You can design a system that’s mathematically unbreakable if the key stays secret and truly random, but in practice you can never guarantee that. Even a single weak bit is enough for an attacker to launch a brute‑force attack or side‑channel exploit. Storing a key forever is a paradox: the longer it’s stored, the more likely it will be compromised, either by human error or by a future breakthrough. So you can get arbitrarily close, but you’ll never reach a true “uncrackable” state.