#5X5 POLYBIUS SQUARE HOW TO#

Our cipher text is long enough and the shortest key we can find to uniquely place all of the plain text within valid polybius coordinates is: opposite corners of the original polybius square, then the key value we choose for that column is unique, and any other key value would result in the plain text being outside the polybius square. But basically, if the plain text for each column for a given key length includes `11` and `55`, or `51` and `15`, i.e. With a shorter cipher text, there might be some ambiguity with respect to key selection. if all `k`s are valid, this is a valid key (and we can stop) if they are all valid polybius coordinates, this is a possible value for `k`

subtract `k` from all values in column `i` for each possible key character value `k` that is a valid polybius coordinate `11, 12, 13, 14, 15, 21. for each column `i` (corresponding to a single key character) for each possible key length `Nk`, `1, 2. We can try to find out what the key length is before doing anything else (see ( )), and this is what I did during the CTF. The first step is to get rid of the additive key. With this in mind, we can start breaking the cipher. `1 ≤ row ≤ 5`, `1 ≤ column ≤ 5`, so `24` is a valid coordinate, while `30` is not any of the numbers from steps 2 and 3 have to be valid polybius square coordinates, i.e. a substitution cipher is trivially broken if we have enough data (the cipher text is quite long here), or we have cribs (we have that as well), or we know that the plain text is English (a fair assumption) There are some serious flaws in the cipher like this. the cipher text is created by taking each number from step 2 and adding it to a number from step 3, repeating the key numbers periodically as needed the key (another one) is also encoded into numbers using the polybius squareĤ. the plain text is encoded into numbers using the polybius squareģ. a 5x5 polybius square is created based on a keyword - basically a substitution alphabet with 25 letters, where each letter in the cipher text is identified by its row and column in the squareĢ. This all makes sense with the nihilist cipher, which works like this:ġ. It is notable that there are no low numbers, and it seems like the lower right corner is actually a continuation (i.e. (Here I was trying to assign the 43 to the most common English letter, and so on, but that is clearly not the right approach.) I plotted the frequency of each number in the cipher text:

#5X5 POLYBIUS SQUARE HOW TO#

I fumbled about a bit still trying to solve it like a substitution cipher for some reason, but then I looked into how to break the cipher properly. While exploring some cryptography pages on Wikipedia for some other challenges, I noticed something in the classical cryptography section: ( )! Sounds like exactly what we need. So having a look at the ( (novel)), we see that the central theme is basically. At this point there were still no solves, and the admin of the challenge hinted that 'knowing what the book was about is useful'. number = word in text, take first letter of wordīut all of it looked rather nonsensical. Given the hint mentioning a book and the cipher text being small numbers, my first thought was that this is a book cipher. `` - containing the cipher text ( ), with 682 pairs of decimal digits separated by spaces Can you help me figure out what it is? NOTE: the flag is not encoded in the usual format, enter it as sctf with the flag in caps I found this strange string of numbers inside a copy of Отцы и дети by Ива́н Серге́евич Турге́нев that Cypher left inside a postbox.