Indecipherable for computers: The Captcha with the password is very grainy, as it is generated in a physical system close to a critical change of state (left). In a chaotic process, it is made completely unreadable. The process can be reversed with an easily remembered password, however. (Photo: Sergej Flach / MPI for the Physics of Complex Systems)
Hacking programmes use so-called brute-force attacks to try out all possible character combinations to guess passwords