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Entropy isn't sufficient to measure password strength

benwr.net

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Re: Entropy isn't sufficient to measure password strength

#51

Post by GoblinSlayer »

mojuba wrote: post #23
krupan wrote: post #18

When will we stop using passwords?! They are an elementary school kid “secret club” game taken way, way too far. They are totally broken. Nobody can come up with and remember good passwords. Nobody can store passwords securely. 100% busted. Instead of continuing to debate what makes a good password, we need to put our energy into better techniques altogether! No more shared secrets! Let’s talk about one-time codes, a…

In one of my current web-based projects I decided to experiment with magic links sent via email. They are pretty convenient (and secure enough) but turns out there's a problem with mobile email clients: they tend to open links in isolated embedded browsers and then forget the cookies. For most non-technical people this is a show stopper unfortunately. I then went with one-time 6-digit sign in codes that are emailed t…

You know that the opened token is linked to the initial client session. You can unblock that session and the user can proceed in the non-isolated browser. You have this workflow with codes anyway: the user must open an email then go back to the browser and type in the code. With the link you will save on the typing.

Re: Entropy isn't sufficient to measure password strength

#52

Post by witrak »

shepherdjerred wrote: post #47
awelxtr wrote: post #43

Earlier quoted context omitted.

> A lot of people (do not trust password managers, case in point the recent last pass scare. That's no excuse. KeePass allows having the database file locally where it's you duty to manage it. It might be less convenient, maybe. But I don't see valid excuses for people to not start using a password manager, even less the less tech savvy people.

It’s completely valid to distrust password managers. No software is free from bugs, or accidentally exposing your passwords. It might take a lot of work, but it’s certainly possible. There’s also the possibility of mismanaging your password database and losing all of your data.

> It’s completely valid to distrust password managers.

Is it, really? And at the same time to trust one's memory? For memorizing hundreds of long passwords? Don't think so...

Re: Entropy isn't sufficient to measure password strength

#53

Post by GoblinSlayer »

Zamicol wrote: post #32
taeric wrote: post #26

Earlier quoted context omitted.

I'm curious how you think these other items work. They ultimately boil down to a shared secret that is beyond what you can remember. Which... isn't the best thing, necessarily. Consider, if I leave my hardware token at home when I go on vacation, I'm basically locked out of all of my accounts. This is fine, as I typically plan for this to be the case. But it is an attack vector. I can't even audit my protected assets…

The point of public key cryptography is that there isn't a shared secret.

A shared secret is not an attack vector though, a reused secret is. If you reuse an asymmetric key, it will identify you across the world. But if you don't reuse an asymmetric key, then the point of asymmetric cryptography is moot and is no better than a password manager.

Re: Entropy isn't sufficient to measure password strength

#55

Post by benwr »

shepherdjerred wrote: post #46
benwr wrote: post #19

Earlier quoted context omitted.

I mostly agree, but I do find myself choosing a new FDE and login passphrases about once a year, and I wish that I could choose these using something like Diceware, but memorable enough that I wouldn't need to write them down at all. Thinking about how I might do that is what ultimately led to this post.

Login, like for your local computer? Why rotate those?

More often for e.g. work computers.

Re: Entropy isn't sufficient to measure password strength

#56

Post by dbaupp »

less_less wrote: post #40
BeefWellington wrote: post #8

There's a bit of a logical flaw here in that the argument is made against average entropy of a set of passwords, rather than individual entropy of each chosen password. This is an argument I can't find anyone making: an aggregate average entropy of the set of all passwords you use is fine for password security, rather than the entropy of each individual password. As far as I can tell this seems to be a (possibly inte…

Entropy and min-entropy are properties of distributions , not of individual samples from those distributions. So there's no meaning to "the entropy of each chosen password".

Despite that slight misuse of terminology, the point stands: the article talks about estimating the entropy of a distribution used for generating a password, but the important thing is the “distribution” an attacker is using for guessing the password.

A single password should instead be treated as a sample from a (plausible) attacker’s distribution, and the complexity of that password can be used to estimate the size of the sample space required for that plausible attacker (as in, how many guesses/how much work they’ll have to do). This is, AIUI, the approached used by libraries like https://zxcvbn-ts.github.io/zxcvbn/

The entropy of a distribution for generating passwords matters when generating them in bulk, such as OTPs or implementing a password manager. This doesn’t seem to be the situation being discussed in the article, which is more about rating a user-provided password.

Re: Entropy isn't sufficient to measure password strength

#57

Post by jjav »

Zamicol wrote: post #32
taeric wrote: post #26

Earlier quoted context omitted.

I'm curious how you think these other items work. They ultimately boil down to a shared secret that is beyond what you can remember. Which... isn't the best thing, necessarily. Consider, if I leave my hardware token at home when I go on vacation, I'm basically locked out of all of my accounts. This is fine, as I typically plan for this to be the case. But it is an attack vector. I can't even audit my protected assets…

The point of public key cryptography is that there isn't a shared secret.

More generically than "shared secret" which is one implementation, the idea is shared trust.

We both (client and server) trust some common background info. It can't be hand-waved away because that trust must exist or be established.

Shared secrets (passwords) are close to an optimal solution when considering all possible criteria. Various forms of PAKEs can be better sometimes, but not very popular. Other solutions address different threat models, often with more significant tradeoffs than a shared secret.

Re: Entropy isn't sufficient to measure password strength

#58

Post by benwr »

croes wrote: post #33

I thought it's the entropy of the chosen password not about the entropy of the possibilities of password you could choose

Entropy of a single password isn't actually a well-defined concept; entropy is always about a distribution. "Entropy calculators" that look at your password and tell you "its entropy" are making assumptions about how you chose the password.

We care about the distribution from which you drew the password, because that lets us analyze how difficult it would be for an attacker who knew your password selection process to brute-force the password. Just knowing the password itself isn't enough information to determine that (though of course you can judge how hard it would be for an attacker once you know their brute forcing strategy).

Re: Entropy isn't sufficient to measure password strength

#60

Post by awelxtr »

shepherdjerred wrote: post #47
awelxtr wrote: post #43

Earlier quoted context omitted.

> A lot of people (do not trust password managers, case in point the recent last pass scare. That's no excuse. KeePass allows having the database file locally where it's you duty to manage it. It might be less convenient, maybe. But I don't see valid excuses for people to not start using a password manager, even less the less tech savvy people.

It’s completely valid to distrust password managers. No software is free from bugs, or accidentally exposing your passwords. It might take a lot of work, but it’s certainly possible. There’s also the possibility of mismanaging your password database and losing all of your data.

> There’s also the possibility of mismanaging your password database and losing all of your data.

The alternatives are same password everywhere or keeping a paper around with the passwords written in plain text. Both are equally disastrous (unless you work at home and don't ever get robbed)

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