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;
}