#!/usr/bin/perl -w

use strict;

if(@ARGV < 3) {
    print "Usage: $0 <key> <input file> <output file>\n";
    exit(0);
}

my $key = $ARGV[0];
open(IN, $ARGV[1]) or die "Can't read $ARGV[1]: $!\n";
open(OUT, ">$ARGV[2]") or die "Can't wrote $ARGV[2]: $!\n";
binmode(IN); binmode(OUT);

my $in;
while( sysread(IN,$in,length($key)) ) {
    print OUT $in^substr($key,0,length($in));
}

close(IN);
close(OUT);