How we Found the Solution

In a large run, we assumed that the first letter of the key was E, an assumption that always can be made without loss of generality. We then tried all possible ways to choose the second letter. Using all these possible bigrams as the preset part of the key we used the program described above. When looking at the best candidate keys for the 25 different trials we found some that looked promising. When deciphering the ciphertext with one of those keys we found the fragments SAYFAIRCIRNER and YOUWILLSEXETHATITI in the plaintext. We had thus found a key square which was partially but not entirely correct. After having played around with the bigrams a little bit, we decided that the letters NKEVI should be one row of the key square. We thus fixed these five letters as the five first of the key and ran our program once more. The best key square found was

NKEVI
RHCTO
GBALU
PDMFS
ZQWXY

which produced the plaintext:

THISSTAGEISNUMBERSIXANDITISAPLAYFAIRCIPHERSTOPTHEFOLLOW
INGSTAGEISNUMBERSEVENWITHINTHECONTEXTOFTHISCOMPETITIONA
NDITISENCRYPTEDACXCORDINGTOANADFGVXTYPECIPHERSTOPWHENYO
UATTEMPTXTOCRYPTANALYSESTAGESEVENYOUWILLSEXETHATITISMOR
ECOMPLICATEDTHANASTRAIGHTFORWARDADFGVXCIPHERSTOPUNFORTU
NATELYICANXNOTGIVEYOUANYMORECLUESSTOPNEWPARAGRAPHINTERM
SOFTHISXSTAGETHESHIBBOLETHTHATYOUSHOULDTAKENOTEOFISMOLY
BDENUMNEWPARAGRAPHYOUWILXLPROBABLYHAVENOTICEDBYNOWTHATE
ACHSTAGEISINADIFFERENTCIPHERANDISUSUALLYINANAPPROPRIATE
LANGUAGESTOPSOTHEVIGENERESTAGEWASINFRENCHANDTHISPLAYFAI
RSTAGEISINENGLISHSTOPIFTHEPATXTERNPERSISTSTHENTHENEXTST
AGEWILXLBEINGERMANSTOPBUTDOESTHEPATTERNPERSISTQUESTIONM
ARKQ

If we format the text and replace the formatting words present in the plaintext we get the following message.

This stage is number six and it is a Playfair cipher. The following stage is number seven within the context of this competition and it is encrypted according to an ADFGVX type cipher. When you attempt to cryptanalyse stage seven you will see that it is more complicated than a straightforward ADFGVX cipher. Unfortunately I cannot give you any more clues.

In terms of this stage the shibboleth that you should take note of is molybdenum.

You will probably have noticed by now that each stage is in a different cipher and is usually in an appropriate language. So the Vigenere stage was in French and this Playfair stage is in English. If the pattern persists then the next stage will be in German. But does the pattern persist?

The codeword for this stage is thus MOLYBDENUM. We also realize that the key was not created at random. If rotate the columns of the key one step to the left we obtain

KEVIN
HCTOR
BALUG
DMFSP
QWXYZ

We guessed that the first two rows emerge from ``Kevin Hector'' and searched for that name to find that it was the name of a front football player in Derby FC during the 1970s. We could not make out the origin of the rest of the key, however. Simon later hinted that the name of the coach of Derby at that time, ``Brian Clough,'' was used for the third row. The fourth row is still a mystery.