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7A from last year

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  • 11 Nov 17 at 4:10 pm #32374
    Anonymous
    Inactive

    What cipher was used for challenge 7A last year? I know 7B and 8A, but I’m currently working through as practice. Thanks!

    12 Nov 17 at 1:25 am #32408
    Anonymous
    Inactive

    Yep, I’m trying 7A too. A bit stuck on what it is. I’ve worked out so far that it is some sort of polyalphabetic or polygraphic cipher. There’s a large peak in Index of Coincidence at period 7 and 14 so I suspect it may be a Vigenere, Porta, Beaufort or Gronsfeld cipher with a key of 7 or 14 characters. I tried breaking it as a Vigenere and Beaufort so that rules them out which leaves a Porta cipher or a Gronsfeld cipher. If it isn’t one of those then I’m not really sure. Hope this helps and if you figure it out please let me know!

    12 Nov 17 at 12:20 pm #32427
    Anonymous
    Inactive

    I’ve had a look and have managed to crack everything in last years challenge except for 6A and 8B, so far. I think that 7A is most probably a combination of multiple ciphers, perhaps a Vigenere and a mono-alphabetic substitution. If anyone can remember from last year, please let me know. Thanks!

    12 Nov 17 at 8:45 pm #32463
    Anonymous
    Inactive

    I think you’re right. Do you mean that it might be a Vigenere which has then been substituted? As for 6A, it is a standard Vigenere cipher. 8B however, is not a classical cipher but one created for the competition. I think there’s a guide to how it was cracked last year, somewhere on the resources page.

    12 Nov 17 at 9:34 pm #32469
    Anonymous
    Inactive

    7A from last year used Vigenere. If you’ve done the earlier challenges from last year, you will know of a trick that was applied to the ciphertext that makes it harder to decipher.

    13 Nov 17 at 1:51 pm #32480
    Anonymous
    Inactive

    Yeah 7A was a Vigenere, never spotted the trick though, as we’ve programmed a Vigenere solver.

    13 Nov 17 at 6:41 pm #32500
    Anonymous
    Inactive

    Thank you. I just worked out what the trick was! I suppose I’m allowed to say it but for anyone who is also working through last year’s challenges and doesn’t want it spoiled, stop reading now…
    It was encrypted as a vigenere cipher and then the text was reversed in order.

    13 Nov 17 at 9:10 pm #32502
    Anonymous
    Inactive

    Thanks guys, I also rather like the hint given in 7a’s keyword for 8a, were there any clues given for 7b that I missed (even though I’ve solved it already)?

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