Home/ Questions/Q 965331
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-16T01:58:46+00:00

I have been trying to solve Project Euler’s problem #59 for a while, and

  • 0

I have been trying to solve Project Euler’s problem #59 for a while, and I am having trouble because some of it seems somewhat more ambiguous than previous problems.

As background, the problem says that the given text file is encrypted text with the ASCII codes saved as numbers. The encryption method is to XOR 3 lowercase letters cyclically with the plaintext (so it is reversible). The problem asks for the key that decrypts the file to English text. How should I restrict the character set of my output to get the answer, without trying to sift through all possible plaintexts (26^3)?

I have tried restricting to letters, spaces, and punctuation, and that did not work.

To clarify: I want to determine, out of all printable ASCII characters, which ones I can probably discard and which ones I can expect to be in the plaintext string.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    Have you tried two of the most “basic” and common tools in analyzing the algorithm used?

    1. Analyze the frequency of the characters and try to match it against English letter frequency
    2. Bruteforce using keys from a wordlist, most often common words are used as keys by “dumb” users

    To analyze the frequency for this particular problem you would have to split the string every third element since the key is of length 3, you should now be able to produce three columns:

    79  59  12
2   79  35
8   28  20
2   3   68
...

    you have to analyse the frequency for each column, since now they are independent of the key.

    Ok, actually took my time and constructed the 3 complete columns and counted the frequency for each of the columns and got the two most frequent item or each column:

    Col1  Col2  Col3
71    79    68
2     1     1

    Now if you check for instance: http://en.wikipedia.org/wiki/Letter_frequency
    You have the most frequent letters, and don’t forget you have spaces and other characters which is not present on that page, but I think you can assume that space is the most frequent character.

    So now it is just a matter of xor:ing the most frequent characters in the table I provided with the most frequent characters in English language, and see if you get any lowercase characters, I found a three letter word which I think is the answer with only this data.

    Good luck and by the way, it was a nice problem!

    • 0