Password length: go longer?
4 November 2011
Time marches on, computing power grows stronger, hackers get cleverer. Every now and again we need to review what we once thought was “safe enough”. Today, the time has come to review what ought to be considered a safe password length.
Pragmatic security and powerful bots
This blog has always taken the pragmatic route to security, recognizing that there will always be a tradeoff between security and time and money. In other words, don’t worry about being 100% safe — instead, focus on being safer than average.
What does that mean for password length? Consider this: computing time is so cheap today that it’s not inconceivable that every one of our email accounts has a bot trying to access it about once per second, every day, 365 days per year.
Still feel safe with the password you’re using right now? Personally, I’m starting to feel queezy, but let’s look at the problem carefully.
Is eight still enough?
I used to recommend an eight digit password. Is that still enough? The Microsoft Password Checker, a tool I’ve recommended before, does not seem to think so. If you type, for example, “t8Uh10xI” into the checker, it tells you that you’ve made a weak password. Is that the case?
To answer that, suppose you found one of those bots that is, in all likelihood, pounding at the gates to your email account. Feeling generous, you give the bot a bit of information. “Look,” you say, “my password is eight digits, so don’t bother guessing passwords of any other length. And I use numbers, uppercase characters, and lowercase characters. I don’t use any special characters.”
Now, how scared should you be?
Well, you’ve made the bots job a bit easier, but let’s take a look at the math. The key statistic is the number of possible permutations of passwords you could have made using those parameters. To find out how many permutations there are, and therefore how many different passwords the bot would have to try, we need to compute the following:
Permutations = (26+26+10)^8
That is, there are 62 ways to pick the first digit of your password (26 uppercase letters, 26 lowercase letters, and 10 numerical digits), 62 ways to pick the second, 62 to pick the third, and so on — eight times.
The solution is that there are 218,340,105,584,896 possible eight character permutations. That’s 218.3 trillion. Supposing that a bot can try one password per second, it would be able to try 31,536,000 in a year. In just under 7 million years, it could try all the possible permutations.
So the answer has to be “yes, eight is still enough”.
On the other hand, if you’re using a software tool like PasswordSafe, the cost of upgrading your passwords to be a bit longer is so low that it’s difficult to think of a reason not to do so. Personally, I’ve begun to use 15 to 30 digit passwords for some applications because it increases my safety without increasing my costs appreciably. But I still feel secure knowing that the master password that unlocks my PasswordSafe database is less than ten characters long. If I lose my PasswordSafe database on the subway again (yes, this has happened once already), I won’t worry.
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