use strict;
use warnings;

my $key = "";

while (length ($key) < 4) {
    print "Please Enter Your Desired Key (5 or more characters) \n --> : ";
    $key = <>;
    chomp $key;
}

my @karray = split (//, $key);
my @offset1;
my @offset2;


for my $ordChar1 (map {ord} @karray) {
    my $sum = 0;

    for my $ordChar2 (map {ord} @karray) {
        $sum += $ordChar1 + ord $ordChar2;
        push @offset1, ($ordChar1 + $ordChar2) % 256;
    }

    push @offset2, int ($sum / length $key) % 256;
}

1 while prompt ();


sub prompt {
    my ($mode, $offset1, $offset2) = @_;

    print "Encrypt or Decrypt (E/D)\n --> :";

    my $inp = <>;

    chomp $inp;
    return 1 if $inp !~ s/^(e|d)$/lc $1/ie;

    print "Enter ", ($inp eq 'e' ? 'plain text' : 'cypher text'), "\n --> : ";
    chomp (my $text  = <>);

    my @chars = split (//, $text);

    if ($inp eq 'e') {
        print encrypt(\@chars, \@offset1, \@offset2);
    } else {
        print decrypt(\@chars, \@offset1, \@offset2);
    }

    return;
}


sub encrypt {
    my ($chars, $offset1, $offset2) = @_;
    my $outText;
    my $len = @$chars;

    for my $index (0 .. $len - 1) {
        $outText .= chr ((
                ord ($chars->[$index]) -
                    ord ($offset1->[$index % $len * $len]) +
                    ord ($offset2->[$index % $len])
            ) % 256 - $index % 6);
    }

    return $outText;
}


sub decrypt {
    my ($chars, $offset1, $offset2) = @_;
    my $outText;
    my $len = @$chars;

    for my $index (0 .. $len - 1) {
        $outText .= chr ((
                ord ($chars->[$index]) +
                    ord ($offset1->[$index % $len * $len]) -
                    ord ($offset2->[$index % $len])
            ) % 256 + $index % 6);
    }

    return $outText;
}