New server for reading Yubikey

My current implementation uses Windows server for reading Yubikey on a front door. I have implemented several security features but I think I need more robust solution for securing keyboard capture.

I believe I have to change Design Rational Decision #1 and Decision #2.

My biggest concern is how I make sure USB on a front door can't be used as an attack vector to get into my system. My new idea is to use another low-end PC for capturing keystrokes and send them into local network as UDP broadcast. Message is seen by the Home Control server, which does validation, opens a door, plays messages etc. There is no need to encrypt messages cause Yubikey itself creates one-time-passwords -- Once used, you can't use it again. I have seen many ideas of using Yubikey for physical access and I believe this kind of approach could be useful more generally.

I found neat Gadget Computer from my closet. It is very small, low power PC. Only disk is missing, I ordered 2.5" SSD drive for that. My plan is install very light weight linux distribution on it and run code like this :

while (1) ( inp=read_standard_input; send_udp_broadcast(inp) )

I'll let you know when this is done - Now I'm waiting for SSD IDE to arrive.

PS. If you have any suggestion for linux distro please comment. My current plan is to install Debian.

Long USB cables

Maximum length of USB cable is 5 meters. It might be too short from your PC to the front door. In my case I needed 8 meters. Before installing I did testing with different kind of setups. Passive 5 meter cable worked but when I extended that, it started to fail. I installed 5 meter active USB cable (it uses 5V of USB to "repeat" signals) and 3 m passive extension. This works just fine, but adding 1 meter extension makes it fail again.

Later I found another option, "Newlink USB 1.1 CAT5E Booster". They promise to extend limit to 50 meters. I bought and tried it out with 20 meters ethernet cable. Works just fine! I'm not using it in my project but this is good to know, if you need to use Yubikey from far away from PC.

PS. This device uses ethernet cable, but it's just a cable. This device has nothing to do with ethernet. If it would, that would possibly provide another neat way to capture Yubikey-logon directly from network traffic...

Security issues

My 13-year old son raised up a question of security. I have had thoughts on that but I believe it's worth of more thoroughly thinking. What are use cases of the Black Hat visitor? What is impact of that threat?

Use case / Threat	Impact
Crashing my Home control server (later HCS)	It's not nice but doesn't cause any big harm
Getting copy of my HCS content	This is bad
Wiping content of my HCS	Not so bad, I use Amazon S3 as a backup backend
Getting copy, changing it and putting back to HCS	This is really bad. Hacker would have possibility to add his own Yubikey as authorized and get access to my house

Wow! We need to think how to make those "misuse cases" impossible.
Next we need to think about what are possible attack vectors. You must remember that Yubikey is acting as a keyboard and I'm using it in a way it's not meant to be used. What does open USB on a front door really mean?

Attack Vector ID	Attack vector	Comment
AVN	Someone get access to HCS through internet connection	Bad problem
AVM	Using mouse on a front door	It would be hard to get something useful to be done moving and clicking mouse blindly. Still I don't like the idea that someone would do that.
AVK	Using keyboard on a front door	Uups, this is bad. Alt-F4 would stop my interactive app, Ctrl-Alt-Del, Alt+T would start TaskManager etc. Really bad.
AVU	Sticking USB stick into front door socket	If my setup won't autostart anything, this is not so bad. But together with other holes this could turn into very big hole

This is a horror story what could happen if I'm not aware of security and don't find enough tricks to secure my system (maybe I will shut down this project if I get too afraid?) :

Mister Black Hat comes to my front door. He plugs a USB hub into my Yubikey socket. He sticks his USB drive and keyboard into the hub. With keyboard he hits Ctrl-ESC to get Start menu up, starts CMD.EXE and makes a wild guess: C is system, D is DVD... E is his own USB drive. He types command that copies all content to his USB drive or sends them to his internet site. Then he plugs USB drive to his own laptop, makes changes to setup, copies configuration files back to my server and opens the door with his own Yubikey ("Hello Black Hat, Welcome! Sorry, your voice box isn't configured yet but you are still welcome!").

This is a list of some of security features I have implemented (or will implement). They are listed in a table that shows Attack Vector ID form previous table and some features to work against those:

Attack Vector ID	Feature
AVN	Firewall (HW based, both IN and OUT), hardened OS, very strict control of installed software
AVM	My interactive app listening keyboard messages will set mouse position to the corner of the screen in 20 ms interval
AVK	In windows there is a system call to disable all special keys (originally meant for screen savers). I'll use that. In a tight interval I'll make sure my APP has focus. My APP does nothing else that listens keyboard messages and sends Yubikey-keystrings to my actual application service sending UDP broadcasts. I'll run it under other apps control that will restart app if is ever stopped.
AVU	Maybe I should find a registry trick, 3rd party app or some other way to disable USB drives? How about creating dummy virtual drives (SUBST) for all drive letters? ;-)

...or, to really nail it, should I setup another PC just for listening Yubikey and sending them as a UDP broadcast to Home Control server?
And finally: Did you find any other holes? Or did you find a better idea against some attack vector? Please, add a comment!

Should I authenticate locally or from Yubico service?

Yubikeys are delivered with secret AES-key which is unreadable from the key. Same secret AES-key is stored both in Yubikey and Yubico's validation server. To be able to validate you have to have internet connection.

Because all code is open source, you can also run your own validation server. In that case you can reprogram Yubikey with your own secret AES-key and store that key locally into your own server. I had to make a decision which way to go.

DR-log:

Question :	Using Yubico validation service or local?
Alternative 1	Yubico validation service + Very easy validation (for example simple http request) + No need to reprogram keys + Access can be provided also friends with Yubikey, even through webinterface - Needs internet connection - You have to trust Yubico to keep your AES secret and state of internal counters
Alternative 2	Running own validation server + Works also when internet connection is broken + No need to trust any 3rd side - Need to reprogram all keys. Same keys can't be used to logon any other Yubikey-enabled service
Comments :	First of all I trust Yubico. Even if internet connections is lost, my PC can also die - I have to have also mechanical key and lock available for those situations.
Decision #3:	I'll use Yubico authentication service

How to capture keystrokes?

Yubikey is the one and only one time password generator hardware that acts as a keyboard. It's great for application logins but for this kind of application it's not so good: My PC has to autologon, my application must be running in interactive mode or I must be able to capture all windows keystroke events. I'm not sure if windows even sends keystroke messages if you haven't logged in. So I needed a autologon solution.

DR-log:

Question :	How to autologon into OS?
Alternative 1	Windows registry based autologon + Free + Unsafe -- Logon username and password is stored into registry uncrypted
Alternative 2	Commercial product: LogonExpert + Encrypts password + Robust solution + Ctrl+Alt+Del, a shutdown event, administrator’s banners, or any other window appearing before logon is bypassed automatically.
Comments :
Decision #2:	LogonExpert

(Just another Yubikey development idea: If Yubikey would act both as a keyboard and serial port, it could be used more robustly also in this kind of applications. It could even act as a "license dongle" sitting in back of a server, providing one time password through serial interface)