Password Generator

Entropy is length multiplied by the bits per character, and length is the term that scales. A 20-character lowercase-only password has about 94 bits of entropy. A 10-character password using all four character classes has about 65. The longer, simpler one is roughly half a billion times stronger, which is not a close call.

This is why the guidance changed. NIST SP 800-63B, revised in 2017 and reaffirmed since, explicitly recommends against composition rules - the requirements to include an uppercase letter, a digit and a symbol. They add little entropy and they push people towards predictable patterns: capitalise the first letter, append a `1`, finish with a `!`. Attackers model those patterns, so the rules make passwords more guessable rather than less.

The same guidance also advises against mandatory periodic rotation, for the same behavioural reason. Forced 90-day changes produce `Summer2024!` becoming `Autumn2024!`, which is worse than a strong password kept until there is evidence of compromise.

A generated password is only as unpredictable as its random source. `Math.random()` is a pseudorandom generator seeded from limited state and designed for speed, not secrecy - its output can be predicted from previous values. A password generator built on it produces passwords an attacker with knowledge of the algorithm can reconstruct.

Browsers expose `crypto.getRandomValues()`, which draws from the operating system's cryptographically secure entropy pool. That is the only acceptable source for anything intended to be secret, and it is what this tool uses.

There is a subtler bug worth knowing about even in generators that use a secure source. Reducing a random number into an alphabet with the modulo operator introduces bias unless the alphabet size divides evenly into the range, because the lower values become slightly more likely. The correct approach is rejection sampling - discard values in the biased tail and draw again. The bias is small but it is a real reduction in entropy.

For a password you have to type or remember, a passphrase of random words is easier to handle at equal strength. Six words drawn randomly from a 7,776-word list - the Diceware approach - gives about 77 bits of entropy, comparable to a 13-character random string but far easier to type accurately and to recall.

The critical word is random. Choosing memorable words yourself collapses the entropy, because human word choice is heavily skewed and phrases from songs, films or books are in every cracking dictionary. The words must come from a generator, not from your head.

For everything you do not need to memorise, a password manager is the better answer. It allows a genuinely unique 20-plus character random password per site, which is the single most valuable property - it means one breach cannot cascade. Reuse, not weakness, is what turns a breach at one service into compromised accounts everywhere else, and credential-stuffing attacks exist precisely to exploit it.