Xkcd Password Generator • Hacker News Board

Re: Xkcd Password Generator

#31

Post by burgerbrain » Thu, Aug 11, 2011, 2:03 PM UTC

ordinary wrote: post #26

burgerbrain wrote: post #15

Earlier quoted context omitted.

https://secure.wikimedia.org/wikipedia/en/wiki/Diceware Properly executed, this will protect you against brute force attacks. No need to do nonsense like adding more spaces. Of course XKCD botched it and said an inadequate minimum length... Notable quote from the article: "This level of unpredictability assumes that a potential attacker knows both that Diceware has been used to generate the passphrase, the particular…

Even 4 words is good enough for many use cases.

Best to error on the side of suggesting more. If the reader is technical enough to to realize you did that, then they're technical enough to estimate a more suitable value themselves.

Re: Xkcd Password Generator

#32

Post by darklajid » Thu, Aug 11, 2011, 2:03 PM UTC

hesselink wrote: post #21

darklajid wrote: post #17

Earlier quoted context omitted.

> he can trivially test combinations of dictionary words in very short amount of time. Explain the reasoning behind this, please. Start with: You don't know the dictionary I used, but have to use one that seems 'good enough' (i.e. a superset of mine, if possible). How many words are in there? How many combinations can you create for 'two word phrases'? (You don't know the length of my phrase) How many for three? How…

The XKCD criticism of the 'bad' password has the same problems. How does the hacker know I have this kind of password (starting with a real english word, for example)?

No, it doesn't have the same problem.

It criticizes a particular way to choose passwords, leading to a result that seems 'secure' and even quite good to lots of people. One that easily satisfies braindead corporate password rules.

The entropy given there is based on that way to choose a password and even explained, graphically.

If you choose your password totally different and from different sets of characters, then the number will be off. That's not a surprise though?

Re: Xkcd Password Generator

#33

Post by nmcfarl » Thu, Aug 11, 2011, 2:05 PM UTC

I've been using phrases and sentences as passwords for a while, and I've found that there are 2 main problems;

1) A lot of sites, still in this day and age, have max password lengths, so I still have a lot of short passwords. Usually this is bank sites and the like.

2) Password entry fields are often very short visually, and with a long password getting lost is much easier. I find I have to type them over A LOT.

The second is actually the more annoying problem.

Re: Xkcd Password Generator

#34

Post by wisty » Thu, Aug 11, 2011, 2:06 PM UTC

hm2k wrote: post #29

What about sites that don't allow spaces? I know hotukdeals.com only allows [a-zA-Z0-9] which sucks.

And also, sites that have a limit on password length. And sites that have a silent limit on password length, and secretly truncate it. And sites that have different truncation, depending on which form you use. And sites that require a Capitals, lowercase, and numbers, because nobody would just use name+birthday.

Re: Xkcd Password Generator

#35

Post by burgerbrain » Thu, Aug 11, 2011, 2:07 PM UTC

adnam wrote: post #6

Such a password scheme provides much less than 44 "bits" of entropy. Considering the use of 4 randomly chosen words from the c.170000 english words in general use, means we can guess the paraphrase in around 2^22 tries - even less than "Tr0ub4d0r3&". EDIT: I'm totally wrong, it's more like 2*10^22 ... oops!

There is no need for scare quotes around bits. The term is being used in a technically correct fashion. https://secure.wikimedia.org/wikipedia/en/wiki/Bit#Informati...

Re: Xkcd Password Generator

#36

Post by drcode » Thu, Aug 11, 2011, 2:08 PM UTC

One slight addition to the xkcd password scheme that would add another order of magnitude of security would be to have your own personal "salt" that you add to all your passphrases. In this case, the salt would be a short, traditional, hard to remember password that you re-use with every xkcd style password. It would be hard to remember, but you'd only need to memorize it once.

So if your personal salt is "@T#23a" you would use "@T#23a correct horse battery staple" on one website and "@T#23a giant bug transistor leech" on another website.

