0
$\begingroup$

I was wondering how I could crack a Playfair cipher with only the alphabet and the ciphered message. Ive tried looking for tools online, but still no luck.

$\endgroup$
0
$\begingroup$

If by alphabet you just mean that you know the letters A-Z are being used apart from identifying I with J, the best place to start is by looking for common digraphs (pairs of letters). There are lists of the most common digraphs in English and if you have a long enough message you should be able to identify likely candidates. Indeed, if your ciphertext is long enough just digraph frequency will be enough.

For shorter ciphertexts, try to chain common digraphs together in the text to form words and guess less common digraphs. For Playfair note that recovering a digraph also gives you the reverse. Also specifically to Playfair, look for plaintext digraphs that contain the same letter (e.g. digraphs of the form E or E) the ciphertexts corresponding to these digraphs should give you strong clues to letters in the Playfair square that are in the same row and column as E. For example, if you see the digraph HE encoded as BX then there's a good chance X is in the same row as E and B is in the same row as B is in the same column.

Some good news is that there are many different Playfair squares that lead to the same encryption method.

Depending how much ciphertext you have, the rest is less science and more art. Don't be afraid to make wild leaps or assumptions, but equally don't be afraid to throw away large amounts of work and start again from scratch. A fuller treatment for breaking Playfair can be found in Chapter XXI of the classic text Cryptanalysis by Helen Fouche Gaines.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.