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Xkcd Password Generator

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Re: Xkcd Password Generator

#261

Post by salvadors »

redxaxder wrote: post #122

When picking a password, you don't just care about the entropy. You also care how far down the password guessing order it is. People who want to guess a password don't just brute force at random. They use a guessing order that goes through more common classes of password first. So if correct horse battery staple becomes a popular password scheme, these will end up attacked before other password schemes. (See http://w…

The point is that this approach pushes brute-force guesses out into territory that makes it unlikely anyone will crack it even if they know exactly what scheme you're using.

People seem to be massively underestimating just how long it would take to brute-force four dictionary words in a row.

Re: Xkcd Password Generator

#262

Post by salvadors »

julianpid wrote: post #116

I find the idea incredibly stupid. If I know someone who used that precise generator to produce his password. Then I know that the generator has less than 2000 words in the dictionnary. It then takes me only a few minutes to guess his password, rather than 550 years. Conclusion: Don't ever use this password generator, write you own, and tell no-one about it.

Let's say the dictionary only has 1000 words in it. A phrase of four words in a row from that is still 1,000,000,000,000 possibilities, which is going to take you significantly more than a few minutes to work through at 1000 tries per second.

Re: Xkcd Password Generator

#263

Post by j_baker »

a3camero wrote: post #201
Cushman wrote: post #189

Earlier quoted context omitted.

This should be higher up. It's scary to see people — intelligent people, I'm sure — saying things like "And that goes even higher when you add punctuation!" No, it doesn't. All of the reasonable punctuation you could add to a sentence adds only a few bits of entropy at best. It also makes the sentence harder to remember— was there a comma or not? Adding unreasonable punctuation or symbols is even worse— you get sligh…

It's certainly a "very long random string" without context but as people have pointed out above, it's actually not a very good password if people adopted this pattern widely (and you said the attacker knows this). 2000^4 = 16000000000000 possible passwords = 1.6E13 = ([A-Z] + [a-z] + [0-9] + [!@#$%^& ()])^7.1ish. So, your four words from the 2000 word list are equal to a 7ish character password that looks like "Av#12…

And yet the point is moot because no one is going to use a password like "Av#12GH".

Re: Xkcd Password Generator

#264

Post by mathattack »

The beauty of this discussion is not just "How to create memorable but hard to break password?" but "How much deep insight can a 4-6 frame cartoon contain?"

The signal to noise ratio of xkcd is fantastic! They've again zipped a great discussion in just a few frames.

Re: Xkcd Password Generator

#265

Post by bigiain »

j_baker wrote: post #263
a3camero wrote: post #201

Earlier quoted context omitted.

It's certainly a "very long random string" without context but as people have pointed out above, it's actually not a very good password if people adopted this pattern widely (and you said the attacker knows this). 2000^4 = 16000000000000 possible passwords = 1.6E13 = ([A-Z] + [a-z] + [0-9] + [!@#$%^& ()])^7.1ish. So, your four words from the 2000 word list are equal to a 7ish character password that looks like "Av#12…

And yet the point is moot because no one is going to use a password like "Av#12GH".

I have a password file here with several hundred passwords just like that (actually, they're all 12 chars with upper/lower case, digits, and "special chars", as chosen and stored by 1Password...)

Joe Public is unlikely to use passwords like that, but I'm 100% sure I'm not the only hackernews reader who does.

Re: Xkcd Password Generator

#266

Post by dspillett »

Dove wrote: post #160

I find the discussion surrounding the XKCD strip alarming for the superstition it reveals about password generation. The particular theme I am alarmed by is that people seem to think that if a password looks alien, or was difficult for them to come up with, it will be hard for a machine to guess. Look, we're working with big numbers here. You need to do the math. In this thread alone, I've seen suggestions to use a c…

> I would trust passwords that come out of a script like this to be far more secure than passwords anyone (myself included) made up, no matter how random they're trying to be.

Definitely agree with you here.

I've been using the "few random words" method for passwords I need to remember for some time (and random 20 character mixes of alpha/numeric/symbol for the other, which I have stored in a keepass db), and I know I'm not all that random in my choice of words so if someone managed to see one or two of my passphrases it would be quite easy to create a script that could brute force the other couple quickly.

I shall have to use a script like this (or throw together my own for paranoia's sake) next time I change one of my passphrases.

Re: Xkcd Password Generator

#267

Post by JonnieCache »

MichaelGagnon wrote: post #176
ScottBurson wrote: post #168

Earlier quoted context omitted.

