Passphrase vs random password: which one should you use?
The short version. Use a random password for every account a password manager remembers for you, which should be nearly all of them. Use a passphrase for the few secrets you have to type from memory: the master password of the manager, a disk encryption key, a device unlock. A six word passphrase drawn from a 2048 word list is worth 66 bits, which is plenty. A passphrase you compose yourself is worth far less and is not a substitute.
What a passphrase is, and what it is not
A passphrase is a password built from whole words rather than individual characters. Harbor-Lantern-Copper-Meadow-Trellis-Quartz is a passphrase. So is correct horse battery staple, the example from the xkcd comic that made the idea popular in 2011.
What makes it a passphrase in the useful sense is not that it contains words. It is that each word was chosen by a random process from a known list. Diceware, the method invented by Arnold Reinhold in 1995, made this literal: you roll five physical dice, read off a five digit number, and look it up in a list of 7776 words. Every roll contributes exactly log2(7776) = 12.9 bits, and nobody, including you, has any influence over which word comes up.
A phrase you thought of is not a passphrase in this sense. MyDogBusterIsTheBest is 20 characters and worth perhaps 20 bits, because the words are not independent, they follow English grammar, and the subject matter is drawn from a very small pool of things people write about. Cracking tools generate exactly this kind of phrase by the billion.
The arithmetic
Passphrase entropy is straightforward when the words are drawn uniformly from a list:
entropy in bits = number of words × log2(size of the word list)
| Word list | Bits per word | 4 words | 5 words | 6 words | 7 words |
|---|---|---|---|---|---|
| 400 words | 8.6 | 35 bits | 43 bits | 52 bits | 60 bits |
| 1296 words (EFF short) | 10.3 | 41 bits | 52 bits | 62 bits | 72 bits |
| 2048 words (Createpass) | 11.0 | 44 bits | 55 bits | 66 bits | 77 bits |
| 7776 words (Diceware, EFF long) | 12.9 | 52 bits | 65 bits | 78 bits | 90 bits |
Two conclusions fall out of this table. First, four words is not enough any more, whatever list you use. The xkcd comic assumed a 2048 word list and quoted 44 bits, which was a defensible number against the guessing rates of 2011 and is not one today. Five words is the floor, six is the sensible default, seven if the secret protects everything else you own.
Second, list size matters much less than word count. Quadrupling the list from 2048 to 7776 words adds 1.9 bits per word. Adding one more word to a 2048 word phrase adds 11. This is the same lesson as with random passwords: length beats variety.
Our generator uses a list of exactly 2048 words, which is a power of two. That is deliberate: 2048 is 211, so each word contributes exactly 11 bits with no rounding and no bias in the selection. The words are common English of three to nine letters, checked against a dictionary, with no duplicates. Duplicates matter more than people expect: a list that contains the same word twice quietly reduces its own entropy and can produce a phrase with a repeated word.
Where each one wins
| Random password | Passphrase | |
|---|---|---|
| Entropy per character typed | 6.6 bits | about 1.7 bits |
| Length for 70 bits | 11 characters | about 40 characters |
| Memorable | no | yes |
| Typing on a phone or a TV remote | painful | easy |
| Reading it out loud or copying it by hand | error prone | reliable |
| Fits a 16 character site limit | yes | often not |
| Survives a site that forbids symbols | yes, with more length | yes |
A random password is denser. If you never have to type or remember it, density is all that counts, and 16 characters is both stronger and shorter than any passphrase. A passphrase trades density for something a machine cannot measure: you can hold it in your head, type it on a games console controller, and read it over the phone to yourself without transcription errors.
The accounts that deserve a passphrase
Most people need between two and four memorised secrets, no more:
- The master password of your password manager. This is the one that matters most. Everything else lives behind it, and nobody can reset it for you. Seven words.
- Full disk encryption on a laptop. Typed at boot, before any manager is available. Six or seven words.
- Your primary email account. Whoever controls it can reset most of your other accounts through the forgotten password flow. Six words, plus two factor authentication.
- The device unlock code on your phone. Six digits is 20 bits, which is weak, but phones throttle attempts and wipe after repeated failures. A short alphanumeric passphrase is better if you can live with typing it.
Everything else goes in the manager as a 16 character random string that you never see and never type.
How to remember one
The trick with a random passphrase is that the words have no relationship, which is exactly what makes it strong and what makes it feel unmemorable at first. In practice a few days of deliberate use is enough:
- Type it, do not read it. Muscle memory forms faster than visual recall. Type the phrase ten times the day you generate it, then once a day for a week.
- Build a mental image, not a sentence. For
Harbor-Lantern-Copper-Meadow, picture a copper lantern on a harbour wall with a meadow behind it. Absurd images stick better than plausible ones. - Keep the word order. Do not rearrange the words to make it flow better. Reordering does not weaken the phrase mathematically, but a phrase that reads like English is one you will paraphrase later, and then you have lost it.
- Write it down once, on paper, and store it somewhere physically safe until you are confident. A note in a locked drawer at home is a defensible backup for a master password. A note in your desk at work is not.
- Do not shorten it after the fact. Dropping a word from a six word phrase throws away 11 bits, more than a quarter of what it was worth.
Common mistakes
- Using a quotation, song lyric or book title. Whole corpora of published text are in cracking dictionaries. A famous line is a single dictionary entry no matter how long it is.
- Picking the words yourself. Human choice collapses the effective list from thousands of words to a few dozen favourites, and puts them in grammatical order.
- Rerolling until you like the phrase. Rejecting phrases you find ugly biases the output. If a phrase is genuinely unusable, discard the whole thing and generate a fresh one rather than swapping a single word.
- Adding
1!at the end and calling it stronger. That is about three bits. A seventh word is eleven. - Reusing the passphrase across the manager, the laptop and the email. These are exactly the accounts where a single compromise is worst.
Generate one now
On the generator, tick Easy to remember. The number of words follows the length you selected, so choose 24 characters for a six word phrase and 28 for a seven word one. Leave the uppercase box ticked to capitalise each word, and the digits box ticked to append a two digit number. Neither adds much entropy, but together they satisfy sites that still insist on a capital and a digit, without you having to think about it.