Security Corner

Aug 24 2009   1:40AM GMT

Un-guessable Passwords—How to Make Them

Ken Harthun Ken Harthun Profile: Ken Harthun

The sheer number of passwords most of us have is a big problem. Even if we have hints written down, how do we know which one created the password for which login? It would appear as though we’re back to writing them down or using a password manager. Don’t worry, though. Here’s how to create secure passwords that you can safely write down; yes, write them down, give them to all your friends–even your enemies–and still be safe. Post them on your monitor at work. Leave them lying around on the bus or train. A simple trick based on cryptographic techniques will conceal your actual password in a form that almost anyone will mistake for the password itself.

Let’s say you found a piece of paper that had this written on it:

Work BDAbe%x#
Home 1941phx!n
email fon!%m

What would you think it was? Bet you’d think you’d found someone’s password list, eh? That’s exactly the deception we want: What those strings of characters really mean is known only to you. So, what DO they mean? Let’s take the first example; in my Ask the Geek blog, my article How to Write Down Your Password and Not Worry About Someone Stealing Them, I explain:

[It's] a substitution cipher based on a date. This one uses two levels of secret "keys": 1. a clue or mnemonic for the date; 2. an abstraction of the encoding algorithm. We’ll use Abe Lincoln’s birthday in numeric form–02/12/1809–for our plaintext, leaving out the slashes, i.e., 02121809, which will result in a strong, eight character password. Now, for the first key, we can use "BDAbe." This immediately reveals the plaintext, but means little or nothing to anyone else. (NEVER use your own birthday, for obvious reasons.) [Note: even if someone guesses that it's Abe's birthday, they still have a long way to go to figure out how it was used - Ken]

Next, we decide to use alternating shifted characters, beginning with the first character. So, for key two, we make an abstraction of that: %x#, for example. It doesn’t matter what characters you use, only that they clearly represent shifted and lower-case characters; you could just as easily use AyT or !2@. The pattern of shift-lowercase-shift on the keyboard is what matters to you; the characters mean nothing else. Put the two keys together and you have this: BDAbe%x#. That’s your cipher pattern, the "something only you know," with an added level of complexity: it’s something only you know (the plaintext) and only you know what it means (the encoding pattern).

Pretty slick, eh? This should give you a clue as to what the second one is: 1941ph means (to me) 12/07/1941, the date of the attack on Pearl Harbor that led us into WWII. Based on the pattern, the actual password is 1@0&1(4!. Can you figure out what the last one might mean? (You won’t guess the actual password unless you know what I know about the first part, but you can figure out what the code hint is.) Post your comments and we’ll see how you do.

I don’t recommend you use these examples, for obvious reasons; you’ll want to come up with your own ways of doing things and your own hints using things that mean something only to you.

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  • MichaelSeese
    That is VERY cool. And it involves calculations which make my head hurt...so it HAS to be good. The only thing I must point out is that the "Pearl Harbor" password, as is, has no alpha characters. That might violate a system's password requirements. -- Michael Seese, author of [A href="http://www.amazon.com/Scrappy-Information-Security-plain-English-Biometrics/dp/1600051324/ref=sr_1_1?ie=UTF8&s=books&qid=1245928166&sr=1-1"]Scrappy Information Security
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