#!/usr/bin/perl
use strict;
use Getopt::Std;
use POSIX;      # because of ceil()

# name:     shift-cipher-ng
# author:   ap0calypse (ap0calypse@agitatio.org)
# version:  0.1 (2008-06-20)
#           0.2 (2008-06-21)
# license:  DO WHATEVER YOU WANT!
# purpose:  reads a sentence from STDIN an returns
#           the ciphered sentence.
#
# changes:  2008-06-21:
#           + added decryption (ap0calypse)

our ($opt_k, $opt_h, $opt_d);

getopt('d:k:h');

sub usage {
    print << "EOUSAGE";
usage:
======
    encryption:
    ***********
    echo "normal text" | shift-cipher-ng -k PRIVATEKEY
     - PRIVATEKEY must be a single lowercase word and must not
       contain any special characters.
    decryption:
    ***********
    echo "encrypted text" | shift-cipher-ng -d PRIVATEKEY
     - PRIVATEKEY must be a single lowercase word and must not
       contain any special characters.
example:
========
    encryption:
    ***********
    echo "i love shifting" | shift-cipher-ng -k abc
        - will result in following output:
    [i nowg tjigvioi]
    decryption:
    ***********
    echo "i nowg tjigvioi" | shift-cipher-ng -d abc
        - will result in following output:
    [i love shifting]
    
explanation:
============
    This is because the sentence is transformed like this:
    input:  [i] [ ] [l] [o] [v] [e] [ ] [s] [h] [i] [f] [t] [i] [n] [g]
    key:    [a] [b] [c] [a] [b] [c] [a] [b] [c] [a] [b] [c] [a] [b] [c]

    the key for each letter represents where the alphabet starts.
    spaces are not transformed.
    
    the result is:
    [i][ ][n][o][w][g][ ][t][j][i][g][v][i][o][i]
    or:
    [i nowg tjigvioi]
    =========================================================================
    For further information search the web or visit my blog:
    http://ap0calypse.agitatio.org
    Have fun!
EOUSAGE
}

if (defined $opt_h) {
    &usage and exit;
}

elsif (defined $opt_k) {
    if ($opt_k =~ m/[^a-z]+/) {
        &usage and exit;
    }

    chomp (my $input = <>);
    $input = lc $input;

    my @a_input = split //, $input;
    my @a_key = split //, $opt_k;

    my (@a_input_num, @a_output_num, @a_output, @a_key_long) = ('', '', '', '');

    my ($i_length, $k_length) = (length $input, length $opt_k);
    my $repeats = ceil($i_length / $k_length);
    my $overhead = 0;

# generate key-array
    for (1 .. $repeats) {
        push (@a_key_long, @a_key);
    }

# ascii-fy the array ;)
    for (0 .. $#a_key_long) {
        $a_key_long[$_] = ord $a_key_long[$_];
    }

# generate output-ascii array
    for (0 .. $#a_input) {
        if (ord $a_input[$_] != 32) {
            $a_input_num[$_] = (ord $a_input[$_]) - 97;
            $a_output_num[$_] = $a_key_long[$_] + $a_input_num[$_];
        }
        else {
            push @a_output_num, 32;
        }
    }

# look for overhead ( values greater 122 (z) )
    for (0 .. $#a_output_num) {
        if ($a_output_num[$_] > 122) {
            $overhead = $a_output_num[$_] - 122;
            $a_output[$_] = chr (96 + $overhead);
        }
        else {
            $a_output[$_] = chr $a_output_num[$_];
        }
    }

# print output
    print "[";
    print for @a_output;
    print "]\n";
}

elsif (defined $opt_d) {
    if ($opt_d =~ m/[^a-z]/) {
        &usage;
        exit;
    }
    chomp (my $input = <>);
    $input = lc $input;

    my @a_input = split //, $input;
    my @a_key = split //, $opt_d;

    my (@a_input_num, @a_output_num, @a_output, @a_key_long) = ('', '', '', '');

    my ($i_length, $k_length) = (length $input, length $opt_d);
    my $repeats = ceil($i_length / $k_length);
    my $overhead = 0;

# generate key-array
    for (1 .. $repeats) {
        push (@a_key_long, @a_key);
    }

# ascii-fy the array ;)
    for (0 .. $#a_key_long) {
        $a_key_long[$_] = ord $a_key_long[$_];
    }

# generate output-ascii array
    for (0 .. $#a_input) {
        if ((ord $a_input[$_]) != 32) {
            $a_input_num[$_] = ord $a_input[$_];
        }
        else {
            $a_input_num[$_] = 32;
        }
    }

    for (0 .. $#a_input_num) {
        if ($a_input_num[$_] != 32) {
            $overhead = $a_key_long[$_] - 97;
            $a_output_num[$_] = $a_input_num[$_] - $overhead;
        }
        else {
            $a_output_num[$_] = 32;
        }
    }

    for (0 .. $#a_output_num) {
        $a_output[$_] = chr($a_output_num[$_]);
    }

    my $underhead;
    for (0 .. $#a_output_num) {
        if ($a_output_num[$_] != 32) {
            if ($a_output_num[$_] < 97) {
                $underhead = 97 - $a_output_num[$_];
                $a_output[$_] = chr (123 - $underhead);
            }
            else {
                $a_output[$_] = chr ($a_output_num[$_]);
            }
        }
        else {
            $a_output[$_] = " ";
        }
    }
    print "[";
    print for @a_output;
    print "]\n";
}
else {
    &usage;
    exit;
}