From ArchWiki

The OATH Toolkit provides one-time password (OTP) components for authentication systems. It contains a PAM authentication module that supports technologies include the event-based HOTP algorithm (RFC 4226) and the time-based TOTP algorithm (RFC 6238). The OTP generator applications are available for Android, iOS, Blackberry and other devices. Similar to Google Authenticator the authentication mechanism integrates into the Linux PAM system. This guide shows the installation and configuration of this mechanism.


Install the oath-toolkit package.

Setting up the OATH

The OATH seed is an hexadecimal number that should be unique per user. To generate a new seed for a user, you could use the following command line:

$ openssl rand -hex 10
Note: The above output seed is used as example in this article and must not be used directly.

There needs to be one OATH per user and link to it in a configuration file /etc/users.oath. While being Root user create the file and insert the user seed:

Tip: Log in to a session with root user privileges in one terminal and modify the PAM configuration in another terminal. In this way, if you have a problem with your PAM settings, you still have an account to log in and fix it, which is especially useful when connecting remotely.
# Option User Prefix Seed
HOTP/T30/6 user - 12345678909876543210
Warning: Do not set T bigger than 60,otherwise you will get error:(OATH_UNKNOWN_USER: Cannot find information about user).

If you need HOTP, use this configuration:

# Option User Prefix Seed
HOTP user - 12345678909876543210

Make sure that the file can only be accessed by Root user:

# chmod 600 /etc/users.oath
# chown root /etc/users.oath

Setting up the PAM

To enable OATH for a specific service only, like OpenSSH, you can edit the file /etc/pam.d/sshd and add at the beginning of the file the following line:

auth	  sufficient usersfile=/etc/users.oath window=30 digits=6

This will allow authentication if you just enter the right OATH code. You can make OATH as a requirement and let the rest of the PAM stack be processed if you use the following line instead:

auth	  required usersfile=/etc/users.oath window=30 digits=6

For SSH login to work, make sure these options are enabled in the file /etc/ssh/sshd_config:

ChallengeResponseAuthentication yes
UsePAM yes

Restart sshd.service to enable the changes.

If you want to force OATH request-response even if there is a working public key authentication and password authentication also add the following in /etc/ssh/sshd_config:

AuthenticationMethods publickey,keyboard-interactive:pam
KbdInteractiveAuthentication yes
PasswordAuthentication yes
Tip: For local login, just edit /etc/pam.d/login.

Logging with an OATH password

For logging in with TOTP:

$ oathtool -v --totp -d 6 12345678909876543210
Warning: There is an error when TOTP mode setting SHA512, but the developers did not fix it. See the open issue on Gitlab.

If you are logging in with HOTP:

$ oathtool -v -d 6 12345678909876543210

Replace 12345678909876543210 by the seed corresponding to your user. It will output something like the following:

Hex secret: 1ab4321412aebc
Base32 secret: DK2DEFASV26A====
Digits: 6
Window size: 0
Start counter: 0x0 (0)


The last string of numbers can be used as a code for login right now, but more interestingly the Base32 secret, because it can be converted to QR code for easily transferring keys. Install the package qrencode to run the following command to convert:

$ qrencode -o user.png 'otpauth://totp/user@machine?secret=DK2DEFASV26A===='

Change user, machine and DK2DEFASV26A==== accordingly. Once done, you can visualize your QR code with your preferred image visualizer application. Alternatively you may generate the QR code directly onto terminal with:

$ qrencode -t UTF8 'otpauth://totp/user@machine?secret=DK2DEFASV26A===='

It is pretty straight forward to use Aegis Authenticator or FreeOTP+ to then take a screenshot of that user.png (or ASCII-art like image) and get it to display OTP password when needed.

Note: The secret key of your users is the most important information in this system. Once you setup a phone to provide OTP, it does have that key, the QR code in that user.png file does have that key. You need to take extra care of this file, they should only be stored on encrypted medium (Your phone need to be using encryption for any sane level of security). If not even confined in a sandbox like Samsung Knox to prevent third party application to potentially access them.

See also