login_yubikey8

Usage

Overview

The login_yubikey utility is called by login(1), su(1), ftpd(8), and others to authenticate the user with the Yubico one-time password (OTP) authentication mechanism.

The options are as follows:

-d

Debug mode. Output is sent to the standard output instead of the BSD Authentication backchannel.

-s service

Specify the service. Currently, only challenge, login, and response are supported. The default protocol is login.

-v

This option and its value are ignored.

The user argument is the login name of the user to be authenticated.

The optional class argument is accepted for consistency with the other login scripts but is not used.

login_yubikey will read the user's UID (12 hex digits) from the file user.uid, the user's key (32 hex digits) from user.key, and the user's last-use counter from user.ctr in the /var/db/yubikey directory.

If user does not have a UID or key, the login is rejected. If user does not have a last-use counter, a value of zero is used and any counter is accepted during the first login.

The one-time password provided by the user is decrypted using the user's key. After the decryption, the checksum embedded in the one-time password is verified. If the checksum is not valid, the login is rejected.

If the checksum is valid, the UID embedded in the one-time password is compared against the user's UID. If the UID does not match, the login is rejected.

If the UID matches, the use counter embedded in the one-time password is compared to the last-use counter. If the counter is less than or equal to the last-use counter, the login is rejected. This indicates a replay attack.

If the counter is larger than the last-use counter, the counter is stored as the new last-use counter, and the login is accepted.

Files

/var/db/yubikey

Directory containing user entries for YubiKey OTP security keys.

Caveats

The login_yubikey utility does not implement the U2F/FIDO2 open authentication standard.