Bébé nageur ⚪

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Description

Vous ressortez de votre premier cours de natation et quelqu'un vous a laissé un petit mot dans votre casier. Vous suspectez votre rival que vous venez juste de battre à plate couture lors d'une course effrénée dans le bassin des bébés nageurs.

Déchiffrez ce message.

all files in resources/ were provided.

Challenge

Looking at the challenge.py file, we see a hashed flag, that plays with two different random numbers a and b, it uses these numbers trough a formula f to cipher the flag.

First step let's find both random values, we can guess that the flag starts with 404CTF{ this is more than enough to find the correct a and b:

HASH = "-4-c57T5fUq9UdO0lOqiMqS4Hy0lqM4ekq-0vqwiNoqzUq5O9tyYoUq2_"
flag = "404CTF{"

for a in range(len(charset[2:n-1])):
  for b in range(len(charset[1:n-1])):

    for i in range(len(flag)):
      h = charset.index(HASH[i])
      x = charset.index(flag[i])

      if h != f(a, b, n, x):
        break

      if x + 1 == len(flag):
        print(f"Found random values: a={a}, b={b}")
        # > 19, 6

Now that we have them it's just a matter of brute forcing the whole charset for each character of the flag and finding the flag:

flag = ""

for h in HASH:
  for c in charset:
    if h == charset[f(19, 6, n, charset.index(c))]:
      flag += c

print(f"Found flag: {flag}")

Flag: 404CTF{Th3_r3vEnGE_1S_c0minG_S0oN_4nD_w1Ll_b3_TErRiBl3_!}