# 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)
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• #92864
Participant

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

#92835
ZB123
Participant

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

#92836
ZB123
Participant

For part a that is

#92837
ZB123
Participant

or for b? I just realised something if so…

#92866
JC
Participant

Is 10b the bifid cipher?

#92767
Harry
Keymaster

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?

#92765
Harry
Keymaster

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!

#92878
snugglyhedgehog
Participant

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!

#92885
Participant

If you’re bored with 10A and 10B, try the HÂ³ challenge, in a separate topic.

#92879
Flappyasdf_2023
Participant

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?

Thanks!

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

#92880
Jake
Participant

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]

#92769
Harry
Keymaster

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.

#92784
Harry
Keymaster

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

14c+3d=20

8a+4b =12

8c+4d=16

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!

#92768
Harry
Keymaster

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:

#92788
Harry
Keymaster

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!

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