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The official Challenge 10 tips and hints thread

The Empty Vault Forums Bureau of Security and Signals Intelligence Forum The official Challenge 10 tips and hints thread


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    @Mattyrat, maybe you will be first this year.

    @all, I noticed a few things this year. There have been more that the usual number of transposition ciphers,
    and the latter ones were divided into blocks and used columns. Maybe Harry and the elves are suggesting that
    10A and/or 10B use columns and blocks?

    p.s. The best blocks are legos.


    A first hint for Challenge 10B

    Frequency analysis will have shown you that this is probably some form of transposition cipher, so the structure of the text itself probably matters.


    (Harry, feel free to omit/delete if necessary)

    Just to make sure that I’m on the right track, is 10B a transposition cipher of some form? (Possibly a variation on the columnar transposition cipher?)


    What’s the official 10B hint?

    [Sorry, it wasn’t picking up the titles so hard to see. Have updated the official hints so you can what they are. Harry]


    Is this the hint for 10B?

    [Hard to say! Sorry for the confusion, I have added titles to the hint posts to make them easier to spot. Harry]


    Hi Harry can we know if there are prizes for people who have had full points until mission 10? And if so what are they? (If you can tell me that is). This is the first challenge we’ve made so far and are curious.

    [There are four main prizes that will be awarded to teams/individuals among those with the highest score. The prize committee takes several things into account in awarding those prizes, including an assessment of the method used. There will also be a small number of runners up medals to be awarded at Bletchley Park on February 7th, and we will be in touch with the winners when the competition closes to discuss this. Hope that helps, Harry]


    Is there any leads at all on challenge B, it seems impossible[There’s a schedule for the hints at the top of the thread. Harry]


    Does anyone who’s managed 10B mind giving us all something to start on? Like others I feel completely stumped since it is very hard to find any obvious vulnerabilities.


    [We will be publishing hints that build up to a complete solution over the three weeks of the challenge. No hints by competitors will be let through before the corresponding official hint is posted as we will hold them back to keep the pace we have set. Good luck, Harry]


    A second hint for Challenge 10B

    The only structure you have at the moment is the number of characters in the text, so why not compute that and factorise it to see if it suggests anything?


    A third hint for Challenge 10A

    If you computed the bigram frequencies then you should have seen that the most common were TA, which appears 42 times  and HL, which appears 39 times. You can read about bigram frequencies at

    The most common bigrams in standard UK English are th, which usually appears just over 3.5% of the time, and he which has frequency about 3.25%, so you can reasonably guess that TA stands for th and HL stands for he. What might you guess is happening?


    Not sure if this is something I you are allowed to comment on. But curious if the line breaks in the actual 10B ciphertext are artificially put in or not? Like, would it make a difference whether we were looking at it as a single line, or exactly the way it is formatted on the webpage?

    [Always worth exploring things like this, but that is not to say anything about your actual question. Harry]


    I also have another question if this one is allowed to be answered? I’ve been experimenting with how column transposition ciphers behave if you don’t add padding to ensure the pt text is a multiple of the key length, and noticed it makes it far trickier to decrypt, since it complicates reconstruction of the shuffled pt rows from the ct.

    Do I have to take this into consideration with 10B, or can I assume that padding has been added (if it is needed in whatever transposition cipher has been used) before encrypting?


    Yep same… But I don’t think it’s going to be that easy…


    Now I wonder what I’ve done wrong I did that already, ahhhhhhh


    (Harry, I don’t know if you will be allowed to answer this at this point, but…)

    Does the cipher used to encrypt 10B, by any chance, appear in

    {I won’t answer that, but I will post it because it is a good opportunity to advertise the book! Harry]

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