#!/usr/bin/perl -w

$#ARGV + 1 || die "$0 <key>\n";

($key = shift) =~ s/[^a-zA-Z]//g;
@offsets = map {$_ < 91 ? (91 - $_) % 26 : $_ - 97} unpack 'C*', $key || 'a';

for($offset = 26; $offset--;)
{
   $translate{$_}{$offset} = $_ foreach 0..255;
   for($input = 26; $input--;
       map {$translate{$_ + $input}{$offset} = $_ + ($input + $offset) % 26}
           (97, 65))
   {
   }
}

for($index = 0; <>;)
{
   foreach (unpack 'C*')
   {
      ($c = chr($translate{$_}{$offsets[$index]})) =~ /[a-zA-Z]/ && $index++;
      $index %= $#offsets + 1;
      print $c;
   }
}