When it comes to authenticating users, most Linux administrators rely on passwords. Of course, this causes no end of problems: If users are allowed to choose their own passwords, they are often too short and too easily guessed. If a random password generator creates the passwords, they are likely to be so complicated that users will forget them, or else the users will write down their passwords then leave the notes out in public places. Secrecy is difficult to ensure in unfamiliar environments where key loggers and shoulder surfers lurk. One-time passwords (OTPs) solve this dilemma: Because one-time passwords become invalid after one use, nobody is likely to worry about whether they are sniffed.

Password generators create lists with huge numbers of passwords and store a copy of the list on the server on which the user needs to authenticate. The idea is that the user carries a second copy of the list. For each authentication, the user and server just use the next password on the list. An example of this technique is the TAN method used by many banks for secure transfers in online banking.

When a user logs in, the system can prompt for, say, the tenth password on the list. Anybody with the password list can look up the matching password and authenticate. The disadvantage of this approach is that it is often impractical. Users need to carry the list around with them at all times; when they log on, they need to grab the list and painstakingly type the password. Of course, this technique also does not prevent the possibility that the list could fall into the wrong hands.

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