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Myths of Password Security

stormpath.com

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Re: Myths of Password Security

#51

Post by mistercow »

mnarayan01 wrote: post #50
mistercow wrote: post #29

Earlier quoted context omitted.

>The password "12345" has the same entropy as "b0g4p" but it would be a mistake to think they are equally secure. Those do not have remotely equal entropy.

At the byte level it does, which is presumably what the OP was talking about when saying that replacing characters with digits (uniformly) does not affect entropy.

Maybe, but I would hope that's not what s/he meant, because that kind of entropy is basically irrelevant to password strength.

Re: Myths of Password Security

#52

Post by mistercow »

venomsnake wrote: post #42
mistercow wrote: post #26

Earlier quoted context omitted.

>The human brain is crappy rng so it cannot create proper high entropy passwords. No way getting around that... Yes there is: you use a real RNG to generate your password. A good option is http://passphra.se . Just remember that you can't go clicking "Generate Another!" until you find something particularly memorable, or you'll be sacrificing an unknown amount of entropy by narrowing the space to memorable passwords.…

If we assume that English has 65535 words a four word password gives you 64 bits of entropy. Of course the real number of words that you can use for mass generation is 2k (or even less) if we search words that everyone that is standard deviation or two below the mean on the vocabulary graph can use and spell correctly. Spelling is going after the ability to memorize telephone numbers too with everyone using the crutc…

Four words chosen from 2048 possibilities gives you 44 bits of entropy (this is what passphra.se does). If you're being specifically targeted in an offline attack, that won't hold up. But in that very rare scenario, you are already in trouble. A 44-bit password will protect you from attackers trying to harvest from a big pool of stolen hashes, or from attackers bruteforcing the login form on a website. That is not the same thing as "fake security through obscurity".

As for the problem of strong, memorable passwords that can stand up to offline attacks, there is still a gap (a 253 bit password made of 11 bit words would be 23 words long), but I don't think the problem is unsolvable. A project I am interested in working on when I have time is one that produces very long, but memorable high-entropy passwords with rigorously quantified entropy by generating evocative, grammatically structured sentences, using a carefully selected word list and a set of mad-libs style templates.

So to get a ~256 bit pass phrase, you might be left to type out a 50 word poem. It would take effort to memorize, but it would be reasonably doable, whereas memorizing a random 39 character password is extremely difficult for most people.

Moreover, since you'd be working with a limited set of words and grammatical constructs, the password entry form could have robust auto-correction so that typos and spelling errors wouldn't be a significant problem.

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