Generate a secure password

Eight characters, one capital, one digit, one symbol: that is still the default rule on a good share of sign-up forms. A password that meets exactly those constraints carries roughly 52 bits of entropy. Set against the realistic scenario — a database leaks, then an attacker runs combinations on hardware of their own, at a scale of a trillion attempts per second — 52 bits fall in a little over an hour, half that on average. The problem is not that you choose your passwords badly: it is that these rules optimise the wrong variable.

The method that works comes down to three decisions: draw the password at random instead of inventing it, make it long rather than convoluted, and give up on memorising it. The password generator covers the first point; the rest is a matter of settings and a few habits.

What a password’s “strength” actually measures

The useful quantity is entropy, in bits: length × log₂(size of the character set). Every bit doubles the attacker’s work.

Character set	Size	Bits per character
lower case only	26	4.70
lower case + capitals	52	5.70
+ digits	62	5.95
+ symbols (28 punctuation marks)	90	6.49

A 16-character password drawn from the 90 available characters is therefore worth 16 × 6.49 ≈ 104 bits. That is the scale behind the bands the tool displays: below 50 bits “weak”, 50 to 75 “medium”, 75 to 100 “strong”, anything beyond that “excellent”.

The formula carries a hidden condition, and it is the decisive one: it only holds if every character is genuinely drawn at random. Paperclip2024! mixes capitals, lower case, a digit and a symbol, so 90 possible characters on paper. But a cracking tool does not try every combination: it works through a dictionary of common nouns, each followed by a four-digit year, then by a punctuation mark. A few tens of millions of possibilities, or about 25 bits. Composition rules measure appearance, not resistance.

Length before complexity

The same formula applied to a few common combinations:

• 10 characters, full set (90 characters): 64.9 bits — medium.
• 12 characters, letters and digits (62 characters): 71.5 bits — medium.
• 16 characters, lower case only (26 characters): 75.2 bits — strong.
• 16 characters, full set: 103.9 bits — excellent.
• 24 characters, full set: 155.8 bits — beyond the reach of brute force.

Sixteen lower-case letters in a row beat ten “complex” characters: adding one character to a full-set password buys 6.49 bits, and nothing stops you adding ten of them. Widening the set pays more at equal length — 1.8 bits per character going from lower case to the full set — but that lever runs out quickly: past the 95 printable ASCII characters there is nothing left to gain. Length has no ceiling. Making it longer is almost always the better investment.

The generator’s slider runs from 6 to 64 characters, with 16 by default. For anything that will end up in a manager and be pasted in automatically, push it to 20 or 24: length costs you nothing, because you will never type the string.

Setting the generator up for the job

Excluding ambiguous characters

The option removes seven characters: capital I, lower-case l, the digit 1, the vertical bar, capital O, the digit 0 and lower-case o. The eye confuses them depending on the font, and the ear cannot tell them apart at all. Switch it on as soon as a person has to read the string out or dictate it: a Wi-Fi key printed on a card, a code tapped in on a television remote, a login dictated over the phone. The cost is trivial — 83 characters instead of 90, so 6.38 bits per character — and one extra character wipes it out.

Turning symbols off

Some systems still refuse punctuation: router admin panels, ageing line-of-business software, forms that strip apostrophes for fear of SQL injection. Others accept it but make it painful to enter, on the on-screen keyboard of a games console or a smart TV, all the more so because UK and US keyboard layouts do not put the same characters in the same places. Without symbols you drop back to 62 characters and 5.95 bits each: it takes 18 characters to recover the 104 bits of a 16-character full-set string. Sliding to 20 settles the question without doing any sums.

The draw itself

The source of randomness matters as much as the settings: the tool uses crypto.getRandomValues, the browser’s cryptographic generator, and rejects the values that would bias the reduction to an index — a naive modulo favours the first characters in the list. If that API is missing, a message says so: generate elsewhere rather than accept the fallback.

The pitfalls that cancel out the benefit

• Reuse. A 120-bit password used on ten sites is worth the security of the worst-protected of the ten. It is the one flaw that length does not fix.
• “Leet” substitutions (a → @, e → 3, o → 0). Cracking tools apply them to the dictionary automatically: P@ssw0rd costs no more than password.
• Forced rotation every 90 days. NIST (SP 800-63B) has dropped it: it produces mechanical variations of the Summer2025! then Autumn2025! kind. You change a password when you have reason to believe it has leaked, not because of the calendar.
• Security questions. The name of your first pet is often public. Treat them as passwords: generate a random string and file it alongside.
• Confusing encoding with protection. A Base64 string is read back in a second — the Base64 encoder/decoder does it in both directions, which is enough to make the point. A SHA digest really is one-way, but a bare SHA-256 falls to a dictionary at billions of attempts a second on a graphics card; serious storage goes through slow, salted functions such as bcrypt or Argon2, on the server side.

Where to keep passwords you cannot remember

A 20-character random string is not memorable, and that is the whole point. The trade-off is a password manager — your browser’s, your operating system’s, or a dedicated app. Everything then sits in one vault, but the balance still tips in your favour: it removes reuse, the leading cause of cascading account compromises.

One password does still have to be remembered: the vault’s. There, pure randomness becomes counter-productive. A passphrase of five or six words drawn at random from a list — the diceware method — gives 65 to 78 bits and takes a couple of days to learn, provided the draw really is random; a phrase you put together yourself falls straight back into the dictionary trap. Add two-factor authentication and print the recovery codes: losing access to your vault is as concrete a risk as having it stolen.

When the password has to travel

A confidential document. You can protect a PDF with a password: encryption in recent PDFs is solid, but it is never worth more than the key you choose — a six-digit code is brute-forced in a few minutes. Generate a proper string and send it by a different channel from the file: PDF by email, password by text message.

A Wi-Fi key. Rather than reading it out, encode it in a Wi-Fi-type QR code: the tool produces a string in the form WIFI:T:WPA;S:network-name;P:key;; that Android and iOS devices recognise natively, connecting in a single scan. But a QR code pinned up in a waiting room can be read by everyone who walks past: it buys convenience, not confidentiality.

If the site rejects your password

• An undeclared maximum length. Plenty of forms silently cap at 16 or 20 characters. The classic symptom: the account is created, but signing in fails — the registration field truncated the string and the login field did not. Generate a shorter one.
• Pasting disabled. You will have to type it by hand: exclude the ambiguous characters and, if the keyboard is an unusual one, drop the symbols and add length instead.
• Characters outside ASCII. Avoid accented letters, and the £ sign too, even where the field accepts them: nothing guarantees a UK layout on the next device.

The generator, like Convertu’s other text tools, runs entirely in the browser: no string ever passes through a server. PDF protection, by contrast, is handled server-side — one free conversion a day without signing up, then a single subscription at €7 a year, with uploaded files erased after an hour.