GottaDecryptThemAll.md

Gotta Decrypt Them All

You are stuck in another dimension while you were riding Solgaleo. You have Rotom-dex with you to contact your friends but he won't activate the GPS unless you can prove yourself to him. He is going to give you a series of phrases that only you should be able to decrypt and you have a limited amount of time to do so. Can you decrypt them all?

nc crypto.chal.csaw.io 5001

We are given a series of plaintexts encrypted using the same scheme, which is a suite of encryptions and encodings.

Solution

We're using CyberChef to decrypt the first message. The magic tool allows us to discover the different encodings used: morse code, decimal encoding, base64.

After those encodings, we are greeted with an RSA instance, which has the following particularities: N is much bigger than c and e=3. Therefore probably the mod part in the encryption scheme has not been used, and we can just compute the cube root of the ciphertext to retrieve the plaintext.

Finally we need to perform a ROT13 on the string.

We automate all this using Python:

from pwn import *
import morse
import base64
from Crypto.Util.number import *
import codecs


def find_cube_root(n):
    lo = 0
    hi = 1 << ((n.bit_length() + 2) // 3)
    while lo < hi:
        mid = (lo+hi)//2
        if mid**3 < n:
            lo = mid+1
        else:
            hi = mid
    return lo

sh = remote("crypto.chal.csaw.io", 5001)

def decode():
    sh.recvuntil("mean?")
    x = sh.recvline()
    x = sh.recvline().decode().strip().replace("/", " ")

    x = morse.decrypt(x)

    x = "".join([chr(int(c)) for c in x.split(" ")])

    x = base64.b64decode(x).decode()

    c = int(x.split("=")[3])
    x = long_to_bytes(find_cube_root(c)).decode()

    x = codecs.encode(x, 'rot_13')
    print(x)

    sh.sendline(x)
    print(sh.recvline().decode())

for i in range(6):
    decode()
sh.interactive()

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