Yubikey

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The Yubikey is a small USB token that generates One-Time Passwords (OTP). It is manufactured by Yubico.

One of its strengths is that it emulates a USB keyboard to send the OTP as text, and thus requires no drivers.

Introduction

How does it work

Yubikey's authentication protocol is based on symmetric cryptography. More specifically, each Yubikey contains a 128-bit AES key, unique to that key. It is used to encrypt a token made of different fields such as the ID of the key, a counter, a random number, etc. The OTP is made from concatenating the ID of the key with this encrypted token.

This OTP is sent to the target system, to which we want to authenticate. This target system asks a validation server if the OTP is good. The validation server has a mapping of Yubikey IDs -> AES key. Using the key ID in the OTP, it can thus retrieve the AES key and decrypt the other part of the OTP. If it looks OK (plain-text ID and encrypted ID are the same, the counter is bigger than the last seen one to prevent replay attacks...), then authentication is successful.

The validation server sends that authentication status back to the target system, which grants access or not based on that response.

Security risks

AES key compromission

As you can imagine, the AES key should be kept very secret. It can not be retrieved from the Yubikey itself (or it should not, at least not with software). It is present in the validation server though, so the security of this server is very important.

Validation requests/responses tampering

Since the target system relies on the ruling of the validation server, a trivial attack would be to impersonate the validation server. The target system thus needs to authenticate the validation server. 2 methods are available :

  • HMAC: This is also symmetric crypto, the target server and validation server share a key that is used to sign requests and responses.
  • TLS: Requests and responses travel via HTTP, so TLS (HTTPS) can be used to authenticate and encrypt the connection.

YubiCloud and validation servers

When you buy a Yubikey, it is preloaded with an AES key that is known only to Yubico. They will not even communicate it to you. Yubico provides a validation server with free unlimited access (YubiCloud). It also offers open-source implementations of the server.

So you can either:

  • choose to use your Yubikey with its preloaded AES key and validate against Yubico's validation server ;
  • or load a new AES key in your Yubikey and run your own validation server.
Note: To authenticate the Yubico validation server, you can:
  • with HMAC: use https://upgrade.yubico.com/getapikey/ to get an HMAC key and ID
  • with HTTPS: the validation server's certificate is signed by GoDaddy, that is cool because it is thus trusted by default in Arch installs (at least if you have package ca-certificates)

Two-factor authentication with SSH

This details how to use a Yubikey to have two-factor authentication with SSH, that is, to use both a password and a Yubikey-generated OTP.

Prerequisites

The necessary packages are on the AUR. The one you need for TFA with SSH is the yubico-pam-git. It depends on:

Install them.

Note: If you are configuring a distant server to use Yubikey, you should open at least one additional, rescue SSH session, so that you are not locked out of your server if the configuration does not work and you exit your main session inadvertently

PAM configuration

You have to edit /etc/pam.d/sshd, and modify the line that reads :

auth		required	pam_unix.so

into

auth		required	pam_unix.so	use_first_pass

Then do one of the following. I personally would highly recommend the HTTPS method, but the choice is yours. --Gohu 17:49, 24 April 2011 (EDT)

If using HTTPS to authenticate the validation server

Insert the following line before the previously modified pam_unix.so line.

auth            required        pam_yubico.so           id=1 url=https://api.yubico.com/wsapi/2.0/verify?id=%d&otp=%s

The id=1 is of no real use but it is required.

Note: If you run your own validation server, modify the url parameter to point to your server

If using HMAC to authenticate the validation server

Insert the following line before the previously modified pam_unix.so line.

auth            required        pam_yubico.so           id=1234 key=YnVubmllcyBhcmUgY29vbAo=

where id and key are your own HMAC ID and key, requested from Yubico as explained above.

Note: HMAC credentials should be unique to a single target server. That way, if an attacker finds them, he will not be able to craft responses to authenticate to other target servers you own
Note: We did not specify the url parameter: it defaults to Yubico's HTTP (non-TLS) server

You should also disallow unprivileged users to read the file to prevent them from seeing the HMAC credentials:

# chmod o-r /etc/pam.d/sshd
Note: If you run your own validation server, add the url parameter to point to your server

SSHD configuration

You should check that /etc/ssh/sshd_config contains these lines and that they are not commented, but I believe this is the default.

ChallengeResponseAuthentication no
UsePAM yes

That is it!

You should not need to restart anything if you just touched the PAM config file.

To log in, at the Password: prompt of SSH, you have to type your password without pressing enter and touch the Yubikey's button. The Yubikey should send a return at the end of the OTP so you do not need to touch the enter key at all.

Note: If you remove use_first_pass from the pam_unix.so line, you can just use your YubiKey first, then it will prompt for your password after the YubiKey line.

Explanation

This works because the prompt is pam_yubico.so's one, since this module is before pam_unix.so, which does basic password authentication. So, you are giving a string that is the concatenation of your password and the OTP to pam_yubico.so. Since the OTPs have a fixed length (let us call this size N), it just has to get the last N characters to retrieve the OTP, and it assumes that the other characters at the start are the password. It tries to validate the OTP, and in case of success, sends the password to the next PAM module, pam_unix.so, which was instructed not to prompt for the password, but to receive it from the previous module, with use_first_pass.