Password strength checker

This measures one thing: how many guesses an attacker would need if your password really were random. It reads the character classes present and the length, and reports the entropy that combination could carry at best.

Be clear about the limitation, because most strength meters are not. This is an upper bound, not a score. "Password1!" contains four character classes across ten characters and scores as though it were random, when in reality it appears near the top of every cracking dictionary and falls in under a second. No meter that looks only at the characters can tell the difference. The number is meaningful for a password you generated randomly and misleading for one you invented — which is the strongest argument for not inventing them.