import base64 as b64

PLAINTEXT = '''{"showpassword":"no","bgcolor":"#ffffff"}'''
COOKIE = 'HmYkBwozJw4WNyAAFyB1VUcqOE1JZjUIBis7ABdmbU1GIjEJAyIxTRg='
FORGED_PLAINTEXT = '''{"showpassword":"yes","bgcolor":"#ffffff"}'''



def xorbytes(x: bytes, y: bytes) -> bytes:
    Lx, Ly = len(x), len(y)
    if Lx < Ly: return xorbytes(y, x)
    
    return bytes(x[i]^y[i%Ly] for i in range(Lx))
        
def extract_key(k: bytes) -> tuple[bytes, int] | None:
    Lk = len(k)
    substr = b''
    length = 0
    for i in range(Lk):
        substr += k[i:i+1]
        length += 1 
        if k == substr*(Lk//length) + substr[:Lk%length]:
            return substr, length
    return None


def main() -> None:
    plaintext = PLAINTEXT.encode()
    cookie    = b64.b64decode(COOKIE)
    decoded   = xorbytes(cookie, plaintext)
    print('Modulated Key:', ''.join(chr(x) for x in decoded))
    key, key_size = extract_key(decoded)

    forged_cookie = b64.b64encode(xorbytes(FORGED_PLAINTEXT.encode(), key))
    print('Forged:', forged_cookie)
    

if __name__ == '__main__':
    try:
        main()
    except (KeyboardInterrupt, EOFError):
        print('\n[!] Interrupt')