=head2 Options

=over 4

=item B<-e>

Input should be treated as cleartext and encoded (default).

=item B<-d>

Input should be treated as ciphertext and decoded.

=item B<-i> infile

Input file (possibly useful if not specifying a keyword).
See also B<infiles>, below.

=item B<-o> outfile

Output file.  Defaults to standard output.

=back

=head2 Arguments

=over 4

=item keyword

The keyword for the cipher (in Peter Wimsey's case, this was "Royalty").
If none is supplied, defaults to "playfair" (as in the example below).

=item infiles

Any number of files may be listed on the command line after the keyword:
no break will be made in the output between input files.  Overrides the 
B<-i> option.If no files are specified, the program reads from the
standard input.

=back

=head1 Disclaimer

B<NOTE WELL>: the Playfair cipher can be solved, for an arbitrary keyword, with 
paper and pencil.  It is not what you would call "secure".  Any ill effects 
accruing to the user as a result of using it as if it were are entirely the 
user's responsibility.


=head1 The Playfair Cipher

=head2 The Square

The square is produced by entering the letters of the keyword, followed by
the remaining letters of the alphabet (in standard order) into a 5x5 matrix.
(I and J are considered equivalent in this cipher.)

With a keyword of "Playfair", the result is this:

   ---------------------
   | P | L | A | Y | F |
   ---------------------
   | IJ| R | B | C | D |
   ---------------------
   | E | G | H | K | M |
   ---------------------
   | N | O | Q | S | T |
   ---------------------
   | U | V | W | X | Z |
   ---------------------

B<Note>: a slightly more complicated method exists for building the square
(I would guess it is the more authentic), based on a transposition of 

  PLAYFAIR
  BCDEGHKM
  NOQSTUVW
  XZ

(yielding a first row of C<P B N X L>).  It was not used by Dorothy Sayers'
murder victim, and hence is not used in this program, though it would be 
relatively simple to implement.


=head2 Encryption Rules

The Playfair cipher is a digraphic cipher: rather than simply translating each
letter of input to a unique letter in the output, we translate pairs of letters.
In the case where two consecutive letters are identical, or where the cleartext has
an odd number of letters, a blank of some kind must be inserted: by convention, 
this blank is X (in the case where a blank must be inserted after an X, we insert 
a Z, because it seems as reasonable a response as a double X in the cleartext
deserves).

=over 4

=item *

In the most general case, each letter is replaced by the letter in 
its own row and in the column of the other letter in the digraph.

    LW -> AV
    RT -> DO
    TA -> QF

=item *

If the two letters of the digraph are on the same row in the chart, 
each should be replaced by the letter immediately to its right (wrapping
as necessary).

    OS -> QT
    GH -> HK
    TO -> NQ

=item *

If the two letters are in the same column, each should be replaced by the 
letter immediately below it (again wrapping as necessary).

    GO -> OV
    UE -> PN

=back

=head2 Decryption Rules

Are left as an exercise to the reader. ;-)

=head1 History

The Playfair Cipher was invented by Sir Charles Wheatstone in the 
neighborhood of 1850, and popularized by his friend Lyon, First Baron
Playfair.  It saw substantial use as a field cipher in the British and 
American militaries over the following century (including by the celebrated
commander of PT-109), owing to its ease of use and the modest difficulty
it presented to cryptanalysts (field ciphers need only stand up to attack
for a matter of hours, rather than the days or weeks of higher-level codes).

=head1 Author

ChemBoy (C<chemboy@perlmonk.org>).

=cut