Re: Xkcd Password Generator

#37

Post by burgerbrain » Thu, Aug 11, 2011, 2:08 PM UTC

TetOn wrote: post #7

adnam wrote: post #6

Such a password scheme provides much less than 44 "bits" of entropy. Considering the use of 4 randomly chosen words from the c.170000 english words in general use, means we can guess the paraphrase in around 2^22 tries - even less than "Tr0ub4d0r3&". EDIT: I'm totally wrong, it's more like 2*10^22 ... oops!

Wouldn't you first have to know that the passphrase consists of four randomly chosen words (eg not three, five, or eight)? To me, that's the underlying strength of the approach that the comic (!) is trying to highlight.

The entropy is actually calculated with the assumption that the attacker already knows those things. If they don't, then it is higher.

Re: Xkcd Password Generator

#38

Post by darklajid » Thu, Aug 11, 2011, 2:09 PM UTC

ZoFreX wrote: post #18

I would actually advise going against this advice. While it isn't a best practice, password sharing can and does happen, as does shoulder-surfing. It would take a LOT of effort to memorise my password, but a simple four word password will probably be remembered by accident. In a year's time if I piss a friend off, I don't want my Facebook password to be readily accessible in their memory. I think more people need to…

How do you share your preferred password? Because I guess everything but sending it per text/mail would be tedious, while it would work better with a couple of words.

Shoulder surfing: It's certainly a risk, but I'd say that prolonged shoulder surfing shouldn't be possible. If I type fast, it will be very hard to make out the phrase. If I type slow, you cannot stand around that long.

And - I'm not a security expert, but how much do you gain if you saw a couple of chars here? My intuition (yeah, shouldn't trust that) says that it's worse if I watch you and know the _first_ character of your password than you seeing the first 1-3 characters of the first word of my passphrase?

(We don't know the name of your cat, so judging the quality of the password or your neighbo(u)r's ability to remember it is hard)

Re: Xkcd Password Generator

#39

Post by commandar » Thu, Aug 11, 2011, 2:09 PM UTC

ZoFreX wrote: post #18

I would actually advise going against this advice. While it isn't a best practice, password sharing can and does happen, as does shoulder-surfing. It would take a LOT of effort to memorise my password, but a simple four word password will probably be remembered by accident. In a year's time if I piss a friend off, I don't want my Facebook password to be readily accessible in their memory. I think more people need to…

>I think more people need to learn to remember arbitrary strings.

The entire point is that humans aren't very good at doing this.

>(Note: This doesn't really apply to me or most of us here in most cases, but for example my WiFi password is of the form "Mycatsname9" and yet my neighbour still has to ask me for it whenever her phone forgets it)

This is actually exactly the kind of scenario where using pass phrases makes the most sense. WPA2 is vulnerable to rainbow table attacks; relatively long passphrases are both easier to remember for mere mortals and less likely to be broken by a rainbow table attack.

Re: Xkcd Password Generator

#40

Post by bunchesofdonald » Thu, Aug 11, 2011, 2:09 PM UTC

ErrantX wrote: post #10

Not a good idea, sadly. In fact I'd go so far to say this is a really bad suggestion ; because it gives a false sense of security. There is potentially a lot less entropy in this password than "Tr0ub4d0r&3", assuming the hacker is smart enough to realise he can trivially test combinations of dictionary words in very short amount of time. (EDIT: I'm way out of touch with this; it's not as trivial as perhaps I figured.…

The comic is already taking worst case scenario into account, that the cracker knows that your password is made of words and which dictionary you used to generate the password.

In the comic he is using a 2048 word dictionary, which gives 11 bits of entropy per word (log2(2048)), with a password made up of four words that gives a total of 44 bits of entropy.

But if we were to assume that the cracker knows nothing about our password, say other than it being all lowercase a-z, then we have an entropy per character of log2(26) or 4.7. For the phrase "correct horse battery staple", which has a length of 28, the bit entropy of that phrase, under those conditions is 4.7 * 28 = 131.6. Which is nearly to the point of the cracker being more likely to find a collision.