Hack The Box Cyber Santa CTF 2021 — Common Mistake

Cyber Santa Capture The Flag

From the Crypto Category of Cyber Santa Is Coming To Town CTF which was going on from December 1st to December 5th 2021, there was a challenge called “Common Mistake”. let’s solve this challenge

Take a look at the challenge description:

challenge description

let’s cat encrypted.txt file, as you can see, we have the same modulus and two different encryption values and ciphertexts Which leads to the Common Modulus attack.

encrypted file content

Generate the first public key using (‘n’ and the 1st ‘e’ value):

key1.pub

Generate the second public key using (the same ‘n’ and the 2nd ‘e’ value):

generate first public key using (n and 1st e):key2.pub

Generate Base64 encoded form of the first cipher text using rsa-cm.py

c1.b64

Generate Base64 encoded form of the second cipher text using rsa-cm.py

c2.b64

Run the script to perform common modulus attacks on RSA to recover the plaintext of the message:

flag

and we get the flag: HTB{c0mm0n_m0d_4774ck_15_4n07h3r_cl4ss1c}

hope you enjoyed this write-up…