UT CTF 2024
Beginner Anti Dcode.fr [100 pts]
I’ve heard that everyone just uses dcode.fr to solve all of their crypto problems. Shameful, really.
This is really just a basic Caesar cipher, with a few extra random characters on either side of the flag. Dcode can handle that, right? >:)
The ‘{‘, ‘}’, and ‘_’ characters aren’t part of the Caesar cipher, just a-z. As a reminder, all flags start with “utflag{“.
By Khael (Malfuncti0nal on Discord).
Caesar’s Cipher is a type of encryption cipher that essentially shifts each character over by a specific offset. i.e., an offset of 1 means A -> B, B -> C, …, Y -> Z, Z -> A.
We could run every possible rotation number of Caesar’s Cipher on the ciphertext, but, there’s one problem. The ciphertext file is 977 kB, meaning that doing so will take a while.
Instead, we can use an online Caesar’s Cipher/ROT site like this one to find all possible results for performing Caesar’s cipher on “utflag”, which is known to be the prefix for all flags. We can then Ctrl+F for each possible result in the txt file to find this string: “cbntio{zqx_lkwlm}”, which decodes to the flag (using the same website):
utflag{rip_dcode}