National Cipher Challenge 2019 › Forums › National Cipher Challenge 2019: Countdown to Catastrophe › Well done … but how?
4th December 2019 at 11:03 pm #44645JimdafishMember
I’d be interested to know how people that solved 7B in a timely manner made the cognitive leap necessary to crack it.
Part A was obviously quite straightforward. But knowing just one of the two ciphers for part B and the fact that it’s a substitution cipher means that you are still left with the possibility of the second being any number of transposition ciphers or more complex block ciphers, Etc. As it turned out, the second cipher was more of a obfuscation than a cipher.
Kudos to the folks that got it early, but would love to understand how you made “the leap” (before the clues).5th December 2019 at 10:13 am #44651S-evanMember
I didn’t make a logical deduction per se. I saw the shape of the text being a 47×47 square and immediately wondered if we needed to transpose it. That’s normally my first step with transposition. Just try a bunch of things and see if any work, then get to hard analysis5th December 2019 at 10:13 am #44652PhilancheMember
Once I had finished 7A, I found that it was a Vigenère from the key. Then I read the second paragraph over and over, as it seemed to have a clue. After some time, I remembered one of the most common tricks to hide a word in a text: looking at the first letter of each sentence. That gave me columns. From that, it was just about reading in columns, and finding the Vigenère key using some ioc analysis.
I hope that helped you, and good luck in the later challenges,
Philanche5th December 2019 at 10:13 am #44654Jemstone15Member
Ok I didn’t solve it in time (cracked it this morning) and I relied on a lot of hints from the forum, but I can tell you the method. By deciphering A, you could see it was encrypted with a keyword ‘VIGENR’, indicating that B had a Vigenere cipher. The main clue for B was that the text had to be read downwards (which I got from forums), in columns, but that also meant it has to be decrypted downwards, eg: using a Vigenere decoder from left to right would not work. I used Excel to rearrange the text so it read from the top to the bottom, then put it into this site: https://www.dcode.fr/index-coincidence and calculated possible period lengths. Both 7 and 21 had a much higher IC so I plugged in 7 as the key length and it deciphered it, showing the keyword to be ‘COLUMNS’. I used the online decoder more than I would have liked, because I’m still not sure how to get the second keyword from part A, but I hope this is vaguely helpful.6th December 2019 at 2:24 pm #44696EpicgeethMember
Well I knew it involved a vigenere and a transposition of some sort. So I googled “types of transposition ciphers”, and eventually found this on wikipedia:
In a route cipher, the plaintext is first written out in a grid of given dimensions, then read off in a pattern given in the key.
Since the text was in a 47 * 47 grid, I was confident that I was close to the solution. I tried to read off the text in a number of ways, and then tried to find the Index of Coimcidence each time. Soon, I found that reading down the columns gave a higher IoC, and every seventh letter gave an IoC of 0.68. I quickly wrote a python script to reorder all the letters and then passed the result into a vigenere solver that I had used in earlier challenges.11th December 2019 at 11:04 pm #44759Diamondbandit23Member
The keyword for part B was Columns. You had to read it like that and use it as the keyword. Was a bit hard but some of us got it. Well done if you guys got it in time. M
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