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Comment by andybak | Hacker News Reader

4 true random choices from a dictionary of 5000 words is 50 bits of entropy. All things being equal that is just as good as any other password scheme with 50 bits of entropy. I think that's about the same as a 10 character upper/lower/numbers/symbols password.

That's your worse case scenario. Add to that the fact that:

a) the attacker doesn't have your dictionary b) they can't be sure you're using this method c) you might add a minor extra step of your choosing

then the attack becomes significantly harder than attacking a 'normal' password with 50 bits of entropy

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> a) the attacker doesn't have your dictionary

How many truly unique sources for dictionaries are there?

My own password generator simply uses `/usr/share/dict/words`, which is not that hard to guess (and documented in the generator's source code).[0]

Add to this that not all languages lend themselves to this algorithm – while my native language is German, you'd need a word stem dictionary, as opposed to a regular dictionary, for this, so I stick to English dictionaries.[1]

[0] I actually made it configurable, but lacking a better source, it'll probably remain at this default: https://github.com/creshal/yspave/blob/master/config.json.ex...

[1] cf. http://creshal.de/News/?id=59 , where I rambled on it a bit more.

My own password generator simply uses /usr/share/dict/words

On my system /usr/share/dict/words has ~45k words. That's 15 bits of entropy per word, significantly more than XKCD (which assumes 11 bits).

So I'd agree that there probably aren't many unique dictionaries. The strength of this password scheme is that it works even assuming the attacker knows your dictionary.

In practice, I guess that an attacker will use a concatenation of all popular dictionaries. This will make things slightly harder for them (e.g. if they have 50k words vs the 30k in your dictionary) but not by a huge factor.

> The strength of this password scheme is that it works even assuming the attacker knows your dictionary.

Indeed. Arguing about whether or not your dictionary is "unique" enough is a red herring. Whatever generator you use should accounts for the dictionary's word entropy, not for the theoretical per-byte entropy.

I just looked at some of the hashcat benchmarks and they are clocking MD5 on one of their test machines at 115840 Mh/s.

(2^50)/(115840*10^6) = 9719.43 seconds, or about 2-3 hours to break one correct horse style password. I think the point is that 50 bits of entropy is not enough when you consider the speed of GPU hashing.

Obviously you shouldn't use MD5 for hashing passwords, but since you often have no control over what algo a site uses (or even information about which one they did pick), assuming they will do something common and bad like unsalted MD5 isn't a bad starting point for this kind of analysis.

Maybe so, but then that doesn't say much about the usual alternative which has even less entropy.

EDIT: Actually I was going by xkcx's claims, which start with a dictionary word and adda random tweaks to it. I don't know off hand how long a random password would have to be to match 50 bits. But ultimately, that is the question; is this method at least as good as the alternative.

Assuming 80 possible characters (A-Z,a-z,0-9 + 18 punctuation symbols), log2(80^8) ~= 50. So a correct horse password would be about the same as a random 8 character alpha-numeric entropy wise.

The XKCD argument is that correct hose style is way easier to remember. It's possible Schneier thought Munroe was arguing that a correct horse was as much entropy as a 20 character random password, even though he isn't. This is the first I've heard of this argument though so I'm not even sure how much of a controversy it is.

IMO, there are two kinds of passwords relevant nowadays:

• Password used to encrypt your password database, whichever it is • Everything else

Since the idea of a password database is that you can give each website unique passwords without having to memorize it, you can go full ASCII printable range 20 digits for site passwords. Those will not be brute forced any time soon – at 128 bit entropy –, and the damage of having them leaked will be minimal.

Your password database, on the other hand, should use a painfully slow algorithm for its PBKDF. If that's not using at least scrypt, you indeed have a problem. But assuming a good password database design, 50 bits shouldn't be too bad.

The attacker will naturally start with a smaller dictionary, most common words, which reduces the search space greatly. So unless you're using some of those SAT words, it's even less secure.

5000 words is pretty small for a dictionary. Of course if you're using really simple words to make your passwords then you're out of luck, but choosing random words from a 5000 word dictionary is reasonably secure.

Language contains a surprising amount of entropy, not as much as random characters, but still enough that I can be reasonably sure that this sentence is unique.

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