InCTF Jr 2023
May - August, 2023


Medium Crypto

Author: Pavani

If you see how this server is working, it is giving you some encrypted data in hex and asking you to give some input. Now, let’s see the given script. Here we used CBC encryption from AES. Here, given plain text is admin=no! . You will get a flag if plain text is admin=yes.let auth_cookie be ciphertext(ct) and auth_message be plaintext(pt).

So now you have to give some input that pretends plaintext has admin=yes.

CBC mode decryption

1.Now you have to change last three bits to get the flag. 2. Use xor operation between nth block of ct,pt and n-1th block of ct. (here change bits of IV to change bits of pt(admin=no!)" 3.change bits of n-1th block(iv in this challenge) in such a way that would give admin=yes in plaintext(pt).

Now, try this challenge again. If you didn't get,then look up the script.