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posted by mrcoolbp on Sunday March 29 2015, @10:38PM   Printer-friendly
from the correct-horse-battery-staple dept.

Micah Lee writes at The Intercept that "coming up with a good passphrase by just thinking of one is incredibly hard, and if your adversary really is capable of one trillion guesses per second, you’ll probably do a bad job of it. It turns out humans are a species of patterns, and they are incapable of doing anything in a truly random fashion."

But there is a method for generating passphrases that are both impossible for even the most powerful attackers to guess, yet very possible for humans to memorize. First, grab a copy of the Diceware word list, which contains 7,776 English words — 37 pages for those of you printing at home. You’ll notice that next to each word is a five-digit number, with each digit being between 1 and 6. Now grab some six-sided dice (yes, actual real physical dice), and roll them several times, writing down the numbers that you get. You’ll need a total of five dice rolls to come up with each word in your passphrase. Using Diceware, you end up with passphrases that look like “cap liz donna demon self”, “bang vivo thread duct knob train”, and “brig alert rope welsh foss rang orb”. If you want a stronger passphrase you can use more words; if a weaker passphrase is ok for your purpose you can use less words. If you choose two words for your passphrase, there are 60,466,176 different potential passphrases. A five-word passphrase would be cracked in just under six months and a six-word passphrase would take 3,505 years, on average, at a trillion guesses a second.

I recommend that you write your new passphrase down on a piece of paper and carry it with you for as long as you need. Each time you need to type it, try typing it from memory first, but look at the paper if you need to. Assuming you type it a couple times a day, it shouldn’t take more than two or three days before you no longer need the paper, at which point you should destroy it.

"Simple, random passphrases, in other words, are just as good at protecting the next whistleblowing spy as they are at securing your laptop," concludes Lee. "It’s a shame that we live in a world where ordinary citizens need that level of protection, but as long as we do, the Diceware system makes it possible to get CIA-level protection without going through black ops training"

 
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  • (Score: 2) by quadrox on Monday March 30 2015, @08:27AM

    by quadrox (315) on Monday March 30 2015, @08:27AM (#164110)

    If you know/can guess that the passwords consists entirely of words from that list, you have suddenly reduced the entropy greatly. You would do a dictionary attack based on that very list and try all combinations of those words (not letters - words!), and suddenly your password is no better than the numerical checksum that represents it.

    Or am I wrong?

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  • (Score: 2) by stormwyrm on Monday March 30 2015, @09:09AM

    by stormwyrm (717) on Monday March 30 2015, @09:09AM (#164130) Journal

    You're wrong. There are 7776 words in the Diceware list. So if you were to mount a dictionary attack, what you would need to do is go through each combination of such words. So if you had five words in your passphrase, that would be 77765 = 28,430,288,029,929,701,376 combinations, which is actually about 64 bits of entropy! The "numerical checksum that represents it" actually has quite a lot of entropy in it, approximately 13 bits per word.

    It's like having a password made up of five Chinese characters. There are about 8105 general-use Chinese characters approved for use in the People's Republic of China, so that's a bit more than 13 bits per character also, so similarly a five-character password using only Chinese characters would have just under 65 bits of entropy.

    --
    Numquam ponenda est pluralitas sine necessitate.
    • (Score: 2) by Gaaark on Tuesday March 31 2015, @01:07AM

      by Gaaark (41) on Tuesday March 31 2015, @01:07AM (#164582) Journal

      and 2 of the words in my password aren't even on that list! i like my odds!

      --
      --- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---