National Cipher Challenge 2019 › Forums › National Cipher Challenge 2019: Countdown to Catastrophe › Practice challenge B
6th November 2019 at 12:07 pm #43880Hethersett-code-crackersMember
Hey Harry, on practice challenge 3 Part b, I have tried to solve it using the frequency analysis, but whenever I think I have found a word, the whole challenge doesn’t see to work out. Am I just not being up to standards, or is there a twist on the cipher?6th November 2019 at 12:08 pm #44018HarryKeymaster
Anyone want to give advice here? No punishment for giving too much away as it is late in the final practice round, , but try to be creative rather than give the answer!6th November 2019 at 3:16 pm #44029MadnessMember
I can think of three things you could try.
1. Look for sequences of letters that repeat. They have to decrypt to the same thing each time, so they might be a common word or something that you could easily guess from knowing what the
story is about.
2. There are three ways to fill in the key alphabet for a keyword cipher. Here are examples where the keyword is KEYWORD.
a. start over at the beginning of the alphabet
b. start with the last letter of the keyword
c. start after the alphabetically last letter of the keyword; in this case ‘Y’
Know how this works, you could do what is called a dictionary attack and try different
keywords until you get one that decrypts the message. For challenge 3B the keyword is not
going to be in a list of common English words, however. (Don’t forget to remove repeated
letters in your keyword.)
3. If you have any programming skills, you could look at this paper. It is the standard
method for doing substitution ciphers. And if you get it working, you can use it again
when another keyword cipher comes up.
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