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Enable multi-factor authentication on OpenBSD

Written by Solène, on 06 February 2021.
Tags: #openbsd #security

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Introduction §

In this article I will explain how to add a bit more security to your OpenBSD system by adding a requirement for user logging into the system, locally or by ssh. I will explain how to setup 2 factor authentication (2FA) using TOTP on OpenBSD

What is TOTP (Time-based One time Password)

When do you want or need this? It adds a burden in term of usability, in addition to your password you will require a device that will be pre-configured to generate the one time passwords, if you don't have it you won't be able to login (that's the whole point). Let's say you activated 2FA for ssh connection on an important server, if you get your private ssh key stolen (and without password, bouh!), the hacker will not be able to connect to the SSH server without having access to your TOTP generator.

TOTP software §

Here is a quick list of TOTP software

- command line: oathtool from package oath-toolkit

- GUI and multiplatform: KeepassXC

- Android: FreeOTP+, andOTP, OneTimePass etc.. (watched on F-droid)

Setup §

A package is required in order to provide the various programs required. The package comes with a README file available at /usr/local/share/doc/pkg-readmes/login_oath with many explanations about how to use it. I will take lot of information from there for the local login setup.

# pkg_add login_oath

You will have to add a new login class, depending on what of the kind of authentication you want. You can either provide password OR TOTP, or set password AND TOTP (in the form of TOTP_CODE/password as the password to type). From the README file, add what you want to use:

# totp OR password

# totp AND password

If you have a /etc/login.conf.db file, you have to run cap_mkdb on /etc/login.conf to update the file, most people don't need this, it only helps a bit in regards to performance when you have many many rules in /etc/login.conf.

Local login §

Local login means logging on a TTY or in your X session or anything requiring your system password. You can then modify the users you want to use TOTP by adding them to the according login class with this command.

# usermod -L totp some_user

In the user directory, you have to generate a key and give it the correct permissions.

$ openssl rand -hex 20 > ~/.totp-key
$ chmod 400 .totp-key

The .totp-key contains the secret that will be used by the TOTP generator, but most generator will only accept it in encoded as base32. You can use the following python3 command to convert the secret into base32.

python3 -c "import base64; print(base64.b32encode(bytes.fromhex('YOUR SECRET HERE')).decode('utf-8'))"

SSH login §

It is possible to require your users to use TOTP or a public key + TOTP. When your refer to "password" in ssh, this will be the same password as for login, so it can be the plain password for regular user, the TOTP code for users in totp class, and TOTP/password for users in totppw.

This allow fine grained tuning for login options. The password requirement in SSH can be enabled per user or globally by modifying the file /etc/ssh/sshd_config.

sshd_config man page about AuthenticationMethods

# enable for everyone
AuthenticationMethods publickey,password

# for one user
Match User solene
	AuthenticationMethods publickey,password

Let's say you enabled totppw class for your user and you use "publickey,password" in the AuthenticationMethods in ssh. You will require your ssh private key AND your password AND your TOTP generator.

Without doing any TOTP, by using this setting in SSH, you can require users to use their key and their system password in order to login, TOTP will only add more strength to the requirements to connect, but also more complexity for people who may not be comfortable with such security levels.

Conclusion §

In this text we have seen how to enable 2FA for your local login and for login over ssh. Be careful to not lock you out of your system by losing the 2FA generator.