Lost.md

🤷Lost - 400pts

💡 Just kinda cryptography

Required files

output.txt

challenge.py

Challenge.py Explanation

#!/usr/bin/python3
from Crypto.Util.number import getPrime, long_to_bytes, inverse
flag = open('flag.txt', 'r').read().strip().encode()

class RSA:
    def init(self):
        self.p = getPrime(512)
        self.q = getPrime(512)
        self.e = 3
        self.n = self.p self.q
        self.d = inverse(self.e, (self.p-1)*(self.q-1))
    def encrypt(self, data: bytes) -> bytes:
        pt = int(data.hex(), 16)
        ct = pow(pt, self.e, self.n)
        return long_to_bytes(ct)
    def decrypt(self, data: bytes) -> bytes:
        ct = int(data.hex(), 16)
        pt = pow(ct, self.d, self.n)
        return long_to_bytes(pt)

def main():
    crypto = RSA()
    print ('Flag:', crypto.encrypt(flag).hex())

if name == 'main':
    main()

Basically, the flag is encoded and they give you a function to decrypt it in a way that you have to repeatedly or recursively decode the message until it is human-readable or at least flag legible

• output.txt that they provide you is the encoded flag you must decode
• It is encoded with hex
• You seriously do not have to understand the majority of these lines

You could write a program to accomplish this: