Re: Password entropy

[Valdis Kletnieks <Valdis.Kletnieks () VT EDU>]

On Mon, 24 Jul 2006 13:01:06 CDT, Roger Safian said:
> At 12:05 PM 7/24/2006, Basgen, Brian put fingers to keyboard and wrote:
> > This isn't going to be strong when combined with regular words. "At the
> > moment" is 13 characters, at 3.5 bits of entropy, gives us 45.5 bits
> > total.
>
> OK...so how long can I expect this phrase to last?  Are there tools or
> spreadsheets that allow you to evaluate various combinations?

Rough guidelines:

40 bits is laughable - a few hours work on a *single* PC will quite possibly
break it (remember - this is "5 or 6 letters/digits" territory, and not
a challenge to brute force in a day on modern CPUs)....

56 bits isn't considered sufficient anymore.  The EFF had a box a while ago
that could break that class in a day, and any zombie net of 10K or so machines
could do it in similar time.

64 bits is far too close to 56 for comfort.  I'd not trust it for anything
that has to survive more that a few hours.

80 bits is 24 bits more (or about 16M times harder) than what the EFF box or
a small zombie net can break in a day.  So a zombie net of a million boxes
would be grinding for several years.  That's probably "strong enough" that
the passphrase itself is your weakest link - going to 96 or 128 is just
piling it on after that. (128 is well into "if we converted the entire planet
into nanomachine computers, we *might* break it before the heat death of the
universe").

Anything over 80 bits or so is "strong enough" - after that (actually, even
before that), your biggest threats are the ones that don't care how long
the passphrase is - keystroke loggers, social engineering, phishing, and
other similar attacks...

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