Optimus Prime has returned to Earth, choose an historical character and overcome this threat.
Information
Challenge: Optimus prime
Category: Crypto
Difficulty: Easy
Environment: Remnux VM
Walkthrough
1. Server Analysis
Connecting with netcat shows a server with four menu options:
All options are useless, except option four, which returns a Public Key and Encrypted Password:
Since we are not given source code, I tried checking the traffic for the ’email’ with Wireshark, but nothing came out of it. Runing the option 4 a couple of times, I noticed that each Public Key has the same GCD, which would be the P value! Given that, we can recover the corresponding q and then decrypt the encrypted password.
2. Common Factor Attack
To recover the plaintext, we need to recover each RSA parameter:
from pwn import *
import math
host = "159.65.49.103"
port = 30243
def get_values(t):
t.recvuntil("option: ")
t.send("4")
out = t.recvuntil("proceed: ").decode("utf-8").split('\n')
values = {'pk': int(out[0].replace('PUBLIC KEY: ','')), 'ep': int(out[1].replace('ENCRYPTED PASSWORD: ',''))}
return values
def recovery_items(val1,val2):
p = math.gcd(val1['pk'], val2['pk'])
q = val2['pk'] // p
assert q * p == val2['pk']
phi = (p - 1) * (q- 1)
e = 65537
d = pow(e, -1, phi)
plaintext = pow(val2['ep'], d, val2['pk'])
message = bytes.fromhex(hex(plaintext)[2:]).decode()
return message
def solve(host,port):
out_vals = []
t = remote(host,port)
out_vals.append(get_values(t))
t.sendline("hi")
t = remote(host,port)
out_vals.append(get_values(t))
message = recovery_items(out_vals[0], out_vals[1])
t.sendline(message)
result = t.recvline()
print(result.decode())
solve(host,port)
#ACCESS GRANTED: HTB{3uc1id_w4z_th3_pr1me_h4x0r}
