Password Generator

Where the randomness comes from

Every character is chosen with crypto.getRandomValues, the browser's cryptographically secure random source, not with Math.random. For each position the tool asks for one unsigned 32-bit integer (a value from 0 to 4,294,967,295) and takes it modulo the size of the pool you selected. With all four sets ticked the pool holds 87 characters, so a 16-character password consumes 16 separate 32-bit draws.

Taking a 32-bit value modulo 87 leaves a tiny bias, because 4,294,967,296 divided by 87 is not a whole number (the remainder is 16). Sixteen of the 87 characters are therefore favoured by one part in roughly 49 million. That is far too small to help an attacker, but it is worth knowing if you are auditing the code.

Each position is drawn independently. Nothing forces a symbol or a digit to appear just because its box is ticked. With all sets on, the chance that a 6-character password contains no symbol at all is (62/87)^6, about 13%; at 12 characters it falls to 1.7%, and at 16 characters to 0.4%. If a site insists on at least one symbol, glance at the result or regenerate.

The exact character sets

The symbol set deliberately leaves out the space, both quote marks, the backslash, the backtick, the tilde, the pipe and the forward slash. Those are the characters most likely to be mangled by a shell, a URL or a badly written sign-up form. The flip side is that a site which demands a specific symbol outside this list will need you to add it by hand.

No similar-looking characters are excluded: lowercase l, capital I, the digit 1, capital O and the digit 0 can all appear. If you will ever read the password aloud or type it from paper, regenerate until the ambiguous ones are gone, or stick to copy and paste.

Entropy for each slider setting

Password strength is measured in bits of entropy: bits = length × log2(pool size). The pool sizes above give 4.70 bits per character for lowercase only, 5.70 for mixed-case letters, 5.95 for letters plus digits and 6.44 for all 87 characters. A 16-character password from the full pool is therefore 16 × 6.443 = 103.1 bits.

What those bits mean against a guessing attack

On average an attacker finds the password halfway through the search, so halve those figures for the expected time. A trillion guesses a second is a reasonable estimate for an offline attack on a leaked database of fast hashes; against a website that rate-limits logins the attacker gets perhaps a few guesses a second, which makes even the 8-character row survive for decades. The slider default of 16 is a comfortable margin for anything that matters, and 20 or more costs nothing when a password manager types it for you.

Things this generator does not do

• It has no passphrase or pronounceable mode; the output is always a string of independent random characters.
• It does not check the result against breach lists, nor enforce 'at least one of each type' rules.
• It keeps no history. Moving the slider or toggling a checkbox replaces the current password instantly, and the previous one cannot be recovered, so copy a password you like before adjusting anything.
• Length stops at 48. For a longer secret, generate two and join them.
• Whatever you copy sits on the system clipboard until you copy something else, where other applications may be able to read it.