SSH authentication: either SSH keys or one time password

Question

I have an Ubuntu 10.04 Linux server that I normally ssh into (from my home machine) using ssh key authentication. However, sometimes I need to ssh remotely from potentially insecure machines (like internet cafes, public computers at the library, etc) on which my password could become compromised. In this case, I'd want to use a one-time password system like OTPW or Steve Gibson's Perfect Paper Passwords.

How can I configure my server to first check for ssh keys, then use an OTPW system for authentication? (Would I have to make two users?)

Answer 1

Public-Key-Authentication with OTP as fallback (that's what you meant, right?):

1. Public-Key-Auth with Password fallback is OpenSSH's default behaivour
2. How your password gets verified is best defined in the PAM configuration files

Answer 2

Edit: sorry for answering my own question! (The other two answers are great, but don't completely answer the question. But still very helpful!)

The OTPassword Pluggable Authentication Module implements Steve Gibson's Perfect Paper Password system in a PAM for Linux. Once you install that, you'll have PPP authentication. But what about a ssh keys bypass? An FAQ on their site answers this question:

If you have a trusted machine from which you often log into your remote system use ssh keys. Generate them with ssh-keygen, and copy your new ~/.ssh/id_rsa.pub into ~/.ssh/authorized_keys on remote computer. When SSH authenticates user with keys it omits PAM.

Conveniently automatic!

Edit: Google Authenticator and Duo Security also seem like good solutions. They don't provide one time passwords on paper; instead they use your smartphone to generate a constantly changing TOTP key. (Duo Security also works with dumbphones by sending them a text message with a couple of one time passwords. However Duo Security is not local; you must rely on their servers...)

Answer 3

This can be simple but there are also a few pitfalls to avoid:

Most of your config changes need to happen in the sshd_config file usually located at /etc/ssh/sshd_config

You already have shared keys running so I will skip that here:

The line you want to pay attention to is:

PasswordAuthentication yes

The caveats you want to be aware of are in limiting who can login and how. These should ALL be in place to restrict access to as small a group of users as possible:

PermitEmptyPasswords no
AllowUsers sshuser@192.168.*
AllowGroups sshusers

There are many options you can set here review the man page for the full set of options available.

I would recommend setting up a group just for ssh permissions.

The following flag is also strongly recommend:

PermitRootLogin no

This will get the daemon to request a password if the key is not sent/reconigized. You can add the hurdle of OTPW if you like but you are theoretically in an encrypted environment so it shouldn't strictly be necessary. Having looked at the OTPW information you linked the code seems to have last been updated 2003 I would be loathe to use it, without getting some peer review. Secure coding practices and indeed the whole environment have changed a lot since then.