Dragons (150)

• Challenge description: Here at RTN be dragons! But where are they really?
• Points: 150
• Contents: dragons.zip (containing image.png and message.txt)

This challenge was fairly easy, as indicated by the points assigned to it.

We're given just an image file. As usual when you're given a single file, you should see if there's some interesting data hidden in it. One way to check that is with the strings command on Linux. If we run that, we see that our output ends on this:

...
!dt!
!dt!
IEND
7 is affine number:
WKU{3UC_0Q_Q1G3_1D_0Q73U_1XU0W3C}

This immediately looks like a letter-based cipher such as the Caesar or Vigenère cipher, so normally I would immediately throw this into CyberChef to try, but let's look up what this "affine" thing means first.

According to Wikipedia, Affine Cipher seems to be like Caesar cipher, but instead of f(x) = (x+a) mod 26, we do f(x) = (ax+b) mod 26. Now I don't know about you, but reading an entire Wikipedia for a 150-point challenge seems like a lot of effort to me.

Luckily Google comes to the rescue. Googling "Affine cipher cracker" brings you to decode.fr, a brilliant site with lots of crypto tools. Putting in the ciphertext in the correct box and clicking "automatic brute force decryption" loads the same page but with tons of possible decrypted flags. Search the page for "RTN" and we find the flag A=7,B=7 RTN{3ND_0F_F1L3_1S_0F73N_1GN0R3D}.

Looks like our 2 parameters were this "fine" number 7. But we didn't need to know, because we are lazy and know how to quickly cheese CTF challenges!