On 7 Mar 2015, at 15:52, Stephen John Smoogen wrote:
On 7 March 2015 at 11:53, Mike Pinkerton <pselists@xxxxxxxxxxxxxx>
wrote:
On 7 Mar 2015, at 10:41, Björn Persson wrote:
Mike Pinkerton wrote:
On 6 Mar 2015, at 23:49, Adam Williamson wrote:
On Fri, 2015-03-06 at 23:09 +0100, Björn Persson wrote:
I hope https://xkcd.com/936/will be among the inputs to that
discussion.
I'm fond of noting that pwquality has not yet blacklisted any variant
of correcthorsebatterystaple. I've been using correcthorse as my stock
anaconda testing password, since the strength check has been
enforced...
It won't stand up to a combinator attack:
<https://www.schneier.com/blog/archives/2013/06/a_really_good_a.html>
It's not entirely clear, but I guess you mean that a two-word
combination like "correct horse" won't stand up. That appears to be
true. A four-word phrase is an entirely different matter. Each
additional word increases the complexity exponentially, so doubling
the
number of words squares the number of possible combinations.
The "combinator" attack that is described in the Ars Technica
article that Bruce Schneier quotes in the above link appears to be
an attack that tries combinations of multiple words from one or
more of the attacker's word lists. Certainly adding more words to
the pass-phrase would make that more difficult. As I don't know
the current state of the art in password cracking, I don't know
whether attackers typically limit their attacks to only two words,
or extend to three or more words.
They limit it to 1-2 words because it takes a LONG time to crack
SHA512crypt passwords. You can do on average 32k -> 128k hash crypt
checks per second per password. A two word dictionary of diceware
would have 2^25.85 passwords in it. A single system is going to
take 256 seconds on 2 words. Add in 3 words (2^38.775) and it is 24
days. Add in a 4th word and it is 544 years. Add in a 5th word and
it is 4.5 million years.
Apparently Diceware's creator is not as confident as you -- he nows
recommends more than 5 words.
<http://arstechnica.com/information-technology/2014/03/diceware-
passwords-now-need-six-random-words-to-thwart-hackers/>
Perhaps improvements in graphics cards have changed the calculus in
recent years.
While writing this up I went and checked that the whole thing is
outlined point for point in wikipedia
http://en.wikipedia.org/wiki/Password_strength
To estimate the time just do the following:
$15,000 computer -> 128k/sec = 2^17. Lets assume moore's law comes
in and we have 2^20 by 2020.
Take the possible entropy and subtract the 2^17 and that will give
you the worst case. I believe it may be 1/4 of that so make it
subtract 2^19 currently for one system and 2^29 for a cluster of
1024 computers (so 15 million dollars).
2 words is going to be (25.85-19) 115 seconds for one system and
0.1 for big ass cluster.
3 words is going to be (38.78-19) 236 hours ). <1 day for big ass
cluster
4 words is going to be (51.70-19) 221 years). < 1 year
5 words is going to be (64.63-19) 1.7 million years) < 1700 years.
(or 1.7 years for a 15 billion dollar investment).
To get equivalent strength from say an all lower case password you
are going to need 14 [a-z] characters.
Now here is the funny thing. All that speed to get 128k is if the
password is less than around 12 characters for most cracking
software due to the way the hardware and algorithms have been
optimized. If the string is longer than that the hardware drops in
speed by orders of magnitude. So correctstaple is actually going to
take longer than I said. In fact all the numbers I put for 3+ words
is probably going to be 10-100 times longer.
All of this assumes that the attacker is trying to brute force the
entire string -- character by character. In the Ars Technica article
I linked to in my previous message, the attackers did not try to
brute force anything over 6 characters. Instead, they used other
strategies, including the combinator strategy that would have broken
correcthorse.
There are 2 caveats.
1) Once again, Adam was being sarcastic. He knows the password
isn't any good because well he TOLD everyone what it was. He was
making fun of the fact that libpwquality does not blacklist it..
which means that correctstaple is the new password of choice (when
the old one might have been 123456)
I saw your first note that Adam was being sarcastic -- although it
probably doesn't matter what password he is using for offline testing
of Anaconda and release candidates.
I was responding to Björn Persson's suggestion that, in discussions
of password quality, correcthorsebatterystaple would be an example of
a safe password. My point is that, if attackers are using strategies
other than brute forcing, which the Ars Technica article suggests is
the case, then constructing long passwords out of known words is
probably not a safe strategy.
Because the word lists used by attackers are lists of strings that
they have scraped from various sources -- human language
dictionaries, password strings found in previous attacks, passwords
publicized by Adam on mailing lists, strings constructed on patterns
(e.g., "7kids", "8kids"), etc. -- a string that one would normally
think of as four words -- correcthorsebatterystaple -- once it has
been discovered as a password once and added to the attacker's word
list, becomes only one word for all future cracking attempts.
2) This is always true http://xkcd.com/538/
And finally. If one were to take the top 1 million known passwords
as the dictionary.. then each word would have about 20 bits of
entropy. A password generator that outputted stuff like
123456 password trustn01 letmein1
would take 256 or more longer to brute force crack than using
diceware. Actually that sounds like a nice project to add to my
EN_RN translation project.
Except that the attackers aren't brute forcing long passwords.
Apparently, they can successfully crack a ridiculously high
percentage (90% in the Ars Technica experiment) in the space of a day
using other techniques.
How much entropy does "rastafarianestablishmentarian" have? With the
techniques attackers are using, I doubt it would take even one hour
to crack it.
That was my point.
--
Mike
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