README.md

Fancy Caesar Cipher

We RSA encrypted the flag, but forgot to save the private key. Is it possible to recover the flag without it?

Download the file below.

fancy_caesar_cipher.py fancy_caesar_cipher.out

We are given the Python source code and the output

It is a typical RSA decryption challenge:

from Crypto.Util.number import getStrongPrime
from fractions import gcd
from secret import flag

def get_key(e=65537, bit_length=2048):
    while True:
        p = getStrongPrime(bit_length, e=e)
        q = getStrongPrime(bit_length, e=e)
        if gcd(e, (p - 1) * (q - 1)) == 1:
            return e, p * q

def encrypt(e, n, m):
    return [((ord(c) ** e) % n) for c in m]

e, n = get_key()

print("Generated key:")
print(e)
print(n)

print("Encrypted flag:")
print(encrypt(e, n, flag))

Notice the encrypt function, it encrypts the flag characters one by one

The public key is also given, means we can just find the plaintext by just brute forcing the ciphertext:

flag = ""
for o in out:
	# Brute force every printable character
	for i in range(32,127):
		# If the ciphertext are equal then the plaintext found
		if pow(i,e,n) == o:
			flag += chr(i)
			break
print flag 

Result:

flag{phwd_iodj_lv_phwd}

Notice the flag is encrypted by Caesar Cipher, simply shift it by 23 become meta_flag_is_meta

Python script

Flag

flag{meta_flag_is_meta}