key;
 11
 12 $xitem =  unpack("H*", $array[1]) ^ $key;
 13 print  "item: $array[1] = hex " . unpack("H*", $array[1]) . "\n";
 14 printf ("xored item \'%x\', want 0x3ba3\n", $xitem);

Thanks, Aaron

[almut]

$msg = "this is a test messagez";
$key = "\x4f\xcb";   # or  $key = pack "n", 0x4fcb;

$xored = $msg ^ $key;

print join("  ", split(//, $msg)), "\n";
print join(" ", unpack("(H2)*", $msg)),   "\n";
print join(" ", unpack("(H2)*", $key)),   "\n";
print join(" ", unpack("(H2)*", $xored)), "\n";

__END__
t  h  i  s     i  s     a     t  e  s  t     m  e  s  s  a  g  e  z
74 68 69 73 20 69 73 20 61 20 74 65 73 74 20 6d 65 73 73 61 67 65 7a
4f cb
3b a3 69 73 20 69 73 20 61 20 74 65 73 74 20 6d 65 73 73 61 67 65 7a

The easiest way to generate your key string for the entire message is probably something like

    $key = "\x4f\xcb" x (length($msg)/2);
or
    $key = pack("n", 0x4fcb) x (length($msg)/2);

That way you can directly XOR both strings without any further ado:

t  h  i  s     i  s     a     t  e  s  t     m  e  s  s  a  g  e  z
74 68 69 73 20 69 73 20 61 20 74 65 73 74 20 6d 65 73 73 61 67 65 7a  
+ $msg
4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb 4f cb     
+ $key
3b a3 26 b8 6f a2 3c eb 2e eb 3b ae 3c bf 6f a6 2a b8 3c aa 28 ae 7a  
+ XOR result

___

 $xitem =  unpack("H*", $array[1]) ^ $key;

What this is doing is XORing the hex-string "7468" (i.e. 4 ASCII chars, hex 37 34 36 38) with the Perl internal representation of the number 0x4fcb ...

[Anonymous Monk]

Brilliant! Pouring over my books I just couldn't grasp it. You made it crystal clear! Thanks, Aaron

[alager]

Last question, then I should be done. :)
For the variable
$key = "\x4f\xcb"; How can I increment it?
I've tried directly with $key++, but that starts counting at 0, I would expect \x4f\xcc as the result.

And I've tried unpacking it. Which starts incrementing with the first number (4 in this case) and loses the rest of the data. eg 4,5,6...
I would like
0x4fcb 0x4fcc 0x4fcd ect.

Thanks,
Aaron

UPDATE: I've found part of the problem, perl doesn't like to add numbers that aren't "numbers", this works fine as long as my bytes are in the range of ASCII numbers, but fails for values above outside of 0x30-0x39.

[almut]

I've tried directly with $key++

The problem with Perl's string-increment is that the string has to match the pattern /^[a-zA-Z]*[0-9]*\z/ for it to work...(see perlop).  So you'd have to increment the corresponding numeric value, e.g.

$key = "\x4f\xcb";

for (1..10) {
    $key = pack('n',unpack('n',$key)+1);  # ++
    print unpack("H*", $key), "\n";
}

__END__
4fcc
4fcd
4fce
4fcf
4fd0
4fd1
4fd2
4fd3
4fd4
4fd5

[Anonymous Monk]

Okay one last item. Using:$key = "\x4f\xcb" x (length($msg)/2); how do I handle odd length messages? We can see in your example too, that the last character is not xored, 0x7a. I would like it to be xored with 0x4f, or the first part of the key. Thanks for your help. Aaron

[Anonymous Monk]

I think I solved this. I just added 1 more, since an extra byte or two in the key doesn't hurt anything.

$key = "\x4f\xcb" x ((length($msg)/2)+1);

Aaron

[JavaFan]

Do you expect 0x3ba3 because 0x3ba3 == 0x7468 ^ 0x4fcb? However, unpack "H*", "th" equals 7468 == 0x1d2c.

And I don't understand the purpose of your split. Wouldn't @array = $msg =~ /(..)/g be much handier?