From: pcl@ox.ac.uk (Paul C Leyland)
Newsgroups: sci.crypt
Subject: A classical Playfair problem
Date: 31 Mar 93 10:06:56 GMT
Organization: Oxford University Computing Services, 13 Banbury Rd Oxford OX2
	6NN
Lines: 34


As a little light relief away from the politicking and flames
wars, I pose the following problem.

	BUFDA GNPOX IHOQY TKVQM PMBYD AAEQZ

You are told only that it is in Playfair, that it was proposed as
a problem in the mid-1930's by a British army officer and was
solved by an American army private.


I came across this in a collection of papers published in
Cryptologia.  Sorry I can't give the full reference, but I forgot
to bring it in with me.

I solved it, but it took me a few hours, most of which was spent
writing a Playfair engine for my PC.  Cracking it with nothing
but pencil and paper looks very laborious, unless there are
tricks I'm not aware of (which is quite likely).

If you're familiar with this one, please don't post the plaintext
until others have had a chance to solve it.

Please do let me know if you succeed in solving it.


Paul

--
Paul Leyland <pcl@oxford.ac.uk>          | Hanging on in quiet desperation is
Oxford University Computing Service      |     the English way.
13 Banbury Road, Oxford, OX2 6NN, UK     | The time is come, the song is over.
Tel: +44-865-273200  Fax: +44-865-273275 | Thought I'd something more to say.
Finger pcl@black.ox.ac.uk for PGP key    |

Newsgroups: sci.crypt
From: gec@nice.usergroup.ethz.ch (Germano Caronni)
Subject: Re: A classical Playfair problem
Organization: Swiss Federal Institute of Technology (ETH), Zurich, CH
Date: Mon, 12 Apr 1993 14:47:07 GMT

Playfair (invented by Wheatstone last Century):

Compose a matrix of 25 characters. (i=j)

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

encrypting:

I am not kidding -> ia mn ot ki dx di ng 
(spaces between words are dropped or changed to x, pairs which contain two
times the same letter like the 'dd' in kidding are splitted by inserting a
'x' between them)
enciphering uses 3 rules:
- if the two letters of the pair are on the same line, the ones right of them
  are taking (wrapping around the matrix if necessary)
  so gh becomes HI, lo becomes MP, and ki becomes FK.
- if the two letters are on the same row: af -> FL, gr -> MW, wb -> BG
- if they are on different lines and rows: take row of first letter, intersect
  with column of second letter, then take colum of first, intersect with row 
  of second. az -> EV, pg -> MK.
deciphering is similar:
just posses the right matrix, then if on same line : shift one to left,
if same row shift one up, if neither just use same step as when encrypting.
(line of first intersected with row of second  etc.)

enciphering 'I am not kidding' thus yields:

ia mn ot ki dx di ng -> FD NO TY FK CY IO MH

which can be written as FDNOT YFKCY IOMHG    (G is a null)

Instead of :
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

you can use an arbitrary matrix, but the 5x5 case is the common.
The letters in the 5x5 matrix can be arranged in any order you want,
but usually you use a keyword like this: (using 'hindenburg')

HINDE
BURGA (write the keyword horizontally, without repeating letters, and)
CFKLM (fill in the letters in alphabethic order, that are not in the keyword)
OPQST
VWXYZ

(Wheatstone proposed a slightly more difficult method for building the matrix,
which seems not to be used often...)

I hope this helps you,

	Germano Caronni