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has 'encrypt_key' => (is => 'rw');

=head1 DESCRIPTION

The Hill cipher  is  an  example of a block cipher. A block cipher is a cipher in
which groups of letters are enciphered together in equal length blocks. The  Hill
cipher was developed by Lester Hill & introduced in an article published in 1929.

The L<Crypt::Hill> module is using block size 2.Acceptable characters are A to Z.

=head1 CONSTRUCTOR

The constructor expects one parameter  i.e.  key to be used in the encryption and
decryption.The key should consists of no more than 4 ALPHABETS.

    use strict; use warnings;
    use Crypt::Hill;

    my $crypt = Crypt::Hill->new({ key => 'DDCF' });

=cut

sub BUILD {
    my ($self) = @_;

    my $encrypt_key = $self->_encrypt_key;
    $self->encrypt_key($encrypt_key);
}

=head1 METHODS

=head2 encode($message)

Encodes the message using the key provided and returns encoded message.

    use strict; use warnings;
    use Crypt::Hill;

    my $crypt   = Crypt::Hill->new({ key => 'DDCF' });
    my $encoded = $crypt->encode('HELP');

    print "Encoded: [$encoded]\n";

=cut

sub encode {
    my ($self, $message) = @_;

    return $self->_process($self->encrypt_key, $message);
}

=head2 decode($encoded_message)

Decodes the encoded message using the key provided.

    use strict; use warnings;
    use Crypt::Hill;

    my $crypt   = Crypt::Hill->new({ key => 'DDCF' });
    my $encoded = $crypt->encode('HELP');
    my $decoded = $crypt->decode($encoded);

    print "Encoded: [$encoded]\n";
    print "Decoded: [$decoded]\n";

=cut

sub decode {
    my ($self, $message) = @_;

    return $self->_process($self->_decrypt_key, $message);
}

#
#
# PRIVATE METHODS

sub _process {
    my ($self, $key, $message) = @_;

    my @chars    = @$CHARSETS;
    my $modulos  = scalar(@chars);
    my $size     = $self->block_size;
    my $table    = $self->table;
    my $_message = to_matrix_2_x_1($message, $CHARSETS, $table, $size);
    my $result   = '';
    foreach (@$_message) {
        my $_matrix = multiply_mod($key, $_, $modulos);
        $result .= sprintf("%s%s", $chars[$_matrix->[0][0]], $chars[$_matrix->[1][0]]);
    }

    return $result;
}

# convert key to matrix (1x2)
sub _encrypt_key {
    my ($self)  = @_;

    my $table   = $self->table;
    my $key     = $self->key;
    die "ERROR: Key should be of length $KEY_LENGTH." unless (length($key) == $KEY_LENGTH);

    my $size    = $self->block_size;
    my $enc_key = to_matrix_1_x_2($key, $CHARSETS, $table, $size);
    die "ERROR: Invalid key [$key] supplied." if (get_determinant($enc_key) == 0);

    return $enc_key;
}

# descrypt key to matrix (2x2)
sub _decrypt_key {
    my ($self)  = @_;

    my $key     = $self->encrypt_key;
    my $modulus = scalar(@$CHARSETS);

    return inverse_matrix($key, $modulus);
}

=head1 AUTHOR

Mohammad S Anwar, C<< <mohammad.anwar at yahoo.com> >>

=head1 REPOSITORY