basic-mod2

Challenge Description

This challenge is similar to the previous challenge, basic-mod1, but instead of calculating the modulus of each number, we need to calculate the modular inverse. There is also a slight difference in the character set instructions that we need to consider.

Getting the Flag

Running cat message.txt after downloading the file with wget displays a string of numbers:

268 413 438 313 426 337 272 188 392 338 77 332 139 113 92 239 247 120 419 72 295 190 131

We need to calculate the modular inverse of each number and map it to a character in the predefined character set in the description. I used a python script to do this. The modular inverse function was created with the assistance of this page.

Decryption Script

chrs = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_"
enc_message = "268 413 438 313 426 337 272 188 392 338 77 332 139 113 92 239 247 120 419 72 295 190 131"
flag = ""
 
def modular_inverse(x: int, m: int) -> int:
   return pow(x, -1, m)
 
for i in enc_message.split():
   mod_inverse = modular_inverse(int(i),41)
 
   # -1 since the character set for this challenge starts from 1, unlike the previous challenge
   flag += chrs[mod_inverse-1]
 
print(flag)

Flag

picoCTF{1NV3R53LY_H4RD_8A05D939}