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Practice challenge B

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  • #43880

    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?

    #44018
    Harry
    Keymaster

    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!

    #44029
    Madness
    Member

    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
    KEYWORDABCFGHIJLMNPQSTUVXZ

    b. start with the last letter of the keyword
    KEYWORDFGHIJLMNPQSTUVXZABC

    c. start after the alphabetically last letter of the keyword; in this case ‘Y’
    KEYWORDZABCFGHIJLMNPQSTUVX

    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.

    https://www.researchgate.net/publication/266714630_A_fast_method_for_cryptanalysis_of_substitution_ciphers

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