open(FILE3, '>file3') or die $!;
  print FILE3 ((`cat file1` + `cat file2`) % 2);
  close FILE3;
[download]

Not what you meant? Yeah, I figured. Maybe you could slow down and explain exactly what you're trying to do?

-sam

Yes I do mean exclusive OR. The 2 binary files are in fact the same file. So I should have said I want to exclusive OR a file to itself giving me a new file that is the original file with a Delta-1 on it.

OIC. If this is what you want to do, then just:

ln -s /dev/null cipher.txt
[download]

Because X ^ X == 0.

~%{${@_[0]}}->{0}&&+++ NO CARRIER

Is there a quick and easy way to modulo-2 add two binary files together to get a third file?

And what to you want it to do when the files are different sizes? And how do you want to handle endian issues?

thulben@bravo:~/perl
125> perl -le 'print "A" ^ "C"' | perl -nle 'print $_^"C"'
A
thulben@bravo:~/perl                                      
126> perl -le 'print "A" ^ "C"' | perl -nle 'print $_^"A"'
C
thulben@bravo:~/perl
127>
[download]

use warnings;
use strict;

my $cipher = shift;
my $key    = shift;

open( my $cipher_fh, $cipher ) or die $!;
open( my $key_fh,    $key )    or die $!;

my $buffer_length = 4_096;

while ( sysread( $cipher_fh, my $cipher_text, $buffer_length, )
     && sysread( $key_fh,    my $key_text,    $buffer_length, ) ) {

         print $cipher_text ^ $key_text;
}
[download]

Close but no cigar, you can't xor arbitrary strings that way. The following will cipher STDIN to STDOUT using the contents of the keyfile on the command line. Assumption is that the key file is much smaller than memory (maybe 5% or less):

use warnings;
use strict;

@ARGV or die "Usage: cipher keyfile < source > dest\n";

open K,shift or die "Can't open keyfile: $!\n";
undef $/; my $k = <K>; close K; # slurp
length $k or die "error reading key file\n";

while (read STDIN, my $t, length $k)
{
  my @k = unpack "C*", $k; 
  print pack "C*", map { $_^shift @k } unpack "C*", $t;
}
[download]

Now let's try it:

# make a key file, ok for demo purposes
$ perl -e 'print chr int rand 256 for 0..10000' > key.file

# make a test source file
$ ls -al /dev > plain.txt

# encipher
$ perl cipher.pl key.file < plain.txt > cipher.txt

# decipher
$ perl cipher.pl key.file < cipher.txt > deciphered.txt

# verify
$ diff -s plain.txt deciphered.txt 
Files plain.txt and deciphered.txt are identical
[download]

---

~%{${@_[0]}}->{0}&&+++ NO CARRIER

I'm not sure that I understand why you can't bit-wise xor two strings:

$foo = "baz"; 
$bar = "foo";

print unpack("B*", $foo); 
print unpack("B*", $bar); 
print unpack("B*", ($bar ^ $foo));
__END__
011000100110000101111010
011001100110111101101111
000001000000111000010101
[download]

Seems to do the right thing to me...

thor