Re: Passphrases v Password

[shanna leonard <ssl () EMAIL ARIZONA EDU>]

On 7/8/13 7:02 AM, Tim Doty wrote:
> I've been resisting, but I will point out that that xkcd significantly overstates the entropy of English
> which ruins his analysis. Relying on simple passphrases as protection against hash cracking

> doesn't work against real threats
> (http://arstechnica.com/security/2013/05/how-crackers-make-minced-meat-out-of-your-passwords/)

I'm not sure that I would call that a "real threat". How real it is 
depends on the format of your password hashes and how simple access is 
to that hash store.

In the offline cracking example given, the crackers had access to simple 
unsalted md5 hashes. This made their task significantly easier (in terms 
of compute time) than it would have been had they only had a copy of a 
well constructed password db, e.g. a good passworddb (e.g. counterpane's 
passwordsafe), or a set of "slower" hashes.

"slow hashes" with a reasonably well-constructed (e.g. mostly english 
18+ character master passphrase with a bit of perturbation, ie typo and 
throw a few symbols in the middle) is still slow to crack  (e.g. 
passwordsafe discussion:) 
http://sourceforge.net/p/passwordsafe/discussion/134800/thread/671fd0d4/#2a53

Likewise bcrypt linux passwords by default use 5000 encryption 
iterations and are computed much more slowly than simple md5 or NThash 
rendering them more difficult to crack than the example.
http://samsclass.info/123/proj10/comparing-hashes.htm
http://arstechnica.com/security/2012/08/passwords-under-assault/4/

"If the LinkedIn passwords had been hashed using bcrypt, I never would 
have been able to crack 90 percent of them," he told Ars in an e-mail. 
"The number of attacks I had to run, combined with the sophistication of 
the attacks I had to run to get many of the passwords [more than] 15 
characters,
would have taken literally centuries to finish.

That said, the race is certainly on - the crackers are nipping at the 
heels of what were recently considered to be well-constructed 
passphrases. Writing a complex completely random 20-character master 
password down on a piece of paper and putting it in your wallet is 
looking better and better! :)

Shanna Leonard
ssl () email arizona edu
University of Arizona