Cracking Playfair code

Question

I need to crack a Playfair encoded text without knowing the keyword. While searching the internet I found a way to do this using a 'shotgun climbing hill' method. Problem is, I can't decide how to quantify one solution against the other. I'm pretty sure the text is in English.

Any help would be appreciated.

UPDATE

This is encrypted text: http://pastebin.com/pEUXDvq1 .

Thanks for pointing me to pastebin.com.

e is usually rather typical, then t, then a (check out http://en.wikipedia.org/wiki/Letter_frequency) — Fabian, May 07 '11 at 15:34
I'm sorry, but the text is more than 3,000 characters long. I can mail the text to you if you like, but I don't want to know the answer, I just want to know how it should be cracked (so basically, I would like to know the codeword). — KevinDL, May 07 '11 at 15:36
To clarify: I want to know the answer, but the way to decode it is more important than the answer itself. — KevinDL, May 07 '11 at 15:49
It might be better to edit that into your question instead of keeping it in the comments... :) — J. M. ain't a mathematician, May 07 '11 at 16:09

Listing · Accepted Answer · 2011-05-07T16:26:26.080

7

Ok I will give a fairly short answer, as this is homework you should figure out the details :-). To begin with the code is

BDEWIKOCGHSMPQRZUVXYNFLAT

The decrypted text is a short story which be found on this website (formatted). You can use this as a online tool for decrypting, still you will (of course) lose all formatting and find the position of the spaced of all words on your own. A detailed approach how to break Playfair can be found here, basically it tries to analyse the frequency of the letters which is of course harder when blocks are substituted instead of single characters.

Finally a ready-coded solver written in C can be found here (coded by J.W. Stumpel). I hope that was helpful.

edited May 07 '11 at 16:26

answered May 07 '11 at 16:20

Listing

13,937

Thank you for your answer! I had already written a Delphi program to decode the text, but I couldn't figure out where it went wrong. The problem was, I didn't know what operations to use on the starting matrix and what stop criterion to use. – KevinDL May 08 '11 at 06:56
@user3123: I found the solution today, thanks for helping out. Studying the C-source from the link that was given, showed me how to calculate a meaningful stop condition. That helped me a lot because that was the main problem I was struggling with in the first place. – KevinDL May 08 '11 at 14:10
Good job, I am glad that I could help. – Listing May 08 '11 at 19:29

Cracking Playfair code

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