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Xkcd Password Generator

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Re: Xkcd Password Generator

#211

Post by Cushman »

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…

My point was that adding a character to something like "valve tangle hastens accept" is like adding a couple bits to something like "Av#12GH", and yet people feel like it's accomplishing something valuable. They feel that way because "Av#12GH" looks random, but "valve tangle hastens accept" doesn't.

Obviously the literal length of the string is not strictly relevant, and it was probably inarticulate of me to include that.

Re: Xkcd Password Generator

#212

Post by jcr »

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 resent your accusation that I use gibberish for my passwords; I actually use perfectly well formed executable code in perl.

Re: Xkcd Password Generator

#213

Post by klipt »

dpark wrote: post #89
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.…

> H4ck3r N3ws H4ck3r News H4cker News Hacker News This is a terrible idea. It's one thing to ask a person to remember which characters they replaced in a word. It's another thing entirely to ask them to remember three different ways they swapped characters. This is a recipe for having to brute-force your own password. It's also still not as secure as you might imagine. There's very little entropy added by swapping ch…

> You've got a ton of entropy there.

Nope - http://news.ycombinator.com/item?id=2873474

Re: Xkcd Password Generator

#214

Post by drblast »

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!

Randall's math is spot on in this comic.

Assuming the attacker knows the the password creation method (and the math assumes the attacker does in the first case), then the attacker knows the word list and the passphrase algorithm.

11 bits per word gives you a grab bag of 2048 possible common words. To guess the password, assuming each word is unique, the attacker needs to try

2048 * 2047 * 2046 * 2045 = 17,540,692,561,920

possible combinations. Initially, you'd think an eleven character password with totally random uppercase, lowercase, numbers and symbols would give you 66 ^ 11 combinations for 66 bits of entropy, but since nobody can actually remember such a random combination, the resulting passwords using these rules are much less secure than that.

Re: Xkcd Password Generator

#215

Post by Ideka »

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…

May I ask how did you find out about that study?

It sounds very interesting. I've got to try it sometime :).

Re: Xkcd Password Generator

#216

Post by Cushman »

LXicon wrote: post #207
MichaelGagnon wrote: post #176

Earlier quoted context omitted.

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…

there might not be 17 trillion songs, but you aren't limited to the first 4 words of the song. there might be 100-300 words per song and you can pick your starting word anywhere you like.

But it falls into the same boat as any dictionary attack. Most people with a passphrase are probably going to use one from a song. 90% of them are going to use one of the top 1,000 songs, 90% of them are going to start at the beginning of a line. If we say there are ~20 unique lines in the average song, and most people won't use more than ten successive words even if it bridges a line, that's 1000 * 20 * 10 = a keyspace of 200,000. Trivial.

What this means is even if you decide you're going to be really secure and pick, say, the 30,000th most popular song, assume all songs have 200 unique lines (to account for sensical starting points in the middle of lines), and use 20 words from it, you're in a keyspace of only 120 million, which even if it takes 1ms to hash will be cracked in a day.

By contrast, four random english words chosen from the 2,048 most common has a keyspace of ~1.75e13, or 17,500,000,000,000.

Choosing a clever, unusual line from the middle of a very uncommon song is the passphrase land version of choosing a rare English dictionary word and replacing the vowels with numbers. If your hash gets compromised, it might as well be "password".

Re: Xkcd Password Generator

#217

Post by kragen »

ubernostrum wrote: post #179
kragen wrote: post #138

Earlier quoted context omitted.

It turns out that you are mistaken. Your favorite book is almost certainly chosen from the 129 million books that Google knows about: http://www.fastcompany.com/1678254/how-many-books-are-there-... That gives you 27 bits of entropy. The average book length is probably not over 400 pages. An average page probably doesn't have over 25 sentences on it. So the whole book contains only ten thousand sentences. That gives y…

Of course, the flip side of this is that we're veering off into attacks where you're targeting one specific person and know a bit about how they've chosen their password. If you want to mount such an attack, fine, but most of us are dealing with the much-more-common threat of someone who gets a file or a database of hashed passwords and wants to crack them all in one go.

The analysis applies to that threat as well; it just adds some constant number of bits of entropy.

Re: Xkcd Password Generator

#218

Post by Dove »

Ideka wrote: post #215
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…

May I ask how did you find out about that study? It sounds very interesting. I've got to try it sometime :).

You mean the statistics demonstration? I'm sure I've seen it in several places.

I know of two tricks for detecting the students. The first is to look for six or seven heads or tails in a row. Over a hundred tosses, a coin will probably do that, but humans "being random" won't. The other is to look at the page as a sequence of "HHH" and "TT" strings and estimate how many there are. A coin, of course, changes from heads to tails 50% of the time, but a human does it more like 70% of the time.

I'm sure there are other characteristics, too, but those two are sufficient to throw out most human attempts at a glance. It's actually kind of obvious, when you see the two side by side.

Me "being random" with the numpad: 10110101001010010101011010100101011010100101010110101

Computer-generated random: 1101110100000000011111110111011000010001010110011111

See? Here's a few more. Try it.

1000100111011001010000001001010110111000011011101011

1101010001010010101110100101000101010111101001001000

1101111010110111100010110001000100001001111001001110

1100000010000010101001000001101001101011111100111001

Re: Xkcd Password Generator

#219

Post by Aloisius »

Personally I think password entry should be done in madlib form. Each user would have a unique madlib prompt like:

Username: ___________

Password: Twelve ___(pl. noun)___ jumped over a ___(adjective)___ ___(noun)___ named ___(proper noun)___.

Re: Xkcd Password Generator

#220

Post by keithnoizu »

Cushman wrote: post #189
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…

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…

This is not entirely correct, we include non alphanumeric characters (punctuation) in our passwords occasionally because it increases the solution space for a brute force attack.

   While this doesn't really improve any individual password the fact that we occasionally include non alphanumeric charecters increases the possible password set from 62 possible charecters ^ password length to a something more like 90^password length.  

  Similarly we dictionary attacks are more efficient than brute force attacks because we're talking maybe 200,000^(words in password) if we allow for some common word permutations versus 90^passwordlength.
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