Reply Cyber Security Challenge - Something is missing

tags: ctf challenge write-up crypto
by Firpo7

Fortunately, the hidden text he’s found is the password for the ZIP file. R-Boy finds another password-protected archive, and another encrypted file. As before, R-Boy decides to focus on the file, maybe he’ll get lucky again! Can you help R-Boy crack the encryption?

The challenge provided only one file with an encripted text. Unfortunately we accoplished this challenge only when the time for the CTF was finished. The hint released from the organizers was very important to us to understand how to find the solution. The hint was:


So we could understand that the encription algorithm was RSA and it has a low exponent weakness.. By googling and doing some research we descovered that gmpy.iroot could find the original message from the cipher text and the exponent

Image of iroot_screen

We are close to the solution… The last pass to do is to decrypt the ROT13

 $ echo 'Guvf vf gur cnffjbeq lbh arrq sbe gur MVC svyr: {SYT:Z0erQ33crE!}' | tr 'A-Za-z' 'N-ZA-Mn-za-m'
 This is the password you need for the ZIP file: {FLG:M0reD33peR!}