This file contains the following information on selecting a password:
A compromised password can easily be used in ways that you are unlikely to notice, such as remote authentication to AFS. Keep in mind that a poorly chosen password not only places all of your own files and data at risk but also places your colleagues and co-workers at increased risk by allowing an outsider to masquerade as you and avoid all restrictions normally in place for external access to the environment. Thus, it is never reasonable to keep an easily guessable password on your account simply because you are willing to personally take the risk.
And the risk of compromise is real: Given enough time, any password can be found simply by trying all possible combinations. For example, an all lower case 6-character password can be found in about 4 days by brute force search on a machine that can try 1000 passwords per second.
However, for well-chosen passwords a brute-force attack is still infeasible even using today's fastest computers, as long as you change passwords from time to time and as long as you follow a few basic guidelines. The following tips come from "Improving the Security of Your UNIX System" by David Curry, SRI International, ITSTD-721-FR-90-21. Please note that Kerberos passwords are not limited to 8 characters.
Although this list may seem to restrict passwords to an extreme, there are several methods for choosing secure, easy-to-remember passwords that obey the above rules. Some of these include the following:
Finally, you should avoid using the same password in multiple administrative domains. This minimizes the damage if the security of one of those domains is compromised. For example, it is common for crackers to accumulate login names and passwords for individuals on one machine and to then look for identical accounts on other machines that these people might use. If you have accounts elsewhere on the Internet you should use a different password than you use at CMU. This protects you from a breach of security at an external site, which you may never even find out about.
You should also use different passwords within the distinct CMU administrative domains (e.g. SCS, SEI, Andrew, ECE, etc.) and even within different SCS computing domains that employ distinct authorization mechanisms. Thus your SCS Kerberos password should not be the same as a password you use for accounts maintained outside of SCS, nor should it be the same as the password you use for non-Kerberos machines within SCS.
For help with any facilities-related problem, or if you have questions and comments regarding Facilities documentation, please send mail to help+doc@cs.cmu.edu