Summary: Everyone needs a good reminder about when it's time to change their passwords. For me, it's March 15th.

Every year on March 15th my mother would act out the death of Julius Caesar. From the warning he received as a young child from the grimy soothsayer, his speech to his "Friends, Romans, Countrymen", the assassin sneaking up behind him for the strike, a truly pained "Et tu Brute?", and a couple of final gurgles. Oh, the last death throes of a great ruler, acted out yearly in the kitchen, or hallway, or -- if you were really late waking up -- your very own bedroom. Now that we're out of the house, March 15th means an early morning phone call from home, but it's lost none of it's potency.

Now you might be wondering why in the heck a security column should begin with the Ides of March. Or, even more interestingly, why it would begin with the Ides of March a week after the Ides of March.[1] Well, fear not, I'll get to the point.

I can't imagine what you might do should you be woken some morning to the retelling of the fall of an empire, but to me it reminds me that it's time to change my passwords. Most security folks suggest you change your passwords at least once or twice a year. One of the most frequently suggested times are the change to and from daylight savings time. But for me, it's the Ides of March.[2]

So what makes a bad password? Anything associated with you or your likes, desires, or quirks. Anything out of a dictionary in any language. The name of your relative, pet, significant other, favourite movie, phone number, birthday, or favourite colour. These things are either easy to guess if someone knows you, or are able to be cracked fairly easily by password guessing programs.

And most importantly, any password that you've used before is right out.

So what makes a good password? It depends somewhat on your password-hashing algorithm. Most new Linux installs use strong password-hashing algorithms such as MD5, which can take an infinite length password. Older installs used the traditional DES algorithm, which only allows 8 character passwords. It's best for you to ask your administrator which kind of hashing algorithm is used on the system before you choose a password.

If you are the administrator, it's not too hard to see which kind of hashing algorithm is the default. For example, change the password for jdoe and then do the following:

  old_des_style# grep jdoe /etc/shadow
  jdoe:m1kbsnKnULUKs:12133:0:99999:7:::
       ^^^^^^^^^^^^^
  md5_style#     grep jdoe /etc/shadow
  jdoe:$1$e0/v1t9O$y/SxZxbiHsesW5HbeZRHq0:12133:0:99999:7:::
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

(I've underlined each password hash above with caret symbols to make it easier to see them.)

On the first host, the passwd program is configured to use the older DES-style password hashes. The password hash is the 13 character long string "m1kbsnKnULUKs". The second host uses MD5 hashing instead, as can be seen by much longer hash "$1$e0/v1t9O$y/SxZxbiHsesW5HbeZRHq0". There are other possible password hashing routines[3] but these are the two you're most likely to have available.

So, once you've determined which kind of password hashing algorithm your machine uses, what makes a good password? Since each type of algorithm has different characteristics, here are my recommendations:

Traditional DES-based

• 6 to 8 characters long
• No more than 8 characters - extras are ignored
• At least one character from the four following characters classes:
  • Lower Letters ( abcd...wxyz )
  • Upper Letters ( ABCD...WXYZ )
  • Numbers ( 1234567890 )
  • Other Characters ( !@#$%^&* etc )
• A good way of creating a short but entropy-laden password is to take the first letter of each word in a phrase, and add some weird characters to it or convert some letters to other characters, for example:
  • Where did I leave my keys? => WdIlmk?
  • Linux kicks Window's a**. => lKW's&**
  • Open Source and LAMP make my day => 0S&lmmd%

MD5-based

• A short password (6-8 chars long) that satisfies the above rules, or

• A 'passphrase' which
  • Should be at least 15 characters long (15-40 is a good rule of thumb)
  • Has a variety of characters, not simply 'aaaabbbbcccc' -- use at least two or three from the four classes above.
  • Can incorporate dictionary words as long as you have a long enough passphrase. For short passwords, using dictionary words is a horrible idea, but with long passphrases it is not as vulnerable to brute force measures. For example:
    • Bridge-dare!prone
    • I'd_rather-run=DJBDNS
    • March^last(if you ask me)

The most important thing, regardless of which password hashing algorithm your system uses, is that your password should be easy for you to remember[4] but hard for others to guess or crack.

And the other most important thing -- never use your password on an unencrypted channel. Use SSH for logins, and SSL-enabled versions of any protocols that use passwords, such as IMAPS instead of IMAP, or STARTTLS for SMTP. A cracker doesn't need to guess or crack your password if he can read it on the wire directly.

NOTES:

[1] I thought I should finish up that three part SSH Port Forwarding thread before I accosted you with sentimental childhood memories.

[2] I use SSH keys for almost all my connections, and I barely ever log in using an actual password, so the chances they'll be snagged are pretty low. Besides, I get copies of all the logs showing my login times and methods, so I'd notice pretty quick if any of my passwords have been compromised. I keep my SSH identities on my laptop only, and change them more frequently.

[3] For example, Owl Linux supports traditional DES, BSDI-style DES, FreeBSD-style MD5, and OpenBSD-style Blowfish hashes.

[4] If you need to write it down on a sticky note, you're in trouble.

Bri Hatch is Chief Hacker at Onsight, Inc and author of Hacking Linux Exposed and Building Linux VPNs. His wife fears that he will continue the yearly ritual with their lovely and impressionable daughter. But hey - it's history, educational, and even contains a little Latin. Can't be all bad, right? Bri can be reached at bri@hackinglinuxexposed.com.

Copyright Bri Hatch, 2003

This is the March 23, 2003 issue of the Linux Security: Tips, Tricks, and Hackery newsletter. If you wish to subscribe, visit http://lists.onsight.com/ or send email to Linux_Security-request@lists.onsight.com.