Password Generator with Passphrases and Entropy

How this random password generator works

Every character comes from crypto.getRandomValues, the browser's cryptographically secure random number generator, which is seeded by your operating system. Math.random is never used. The page runs entirely in this tab: no password is sent to a server, written to storage or placed in the URL, and there is deliberately no share link, because a link containing a secret ends up in browser history and chat logs.

A detail many generators get wrong is modulo bias. Turning a random 32-bit number into a position in an 89-character alphabet with value % 89 makes the first few characters slightly more likely, because 2^32 is not a multiple of 89. This generator uses rejection sampling instead: it computes the largest multiple of the alphabet size that fits in 32 bits, throws away any draw above it and draws again. Every character, and every word in passphrase mode, is then exactly equally likely. The same unbiased draw is used to shuffle the characters when "at least one of each selected set" is on, so the guaranteed digit or symbol does not always sit in the same position.

The entropy figure is the strength of the generation process, not a guess about the string: length × log2(alphabet size). Sixteen characters from all four sets (89 characters) gives about 103 bits; excluding look-alike characters such as I, l, 1, O and 0 shrinks the alphabet a little and the meter updates to match.

Strong password generator settings: 12, 16 or more characters?

Length matters more than any other setting. A 12 character password generator using upper and lower case, digits and symbols yields roughly 77 bits, which is fine for an online account protected by rate limiting. A 16 character password reaches about 103 bits and is a sensible default for anything stored in a password manager, because you never type it. For keys that protect data offline, such as a disk encryption password or an SSH key passphrase, aim for 100 bits or more, since an attacker with a stolen file can guess billions of times per second without being slowed down.

If a site rejects symbols or caps the length, keep the length as high as it allows and turn off only the set it refuses; each extra character adds more strength than any single character class does. Use the bulk option (1-50) when you need several credentials at once, for example seed users for a test environment or one-off database passwords for a staging deployment. The crack-time estimate assumes ten billion guesses per second against a fast hash and is meant for comparison, not as a guarantee.

Passphrase generator: the xkcd approach

xkcd comic 936 made the case that four random common words are easier to remember than a short string of symbols and can be stronger. The key word is random: the words must be picked by a machine, not by you, because humans choose predictable phrases. Passphrase mode draws each word uniformly from a built-in list of just over 2,000 common English words, so each word contributes about 11 bits. Five words give roughly 55 bits, six give 66 and eight give 88. Capitalising words does not add entropy here because it is applied to every word, while "add a digit" adds a few bits by placing one random digit after one random word.

Use a passphrase where you actually have to type or remember the secret: a password manager master password, a laptop login or a Wi-Fi key you read out to guests. For everything else, let your password manager store a long random password instead.