Tighter SSH Security with Two-Factor Authentication

How to set up two-factor authentication using a USB pendrive and ssh-agent for root logins.

Static passwords are quickly becoming more trouble than they're worth. We need to break the static habit and start using two-factor authentication. OpenSSH is a powerful system that provides the tools necessary to make that step. By using public/private keys, agent forwarding and removable media, we can use OpenSSH as a key “safe”. This, in turn, allows us to create a simple, inexpensive and effective host-based, two-factor authentication system.

This two-factor system requires a moderate amount of work to configure and use, but it is well worth the extra security. However, using the tfssh script makes the process easy to use. Using the script means you get all the benefits of two-factor authentication but almost none of the hassle.

Paul Sery has been a UNIX and Linux System Administrator for more than 20 years. He's written several Linux books, including Network Linux Toolkit and Knoppix for Dummies. He's also co-authored several Red Hat Linux for Dummies and Fedora Core for Dummies books with Jon “maddog” Hall. Paul lives in Albuquerque, New Mexico, and can be reached at pgsery@swcp.com.



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Two-Factor ssh

Mike Kachline's picture

Agree with Nicholas above. An ssh key + "passphrase" is indeed, not "two-factor" because, as he mentioned, the presence of a passphrase is never validated on the server.

Furthermore, an ssh key (something you have) which is stored onto a USB thumb drive (something you have) is still, technically speaking, single factor authentication. Much like requiring multiple "chained" passwords is still single-factor authentication.

A possible workaround is to use an SSH key which "forces" a command of "sudo /bin/login". By doing so, one would first authenticate with the SSH key (something you have), and then need to authenticate through the "regular" PAM stack (Something you know.) This has the added bonus that the "pam stack" authentication would enforce other rules such as password complexity, fail counts, and so forth.

This is single factor, not two factor authentication

Nicholas Sushkin's picture


RSA key authentication is a single factor authentication. The server only verifies your RSA key, which is the only authentication factor. The key's passphrase is only relevant for protecting your private key in your client machine. You can set empty passphrase, the server doesn't care.

The authentication would have been two factor, if the server verified both the key and a static password independent of the key.

mv not quite secure :)

witek's picture

Don't move cryptokeys using "mv". Use "cp", then "shred" or "wipe".

Excelent article.

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