Skip to main content

The official Challenge 10 tips and hints thread

The Body in My Library Forums Harry’s Meeting Place The official Challenge 10 tips and hints thread

Viewing 15 posts - 46 through 60 (of 106 total)
  • Author
  • #92864

    As funny as it was, I did question the initial artistic decision behind allocating only 4 pixels towards the clues


    Harry, are there 4 ways of going about this? Or less… If you know what I mean


    For part a that is


    or for b? I just realised something if so…


    Is 10b the bifid cipher?


    The fourth hint for Challenge 10A


    Challenge 9A was a very simple cipher in which the letters in positions 1,3,5, 7, 9 etc were left along and the letters in positions 2,4,6,8,10 … were shifted by the letters to their left. Many of you noted that this is a special case of a Hill cipher, Why not look that up?


    The third official hint for Challenge 10B


    Since the fragments showed plaintext squares top left and bottom right of the page, we might guess this is a Four Square encryption. We can use that along with our bigram count to start filling in the square. At this stage these are only guesses and further analysis will be needed!



    I thought my hill-climbing code was wrong… turns out that there’s only so much you can find with randomness in one go. finally found the main bit, now onto more specific decryption. thanks harry for another great year!


    If you’re bored with 10A and 10B, try the H³ challenge, in a separate topic.


    Hey Harry,

    In terms of prizes for induvial; how fast must we have solved 10B by? (I did it yesterday). Also is it simply the first 2 people to solve, or is the explanation of how it was solved also relevant in making that decision?


    [We do take the explanations into account, and have asked some people to write to us to help with that. Best wishes, Harry]


    Thanks for the hints!

    I had guessed that it was one of the so-and-so square ciphers, such as Playfair and its relatives, so it’s nice to be correct there. But from my searches I couldn’t find a guide on cracking it with pencil and paper – all seem to need a certain amount of programming skill. Do you know of a good outline I can read to tackling these kinds of ciphers in a traditional slow path way?

    [The hints we have lined up over the next two weeks will (we hope) guide you through it. All the best, Harry]


    The fifth hint for Challenge 10A


    If we think this is a Hill cipher and that the names JODIE and HARRY are likely to appear at the start or the finish of the text, then we might see the pairs

    JO, DI

    OD, IE

    HA, RR

    AR, RY

    in those positions. Assuming that we are using the convention A->0, B->1, C-> 2 and so on this would give us a series of equations that might allow us to work out the Hill cipher matrix. Take a look at Unit 88 in Madness’s book to see how to use that information.


    The sixth hint for Challenge 10A


    If Jodie sent this message and signed it we will see the bigrams OD, IE at the end and these are represented by the vectors (14, 3) and (8, 4).

    What we actually see are the letter pairs RU, MQ which correspond to the vectors (17,20) and (12,16), so the encryption works to give

    14a+3b = 17


    8a+4b =12


    We can solve these four equations in four unknowns to get the encryption parameters. Try looking up solving systems of equations online, but be careful doing so in mod 26 arithmetic!


    The fourth official hint for Challenge 10B


    Now we think we know a couple of entries in the encryption scheme we need to look for some more. The traditional way to do this is using cribs. Here is a suggestion of how to start:


    The seventh hint for Challenge 10A


    The inverse matrix is given by switching the diagonal entries and negating the off diagonal so we get 2,-1,-1,1.


    Now you use that matrix to decrypt the cipher text as if you were encrypting the plain text, and out should come the message. You can do it by hand, or you can use a python script or a spreadsheet. Have fun!

Viewing 15 posts - 46 through 60 (of 106 total)
  • You must be logged in to reply to this topic.
Report a problem