But there's a long tail of song lyrics. If you pick something obscure, the odds of the attacker even having heard of it become very small (particularly if the attacker is from a different culture than your own). Pick something arty and incomprehensible, and the odds against someone else accidentally stringing those words together in some other context become astronomical. For instance, I'd wager no cracker has ever h…

This gets into the whole "security through obscurity" thing. Ideally, you should use a password-generation system such that if the attacker knows your pasword-generation system (e.g. lines from songs) it would still be infeasible to guess your actual password. Thats why the 4-random-words technique is good. According to XKCD, the 4-random-words technique generates about 17 trillion passwords---all equally likely. But…

The answer is obviously to write your own song or poem and not tell anyone about it. A passpoem, perhaps in the style of Lewis Carrol.

Re: Xkcd Password Generator

#268

Post by xkcdentropy »

demallien wrote: post #52
ErrantX wrote: post #25

Earlier quoted context omitted.

> 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). People are likely to use a standard English dictionary. In my experience (which is exactly within this field) people use a fairly tight subset of the English vocabulary. So I would be quite happy to test for a dictionary of, say, 100,000 words and be hopeful of a good hit rate (note…

If the dictionary really has 100 000 words, you're looking down the barrel of 52 bits of entropy for a three word phrase In a more likely dictionary of the 5000 most commonly used words in the English language, you still get a three word pass phrase of about 40bits of entropy. Make that a four word passphrase, and you're back up around 52 bits.

This is simply incorrect. If you assume you really do have 100 000 "characters" in your alphabet this is correct. However, your alphabet follows a certain pattern: It's English text.

At that point its easier to brute force the individual characters. English text has about 1 to 2 bits of entropy per character. Lets assume 1.5 bits per character on average. That means that to really get 52 bits of entropy for a 3 word phrase you need to have at least 35 characters in your 3 word phrase, or about 12 per word.

Your password is only as strong as the weakest link. In this case English is easier to brute force than your 100 000 character alphabet.

It's simply false to assume that "yes no one three" has 44 bits of entropy because it got randomly selected out of a 2048 word dictionary.

Re: Xkcd Password Generator

#269

Post by khafra »

lukev wrote: post #181
ddlatham wrote: post #23

A lot of comments here seem to be missing the point. The main point is to use passwords that give you the most "bang for the buck" in the sense of adding the most bits of entropy for the least difficulty of remembering. Adding an extra number, or punctuation, or certain numbers of repetitions generally adds only a little bit of entropy for a significant cost in additional challenge to your memory. Our minds are well…

I don't think entropy is the whole story. I would argue that although security-through-obscurity is a terrible, awful practice for systems, it's not that bad for personal password schemes. Using a nonce "system" for passwords, even if it's mathematically low-entropy, is still secure, at least enough for personal use. For example, if I use single dictionary words fed through a trivial ceasar cipher, then that is mathe…

You'd be surprised. JTR does l33tspeak substitutions, one-row-up substitutions, keyboard walks, pretty much all of the common things everyone does because "no hacker would ever think of that."

Re: Xkcd Password Generator

#270

Post by nait »

benmathes wrote: post #165
gjm11 wrote: post #154

Earlier quoted context omitted.

Be aware that adding to the length simply by taking more of the lyrics adds very little entropy. If you're trying "Oh say can you see" then it doesn't take a lot of extra bits also to try "Oh say can you see by the dawn's early light what so proudly we hailed at the twilight's last gleaming". Similarly, extended passages of text -- even if they don't come from a restricted corpus like that of song lyrics -- have less…

I can see your point in that the kolmogorov complexity of two lines in a song isn't much larger than one line. Similarly, 30 digits of pi and 300 digits of pi have very little difference in kolmogorov complexity. What I don't know is if state-of-the-art password guessers are great at recognizing larger patterns in the entire canon of human knowledge. I.e. is there a "common phrases" attack that's analogous to a "dict…

Given that the knowledge about correct parts of a password based on known sources (pi, peace and war, song lyrics etc) drastically reduces the amount of possible solutions. But how would an attacker figure out the first part of such a password? What comes to mind are timing attacks http://en.wikipedia.org/wiki/Timing_attack What other possibilities did I miss?

EDIT: I get that having a long streak of my pass in a dictionary would reduce overall security but it's still unclear how a partial match in the dictionary would be detected.

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