Questcon writeups 2023 crypto

encoded text : VUUEV2QGW364QGN3YE:MN16eUGMpaE:La2:VMDty`03>

I used the magic function in cyberchef

and that’s how we got the flag : QUESTCON{D34d_M3n_T3ll_No_T4l3s}

N = 882564595536224140639625987659416029426239230804614613279163
E = 65537
C = 164269225538436495685306542268826436068505673594249194166792

This is the content of the file

I used https://www.alpertron.com.ar/ECM.HTM to get the p and q, which are the values that makes N.

N = p * q

now we write a script to get decipher the C

from Crypto.Util.number import inverse, long_to_bytes

n = 882564595536224140639625987659416029426239230804614613279163
q = 857504083339712752489993810777
p = 1029224947942998075080348647219

e = 65537

ct = 164269225538436495685306542268826436068505673594249194166792

phi = (p - 1) * (q - 1)

d = inverse(e, phi)

message = pow(ct, d, n)

print(long_to_bytes(message))  

┌──(mahmoud㉿kali)-[~/…/CTF/Questcon2023/crypto/chall2]
└─$ python3 sol.py 
b'QUESTCON{1_HaT3_RS1}'