tinyCTF – Cry100

Cry100 Challenge

Cry100 Challenge

This was one of the first crypto challenges I’ve done for a CTF, and thankfully it was basic enough (it was only worth 100 points, after all)! The challenge file provided was a text file which looked like it contained words and sentences, only the letter values were jumbled up. Since the challenge was very likely an easy one, I didn’t overthink the possible solutions. My first thought was that this could have been a simple character substitution problem.

I searched the text for anything to help identify common english words. I used double letter combinations (GG, II) and tried substituting them for common double letters seen in english (LL, EE, OO, SS). I also looked for single character words in the text and only found ‘A’, which appeared to be a literal A in the translated text. Finally, the last thing I wanted to look for were four letter words which I could translate into ‘Flag’, since all prior challenges contained the word ‘Flag’ as part of the solutions.

With these rules to work from, I bruteforced the remaining text with a lot of trial and error to come up with the translated text and subsequently, the flag (Figure 1)!

Figure 1 - Garbled text and result with flag.

Figure 1 – Garbled text and result with flag.

Flag: flag{no_this_is_not_crypto_my_dear}

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