Randomness

A surprising amount of security rests on values an attacker cannot predict. Session tokens, keys, password reset links.

The word random is doing heavy lifting there. Computers are deterministic, so producing genuinely unguessable values takes deliberate effort and the right tools. When the wrong ones are used, the values look random to a human and turn out to be perfectly predictable to somebody who works out the pattern. There is a long history of exactly this: keys that could be regenerated, reset links that could be guessed in order, tokens that repeated.