I designed an algorithm to encrypt a message. No one can decrypt it without any information about my algorithm.
Solution
We are given a string that looks like this: 0x56455a7059574e7459584a685a576c756557646f59565637633364765a57647664474e305a48427965584e4464476c6c62584e6c6257463165574e3059574e7a5647687a6247567a5a6d527a636d46796233427366513d3d
Judging from the 0x, it looks to be hexadecimal. After decoding it we end up with:
Now we have something starting to resemble the flag format:
TUCTF{3x4mpl3_fl4g}
From here we simply need to figure out how the flag is scrambled. We know that it should start with “TUCTF”, so lets quickly take a look at the position of those characters in the string. Assuming the first character “T” is in its correct place (index 0), we notice that the next character we need “U” is at index 16. If we move over another 16 indexes, we get the next letter “C” and so on.
Ok, so we know the pattern that we can use to unscramble the letters, now we just have to code it. My solution will use Java but this can be done in any language:
publicclassSheepDcode { publicstaticvoidmain(String[] args) { Stringenc="TFiacmaraeinyghaU{swoegotctdprysCtiemsemauyctacsThslesfdsraropl}"; //our scrambled flag Stringflag=""; //the unscrambled flag we will make. intidx=0; //we will start with an index of 0, since we assume the first letter is positioned correctly
/* This loop will simply go through our letters from index 0 to our last index, moving over 16 indexes at a time. * When our idx goes over the maximum idx of enc, we simply mod it by 16, then add 1, shifting our letters so * we don't end up with the same letters repeating each time we go out of bounds.*/ for(inti=0; i < enc.length(); i++){ flag += enc.charAt(idx); System.out.println(flag); //debug print idx += 16; if(idx >= enc.length()){ idx = idx % 16 + 1; } } System.out.println(flag); } }
Running the code, we get this satisfying output as it puts together our flag, letter by letter:
Big thanks to the TUCTF organizers for putting on such a great CTF this year! This was my first year doing it, and I’ll definitely be back again next year.
