How Secure is Your Password? Calculating Shannon Entropy in the Browser

We've all seen password strength meters on sign-up forms. Most of them rely on simplistic, static rules: "Must contain at least 8 characters, one number, and one special character." But from a mathematical standpoint, these rules are a poor proxy for actual password security. A password like Tr0ub4dor&3 conforms to these rules but is far easier to compromise than a randomly generated four-word passphrase like correct-horse-battery-staple. To truly measure password security, we have to look at in