Matthew GreenSomebody asked whether dictionary-word passphrases (“correct horse battery staple”, like the ones generated by 1Password) are any good. Short answer: good means different things. Shorter answer: yes!
I’ll talk about why in a thread below.
The basic idea of these passphrases is that you have a dictionary of D words. You pick N words at random. That’s the whole idea. Example: “overlook-hooey-valance-flood-useless-ladyship”.
Cryptocurrency BIP32 passwords use a 2048 (2^11) list, and use 12-24 words per passphrase. 1Password seems to use a larger list, between 18000-18500 words (2^14.15) and you can pick your length (6-8 is common.) https://github.com/1Password/spg/blob/master/agilewords.go
Someone in my timeline asked for papers saying these were good passwords. From a purely mathematical perspective we don’t need a paper, just a toot. But there’s more than math here.
Password quality is about three things: strength (how long til Mallory guesses it, perhaps with a powerful computer), memorability (can you keep it in your head) and usability (can you enter it into a website or device.) Only the first one involves any math.
The math for dictionary passphrases is pretty simple. Assuming you choose words uniformly at random: if your dictionary has D words and your oassphrase is N words long, then there are D^N total passphrases.
The total matters because for a random passphrase the best strategy for guessing is to try all (or most) of them. This D^N determines password cracking time.
A simpler way to do this math is with powers of 2. The 1 password dictionary is about 2^14 in size, so for a 6 word password we get 2^{14*6} = 2^84.
Cryptographers tend to treat anything over 2^80 as “probably good enough to secure your Bank of America account” and anything over 2^128 as “probably good enough to secure really important stuff”. I told you there’d be science.
For comparison, last I checked the Bitcoin network was computing about 2^64 hashes every 10 minutes and using as much electricity as Argentina.
Bitcoin doesn’t crack passwords, but if it could & the entire Bitcoin network was cracking your 6-word 1Password phrase, it would take about 9.5 years on average.
But what about human memorability? Can people memorize such complex passwords? The answer is “yes”, because I just memorized one.
If you don’t accept N=1 studies, then there are a few studies. This one looks at 3-4 word passphrases: https://cups.cs.cmu.edu/soups/2012/proceedings/a7_Shay.pdf
Here is another more recent study that focuses on 56-bit (2^56 strength) 6-word passphrases and discusses strategies that help people memorize them. It turns out that “spaced repetition” (making people learn the password over a period of time) works well enough that many don’t have to write it down. https://www.usenix.org/system/files/conference/usenixsecurity14/sec14-paper-bonneau.pdf
The final barrier is usability: can people actually use passphrases on the Internet? Sadly here the answer is “it depends.” The problem is that many website designers have decided on cargo-cult security procedures like “letters, numbers, special characters required”. Some even institute character limits.
If you’re looking for a recommendation here, I would urge you to do the following:
1. Use a good password manager with a strong random 6-8 word master passphrase.
2. Write it down (one safe place) and practice entering it from memory on a regular basis. You will eventually remember it.
3. Let the password manager generate passwords for individual sites.
There are no guarantees, but this is probably the safest way to keep passwords online.
Jeffrey Vagle@matthew_d_green I find it's helpful to build a (often dadaist) narrative that somehow links the seemingly disjoint words.