#!/usr/bin/perl -w
use strict;

# Vigenere cipher toolkit. Encrypt and Decrypt.
# Cracking not included.
# 

my $plaintext	= "This is not secure, but it's kind of fun.";
my $key			= 'perlmonks';		# By definition, letters only.
my $ciphertext;
my $derived_plaintext;

# For convenience, do the work in lowercase...
$plaintext = lc($plaintext);
$key       = lc($key);

# By definition, only encipher letters, and only use letters in the key...
$plaintext =~ s/[^a-z]//g;
$key       =~ s/[^a-z]//g;
if(length($key) < 1){
	die "Key must contain at least one letter.\n";
}

print "plaintext is [$plaintext]\n\n";
$ciphertext			= encrypt_string( $plaintext, $key);
$derived_plaintext	= decrypt_string( $ciphertext, $key );

print "Plaintext used      :$plaintext\n";
print "Ciphertext          :$ciphertext\n";
print "Recreated plaintext :$derived_plaintext\n";

sub encrypt_string{
	# encrypts a full string of plaintext, given the plaintext and the key.
	# returns the encrypted string.
	my ($plaintext, $key) = @_;
	my $ciphertext;
	$key = $key x (length($plaintext) / length($key) + 1);
	for( my $i=0; $i<length($plaintext); $i++ ){
		$ciphertext .=
			encrypt_letter( (substr($plaintext,$i,1)),  (substr($key,$i,1)));
	}
	return $ciphertext;
}

sub decrypt_string{
	# decrypts a full string of ciphertext, given the ciphertext and the key.
	# returns the plaintext string.
	my ($ciphertext, $key) = @_;
	my $plaintext;
	$key = $key x (length($ciphertext) / length($key) + 1);
	for( my $i=0; $i<length($ciphertext); $i++ ){
		$plaintext .=
			decrypt_letter( (substr($ciphertext,$i,1)),  (substr($key,$i,1)));
	}
	return $plaintext;
}


sub encrypt_letter{
	# encrypts a single letter of plaintext, given the plaintext
	# letter and the key to use for that letter's position.
	# The key is the first letter of the row to look in.
	my ($plain, $row) = @_;
	my $cipher;
	# in row n, ciphertext is plaintext + n, mod 26.
	$row 	= ord(lc($row))   - ord('a');	# enable mod 26
	$plain  = ord(lc($plain)) - ord('a');	# enable mod 26
	$cipher = ($plain + $row) % 26;
	
	$cipher = chr($cipher + ord('a'));

	return uc($cipher);	#ciphertext in uppercase
}

sub decrypt_letter{
	# decrypts a single letter of ciphertext, given the ciphertext
	# letter and the key to use for that letter's position.
	# The key is the first letter of the row to look in.
	my ($cipher, $row) = @_;
	my $plain;
	# in row n, plaintext is ciphertext - n, mod 26.
	$row 	= ord(lc($row))    - ord('a');	# enable mod 26
	$cipher = ord(lc($cipher)) - ord('a');	# enable mod 26
	$plain  = ($cipher - $row) % 26;
	
	$plain = chr($plain + ord('a'));

	return lc($plain);	#plaintext in lowercase
}