Lock Up Your Raspberry Pi with Google Authenticator

Raspberry Pi boards (or any of the many similar boards) are handy to leave at odd places to talk to the network and collect data, control things, or do whatever other tasks you need a tiny fanless computer to do. Of course, any time you have a computer on a network, you are inviting hackers (and not our kind of hackers) to break in.

We recently looked at how to tunnel ssh using a reverse proxy via Pagekite so you can connect to a Pi even through firewalls and at dynamic IP addresses. How do you stop a bad guy from trying to log in repeatedly until they have access? This can work on any Linux machine, but for this tutorial I’ll use Raspberry Pi as the example device. In all cases, knowing how to set up adequate ssh security is paramount for anything you drop onto a network.

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Taking a U2F Hardware Key from Design to Production

Building a circuit from prototyping to printed circuit board assembly is within the reach of pretty much anyone with the will to get the job done. If that turns out to be something that everyone else wants, though, the job gets suddenly much more complex. This is what happened to [Conor], who started with an idea to create two-factor authentication tokens and ended up manufacturing an selling them on Amazon. He documented his trials and tribulations along the way, it’s both an interesting and perhaps cautionary tale.

[Conor]’s tokens themselves are interesting in their simplicity: they use an Atmel ATECC508A specifically designed for P-256 signatures and keys, a the cheapest USB-enabled microcontroller he could find: a Silicon Labs EFM8UB1. His original idea was to solder all of the tokens over the course of one night, which is of course overly optimistic. Instead, he had the tokens fabricated and assembled before being shipped to him for programming.

Normally the programming step would be straightforward, but using identical pieces of software for every token would compromise their security. He wrote a script based on the Atmel chip and creates a unique attestation certificate for each one. He was able to cut a significant amount of time off of the programming step by using the computed values with a programming jig he built to flash three units concurrently. This follows the same testing and programming path that [Bob Baddeley] advocated for in his Tools of the Trade series.

From there [Conor] just needed to get set up with Amazon. This was a process worthy of its own novel, with Amazon requiring an interesting amount of paperwork from [Conor] before he was able to proceed. Then there was an issue of an import tariff, but all-in-all everything seems to have gone pretty smoothly.

Creating a product from scratch like this can be an involved process. In this case it sounds like [Conor] extracted value from having gone through the entire process himself. But he also talks about a best-case-scenario margin of about 43%. That’s a tough bottom line but a good lesson anyone looking at building low-cost electronics.

Turning A Teensy Into A U2F Key

Last month, GitHub users were able to buy a special edition Universal 2nd Factor (U2F) security key for just five bucks. [Yohanes] bought two, but wondered if he could bring U2F to other microcontrolled devices. he ended up building a U2F key with a Teensy LC, and in the process brought U2F to the unwashed masses.

Universal 2nd Factor is exactly what it says on the tin: it doesn’t replace your password, but it does provide a little bit of extra verification to prove that the person logging into an account is indeed the person that should. Currently, Google (through Gmail and Google Drive), Github, Dropbox, and even WordPress (through a plugin) support U2F devices, so a tiny USB key that’s able to provide U2F is a very useful device.

After digging into the U2F specification [Yohanes] found the Teensy LC would be a perfect platform for experimentation. A U2F device is just a USB HID device, which the Teensy handles in spades. A handy library takes on ECC for both AVR and ARM platforms and [Yohanes’] finished U2F implementation is able to turn the Teensy LC into something GitHub was selling for $5.

It should be noted that doing anything related to security by yourself, with your own code is dumb and should not be considered secure. Additionally, [Yohanes] didn’t want to solder a button to his Teensy LC, so he implemented everything without a button press, which is also insecure. The ‘key handle’ is just XOR encryption with a fixed key, which is also insecure. Despite this, it’s still an interesting project and we’re happy [Yohanes] shared it with us.