Password generator: strong, random passwords

NIST SP 800-63B, the US federal guide to passwords, sets the minimum at 15 characters when a password is the only factor, and 8 when it is used with a second factor. It also says sites should accept at least 64 characters and should not force rules like "one uppercase, one symbol". Length matters more than variety.

Entropy is length × log2(pool size). Here is what common settings give:

The default here is 20 characters from upper and lower case, digits and 25 symbols (87 characters), which is about 129 bits. With "Require at least one from every set" on, the tool counts only the passwords that pass that rule, so the number shown is exact and a little lower.

A random password is the best choice for anything a password manager fills in. A passphrase is a few random words, like a sentence nobody would write. It is easier to type on a phone or TV and to remember, so it suits the few passwords you type by hand.

The word list here has about 1,040 short, common English words, so each word adds about 10 bits. Six words give about 60 bits, plus about 6 more for a digit in a random position. Choose 8 words or more for a master password. The EFF large word list has 7,776 words (about 12.9 bits each), which is why EFF dice phrases can be shorter for the same strength.

A PIN is digits only and weak on its own: 6 digits is under 20 bits. It is fine where the device locks you out after a few wrong tries, like a phone or a bank card, and nowhere else.

Every character comes from crypto.getRandomValues(), the browser's cryptographically secure random number generator. It is not Math.random(), which is fast but predictable and must never be used for passwords.

Picking a character from random numbers has a trap called modulo bias. A random byte has 256 values. If you take byte % 87, the first 82 characters can come up 3 ways and the other 5 only 2 ways, so some characters show up more often. This tool draws 32-bit numbers and throws away the few at the top that do not divide evenly (rejection sampling), so every character is exactly as likely as any other.

"Require every set" works by drawing the whole password again until it contains each set, instead of forcing one character from each set into fixed places. That keeps every allowed password equally likely. The crack-time table assumes the attacker knows your settings and has to try half the possible passwords on average.

Use the secure random source of your language, never a general-purpose one like random in Python or Math.random in JavaScript.

# Python 3 (secrets is the module for passwords and tokens)
import secrets, string
alphabet = string.ascii_letters + string.digits + string.punctuation
password = ''.join(secrets.choice(alphabet) for _ in range(20))

// Node.js (crypto.randomInt is uniform, no modulo bias)
const { randomInt } = require('node:crypto');
const a = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*';
const password = Array.from({ length: 20 }, () => a[randomInt(a.length)]).join('');

// Go (crypto/rand.Int returns a uniform value in [0, max))
const a = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
b := make([]byte, 20)
for i := range b {
    n, _ := rand.Int(rand.Reader, big.NewInt(int64(len(a))))
    b[i] = a[n.Int64()]
}

# Shell: tr drops bytes outside the set, so the result is unbiased
LC_ALL=C tr -dc 'A-Za-z0-9!@#%^*_+=-' < /dev/urandom | head -c 20; echo

To store passwords on a server, hash them with a slow password hash such as bcrypt, never as plain text. The bcrypt generator makes and checks bcrypt hashes, and the htpasswd generator makes Apache and Nginx password files. For API keys and tokens instead of passwords, use the random string generator.

• Use a different password for every account. A leak on one site is then only one leak.
• Save passwords in a password manager right away. Copying from this page puts the password on your clipboard; paste it, then copy something else.
• Turn on two-factor login where you can. Even a perfect password can be phished.
• Turn on "Avoid look-alikes" for passwords you will read aloud or type from paper, so l, 1 and I are never mixed up.
• Do not reuse a generated password after it has appeared in a screenshot, a chat or a ticket.

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