Securing SSH with two factor authentication using Google Authenticator

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Two-step verification (also known as Two-factor authentication, abbreviated to TFA) is a process involving two stages to verify the identity of an entity trying to access services in a computer or in a network. This is a special case of a multi-factor authentication which might involve only one of the three authentication factors (a knowledge factor, a possession factor, and an inheritance factor) for both steps. If each step involves a different authentication factor then the two-step verification is additionally two-factor authentication.

Google’s two-step verification process

Google was one of the first Internet companies to introduce a two-step verification process. To access a Google service using the two-step verification process, a user has to go through the following two stages:

The first step is to log in using the username and password. This is an application of the knowledge factor.
The implementation of the second step requires a mobile phone or the Google Authenticator application, which is an application of the possession factor.

If the user opts to use a mobile phone, he/she has to register his/her phone number with Google. When one attempts to authenticate with username and password, Google will send via SMS a new, unique code to the phone. Receiving the SMS demonstrates that the user has the phone (or, in the case of GSM like networks the appropriate SIM chip).

If the user opts to use the Google Authenticator (or another supported code generator application), he/she simply opens the application, which generates a new code every 30 seconds. This code is to be entered to complete the log in process. As a backup option in case the registered mobile phone or device running Google Authenticator is lost, stolen, or otherwise unavailable, the user can print a set of static single-use backup codes (also the knowledge factor) and store them in a safe place.

The following are some other sites which offer two-step verification service:

1. Amazon Web Services
2. ANX
3. App.net
4. Apple ID
5. Authy
6. Battle.net
7. Bitstamp
8. Blockchain.info
9. Box
10. Dropbox
11. eBay
12. Etrade
13. Evernote
14. Facebook
15. GitHub
16. GoDaddy
17. HootSuite
18. Lastpass
19. LinkedIn
20. LocalBitcoins
21. Microsoft
22. MongoLab
23. Namecheap
24. PayPal
25. SocialFlow
26. timetotrade
27. Tumblr
28. Twitter
29. Viaduct
30. WordPress
31. Yahoo! Mail
32. innoview.gr
33. zoho

Google Authenticator

Google Authenticator implements TOTP security tokens from RFC6238 in mobile apps made by Google, sometimes branded “two-step authentication”. The Authenticator provides a six digit one-time password users must provide in addition to their username and password to log into Google services or other sites. The Authenticator can also generate codes for third party applications, such as password managers or file hosting services. Some versions of the software are open source.
[Source: WikiPedia]

Note: Google Authenticator doesn't “call home” to Google — all the work happens on your SSH server and your phone. Google Authenticator is a open-source software, that means you can check the codes yourself. I guess after what happened with openSSL, that’s not a bad idea after all.

In this article I will demonstrate how to secure SSH by setting up Two Factor Authentication (TFA) with Google Authenticator for the following Linux operating systems:

1. Debian
2. Ubuntu
3. Linux Mint
4. Kali Linux
5. Red Hat
6. CentOS
7. Fedora

Google Authenticator can be used in the following smartphones:

1. Android,
2. iOS and
3. Blackberry.

I am not too sure if you can use this on a Windows Based phone (i.e. new Nokia phones or Windows Mobiles devices). Feel free to try and report back.

App Download Links as follows:

1. Android: https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2&hl=en
2. iOS: https://itunes.apple.com/au/app/google-authenticator/id388497605?mt=8
3. Blackberry: http://www.google.com/mobile/other

More detailed per device install instructions can be found on this page:
https://support.google.com/accounts/answer/1066447?hl=en

The implementation is exactly same except for some package name differences. So you can implement this in any Linux Operating System that uses these packages.

Requirements

Followings are the requirements for using Google Authenticator on your system:

1. A PC running Linux
2. A smartphone
3. A running SSH server on your machine.

In case you don’t have SSH server running, just follow this guide to install openSSH server.

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