Hello dudes,
today we 'll learn how to break a very interesting crackme from Waganono. It shouldn't be too hard if you know how to break a Vigenere cipher.
In the first part, you have to guess the key's length. To do that, i did the Kasiski test's : you can find some infos about this test here. I found that the key was 8 chars long.
Then, i did a frequence analysis (For that, I coded a home made tool) The author said that the text is french. We all know that the most popular characters in french are " " and "e". So then it's easy to guess the key.
First character :
72 52 f=0.227074 22.71%
37 27 f=0.117904 11.79%
So the first char is 72h XOR 20h = 52h = 'R'
Second character:
65 62 f=0.270742 27.07%
20 21 f=0.091703 9.17%
So the second character is 65h XOR 20h = 'E'
...
At the end, we have the key which is an anagramm for reverser : RESREVER.
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Wow I would like to see source for real frequency analysis.
RépondreSupprimerMore weak is to consider a whole character set [A-Za-z] as the common "e". At 41% of 7-bit ascii set, it is still limiting enough to derive the key.
"RESREVER" is "REVERSER" in REVERSE, an easy anagram :)
lol man, it's really lame, i can't even publish it
RépondreSupprimeri count the number of letters on the buffer (in fact BYTE here) and i do a division in order to get the ratio, and that's all