9A/9B
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Guys, can someone please help me understand the 9th challenge?
I didn’t quite get how they were en/decrypted
thanks in advance
@artharvdagoat
I’m pretty sure @upsidedown explained it in good detail in the ‘Welcome’ topic in post #114632. Is there anything specific you don’t understand about the cipher?
I believe there’s a much more detailed description of 9A & 9B 9n the third page of the welcome thread if that helps?
9A was a Vigenere cipher, which was encrypted using the keyword TRAITOR, then reversed (so decrypt by reversing, then use keyword TRAITOR to decrypt (we used the crib of MY DEAR))
9B was a playfair cipher with altered rules. The keyword for the playfair square was COMPANY (found as the second word on line 17 of page 42). The diagonal rules for encryption remained the same, but if the two letters were on the same row or column, each letter in the pair would instead be encrypted as the letter to its right.9A: A Vigenère cipher with keyword TRAITOR, then reversed.
9B: A Playfair cipher with keyword COMPANY, but if the two letters in the digraph were in the same row/column, you would decrypt it by replacing it with the letter to the right of the encrypted text (wrapping around at the end).9A is vigenere but inverted text, 9B is playfair company is the keyword however the rules are different
@atharvdagoat did you see my reply #114632? Let me know if anything is unclear with that explanation.
9A was encrypted with a Vigenere cipher and then reversed. To decrypt it, first reverse the text and then decrypt Vigenere with the keyword TRAITOR.
9B was a 5×5 playfair cipher similar to 8B however with one of the rules changed. The grid can be found in case file 9.5. The rule change was that if 2 letters were in the same column as each other, the decrypted letters were the ones to the RIGHT not below as usual. Madness did a great job at explaining how the playfair works (and how to modify it) in the ‘Let’s talk about the Playfair cipher’ thread.
I hope this helps.
I believe all the challenges so far are available in the GitHub archive (https://github.com/themaddoctor/BritishNationalCipherChallenge/tree/master/2025) with solutions too for anyone who doesn’t understand any of them.
Request, Harry would you, or anyone, please supply me with the text that was given after a successful decrypt of 9A
I solved it pretty quickly but somehow I forgot to post my answer for it.
9A: How long did it take you to figure out that the text was backwards? It makes it a lot harder to crack until you notice that, though we suspect that those of you who have been busy automating all this codebreaking might not have realised there was a problem at first!
Congratulations on getting this far, and good luck with Challenge 9 part B!
Jodie
9B: Congratulations on getting this far in the competition, we hope you are still having fun. With just one more Chapter to go the story is racing to its conclusion, and we have to say things are looking bleak for our heroes. Which Charles is right? And can they make up in time to complete their mission – tune in next week to find out in the final thrilling instalment!
Harry
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