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Choosing Good Passwords

By Jose Nazario

Right now I'm running Crack on some people, and I'm doing a lot of thinking about passwords and how to generate good ones. Specifically, what sorts of things I can do to get better ones. I was recently asked by a friend about any ideas on passwords and about sharing them at our local event "LinuxDay". I'll take some time now and discuss passwords with you now. Passwords provide our most major defense against unauthorized use of our systems, so let's keep them good, even in the presence of crypto, firewalls, and rabid dogs.

OK, so this is how I generate passwords for myself: I reach over, grab the nearest issue of "Nature", open it up to a genetics article and point to some gene construct name and use that. No lie. It's how I chose good passwords. Random, complex, easy to generate. Granted, a lot of dictionaries have now included gene names, but these are construct names, which differ from gene names. So, instead of something like "Brc1" it's more like "pRSET5a::20STa::6xHis". You can shove the latter in any cracking program and it will not fall out quickly, I can almost garauntee it.

The trick is this: users dislike complex passwords. They're difficult to remember, they'll cry. And they're right. To overcome that, they'll either write it down on some post-it note on their monitor or change it to something silly, like "LucyDoll".

Most importanly, a password should roll off the fingers. It should be typed quickly and efficiently, and of course corectly. For that matter, I sometimes will type it ten times quickly to generate the rythm of it, and that works.

Quickly, a diversion to the Crack 5.0a documentation, this is ripped from the appendix. It deals with password security and security in general, and is some good wisdom:

At the bottom line, the password "fred" is just as secure (random) as the password "blurflpox"; the only problem is that "fred" is in a more easily searched part of the "password space". Both of these passwords are more easily found than "Dxk&2+15^N" however. Now you must ask yourself if you can cope with remembering "Dxk&2+15^N".

OK, great, we've chosen a good password... oh crap. We have about ten accounts, some on BBS's, some on systems we can't ssh to, and some are the root passwords on systems we administer for major businesses. or we have to rotate them often. How do we keep track of them all?

Myself, I do two things: I keep levels of passwords. I have a handful of passwords for disposable things. Yeah, if someone grabs a password of mine I use on a BBS and posts some flamebait, I will get some flack. But honestly, I doubt anyone will do that, it's the systems I care about and administer that I really protect. Hence, I cycle passwords there, using very strong passwords that never go out on the wire without some strong crypto behind them (ie secure shell). A new password is chosen (randomly), and the old ones are bumped down the chain to less demanding positions, system and accounts. I use the tricks I outlined above, and it has paid off. Sometimes I forget them, and that's always a scary moment, but it's usually no more than a minute or two.

Keeping track of multiple passwords is easily handied using the Password Safe from Counterpane systems, but that only works on Windows systems. I once started writing the implementation for Linux, but given my poor programming skills and heavy load of other things, I doubt it will ever see the light of day (it's sitting idle now, if anyone wants to know). I do, however, often reccomend this program to people with lots of passwords to remember. Other similar applications exist for the Palm Pilot of other PDAs, which protect a bank of passwords with one password. Since most Linux geeks I know also have PDAs, this would be a handy solution.

For some real fun, though, check out FIPS 181 (1), a scheme the government uses to generate passwords based on pronounceable sounds. It's pretty cool, and I started playing with it (via Gpw, a related tool(2)). And check out how Crack (3) works, it's chez pimp. For comparison's sake, find out how L0phtCrack (4) works, and you'll snicker at NT's security. If you're feeling particularily brave and have some computing power to burn, consider brute forcing passwords (6), which is an interesting problem in dictionary generation and optimization of routines.

Notes and Links:

1. FIPS 181 is Federal Information processing Standard 181. The document can be found (with source for DOS) at A companion FIPS document, FIPS 112, discusses the usage and considerations of passwords.

2. Gpw is a UNIX utility in C/C++ (and Java, too) to generate pronoucable passwords. Handy and fun. . An additional one can be found on

3. Crack 5.0a source can be found at It can also be found at

4. L0phtcrack... how I love thee. . Mudge tells us how L0phtcrack works at this realaudio presentation from beyond Hope, 1997, NYC (1 hour) (Note that since this piece was originally written, L0phtcrack version 3 has been released. Several people have noted a dramatic drop in the efficiency of cracking passwords, though new extraction tools have been incoporated into the code. Many people I know who use L0phtcrack use version 2.52 for cracking after extractions with version 3.)

5. John the Ripper is another useful password cracking utility. Several modules for cracking S/Key and MD5 passwords have been introduced lately.

6. This is a great description of brute forcing passwords and some of the techniques involved... I may have to try it! The ambitious amateur vs. crypt(3)

Jose Nazario

José is a Ph.D. student in the department of biochemistry at Case Western Reserve University in Cleveland, OH. He has been using UNIX for nearly ten years, and Linux since kernels 1.2.

Copyright © 2001, Jose Nazario.
Copying license
Published in Issue 68 of Linux Gazette, July 2001

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