[Harry says] Time for a hint (or, more accurately, a tutorial on cracking 6B).

Challenge 6B is technically the hardest yet. As Trinity says in part 6A it is double encrypted. In this case it has been encrypted with a substitution cipher then jumbled up with a transposition. In fact it doesn't matter which order these two were done in, it could have been jumbled first then Ben could have applied the substitution second. If you look at the frequency distribution of the cipher text characters you will see that no one letter stands out as much more common than the others, this tells you that he can't have used a standard substitution like a keyword cipher but must have used something polyalphabetic like the Vigenere cipher. You might want to look it up online and in our Teachers' Pack. The key to breaking it is to figure out or guess the length of the keyword used. It is likely to be a small prime, and, for reasons I won't go into here, is also likely to divide the gap length between repeats of triples of letters (you will see why if you read up about the Vigenere) so the first step you should take is to look for repeated triples of characters in the text and write down how many characters there are from the start of the first triple to the last character before the start of its repeat. Do this wherever you see a triple repeating. (You could try couples of letters instead, its up to you.) Then look for common factors. The most common factor is likely to be the length of the keyword. You can use this to divide the text up into pieces. If the keylength is seven the first piece consists of the first, the eighth, the fifteenth letters and so on, and the second consists of the second, the ninth, the sixteenth, ... You should then pretend each piece is a cipher text in its own right, encrypted with a Caesar shift, and use frequency analysis to work out which one. You might not get it right first time but try this for each piece anyway and put the text together to see what you get. If Ben had not used a permutation on top you would now be done, but unfortunately he did so even if you got everything right it will still look wrong. However, Ben used a very simple transposition cipher and if you look carefully you will see which one he used! Even if you got one of the Caesar shifts wrong you should be able to work out which one you ought to use in its place. This is very tricky, but you will feel great if you get it out so give it a go. Good luck,