What a passphrase is
A password made of several words picked at random from a long list, like velvet-canyon-lantern-pebble-anchor-steam.
Its advantage is that it fits how memory works. Remembering q8#Lv!2mWz9e means memorizing twelve unrelated symbols; remembering six words means remembering six pictures, and with a little story linking them they stick within a few days.
Where the idea comes from
In 1995 Arnold Reinhold published the Diceware method: a list of 7,776 numbered words and five dice. Each roll of five dice gives a number between 11111 and 66666 (65 = 7,776 possibilities) that points to a word. Repeat the roll a few times and you get a phrase chosen by pure chance, with no computer involved.
The idea became famous in 2011 thanks to an XKCD comic comparing Tr0ub4dor&3, hard to remember and easy to guess, with correct horse battery staple, the other way round. In 2016 the Electronic Frontier Foundation published a new list with words that are easier to type and remember, which is the one we use.
Our passphrase generator does the same as the dice, but with the browser’s cryptographic generator: every word is exactly as likely to come up as any other. If you would rather use real dice, the EFF publishes its list with the dice numbers next to each word.
Passphrase or random password
Each word from a 7,776-word list adds 12.92 bits. Each random character with letters, numbers and symbols adds about 6.55. That gives these equivalences:
| Passphrase | Bits | Equivalent to a random password of |
|---|
| 4 words | 51 | 8 characters |
| 5 words | 64 | 10 characters |
| 6 words | 77 | 12 characters |
| 7 words | 90 | 14 characters |
| 8 words | 103 | 16 characters |
A passphrase takes longer to type, but it is much easier to remember and to enter without mistakes on a phone. For passwords a manager stores, remembering does not matter, so the random one wins there. The full calculation is in the guide to entropy.
Mistakes that make it predictable
Choosing the words yourself
The most common mistake, and the most serious. People pick frequent words, related words or words from their own life (the dog, the team, the hometown). All of a passphrase’s strength depends on the words coming from a random draw.
Using a phrase that makes sense
“tobeornottobe” has 13 letters and is one of the first things tried. Sayings, verses, song lyrics and movie lines are all in attack lists.
Too few words
Three or four words are not enough against an offline attack. Use six at the very least, and seven or eight for a master password.
Tweaking the result
Swapping a word you dislike for one you pick, or reordering them so they sound better, brings the human factor straight back. If you do not like a phrase, generate a whole new one.
When a site wants capitals and numbers
Some services still require a capital letter, a number or a symbol. With a passphrase that is easy: turn on “capitalize” in the generator, and if a number is required, add one at the end. It adds no real strength (an attacker tries it straight away), but it takes none away either, and it lets you use the phrase. The separators (hyphen, period, space) count as a symbol on most sites.