#!/usr/bin/perl

use strict;

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

my $key = $ARGV[0];
open(IN, $ARGV[1]) or die "Can't open infile";
open(OUT, ">$ARGV[2]") or die "Can't open outfile";

my $kp = 0;
while(<IN>) {
    for my $i (0..length($_)) {
	if($kp > length($key)) {
	    $kp = 0;
	}

	my $kc = substr($key, $kp++, 1);
	my $char = substr($_, $i, 1);
	
	$char ^= $kc;
	print OUT $char;
    }
}

close(IN);
close(OUT);