## replace modifyKeys with below code
sub modifyKeys {
  my $self = shift;
  my $S_KEY = "";
  my ($i, $x);

  for my $x (0..length($self->{R_KEY})-1) {
    $i = ord(substr($self->{R_KEY}, $x, 1)) - 32;
    $S_KEY .= chr(($i ^ $self->{RSORT}[$i] ^ $self->{ATT}) + 32)
  }
  return $S_KEY;
}

## fix up the following line in sub cipher() ##
  my $i = (length ($self->{xString}) % $keylen); # DATA DEPENDENT SHIFT