#!path-to-perl
use warnings;
use strict;

die "usage: playfair <'key'> <'message'>" unless(@ARGV == 2);

my $key = lc($ARGV[0]);
my $message = lc($ARGV[1]);

my @alphabet = qw(a b c d e f g h i k l m n o p q r s t u v w x y z);
my %square_hash = ();
my $x;
my $y;
#replace all 'j's with 'i's
$key =~ s/j/i/g;
#build a key with unique letters (no repeats)
while($key =~ s/(.)(.*)\1/$1$2/) {}
print "Reduced key: $key\n";

#form the mapping 
my @keys = split(//,$key);
$x = 0;
$y = 0;
for my $char (@keys) {
    $square_hash{$x . "," . $y} = $char;
    $square_hash{$char} = $x . "," . $y;
    $x++;
    if($x==5) {    $x=0;$y++;}
}
#put in the rest of the letters
for my $char (@alphabet) {
    next if exists($square_hash{$char});
    $square_hash{$x . "," . $y} = $char;
    $square_hash{$char} = $x . "," . $y;
    $x++;
    if($x==5) { $x=0; $y++;}
}
print "5x5 playfair square:\n  +-----------+\n";
for $y (0..4) {
    print "  | ";
    for $x (0..4) {
    print $square_hash{$x . "," . $y} . " ";
    }
    print "|\n";
}
print "  +-----------+\n";

#change all j's to i's
$message =~ s/j/i/gi;
#remove spaces from the message
$message =~ s/\s+//g;

#insert an X in between identical letters
$message =~ s/(.)(\1)/$1x$2/g;
$message =~ s/(.)(\1)/$1x$2/g;

#make it even length
$message .= "x" if length($message)%2==1;
print "Formatted message: $message\n";
#now encrypt everything
my $encrypted = "";
while($message =~ s/^(.)(.)//)
{
    my @xy1 = split(/,/,$square_hash{$1});
    my @xy2 = split(/,/,$square_hash{$2});
    #3 rules. 1: same row.
    if($xy1[0] == $xy2[0])
    {
    #same col.  move down 1
    $xy1[1] = ($xy1[1]+1)%5;
    $xy2[1] = ($xy2[1]+1)%5;
    }
    elsif($xy1[1] == $xy2[1])
    {
    #same row move right 1
    $xy1[0] = ($xy1[0]+1)%5;
    $xy2[0] = ($xy2[0]+1)%5;
    }
    else
    {
    #same corners.  chose element on other column same row.
    my $tempcol = $xy2[0];
    $xy2[0] = $xy1[0];
    $xy1[0] = $tempcol;
    }
    $encrypted .= $square_hash{join(",",@xy1)};
    $encrypted .= $square_hash{join(",",@xy2)};
}
$encrypted = uc($encrypted);
print "Encrypted message E:\n$encrypted\n";
#OUTPUT
# .\playfair "playfairexample" "hide the gold in the tree stump"
# Reduced key: playfirexm
# 5x5 playfair square:
#   +-----------+
#   | p l a y f |
#   | i r e x m |
#   | b c d g h |
#   | k n o q s |
#   | t u v w z |
#   +-----------+
# Formatted message: hidethegoldinthetrexestump
# Encrypted message E:
# BMODZBXDNABEKUDMUIXMMOUVIF
[download]

The playfair cipher should not be used in a modern encryption scheme. It is solely a classic cipher. It is vulnerable to frequency analysis (digraph rather than monograph) yet is still quite secure if short messages are encrypted.

I hope to post a better-than-bruteforce decryption algorithm shortly.

I welcome any and all comments regarding ways I could improve my Perl scripting.