Securing HomeAssistant with client certificates (works with Safari/iOS)

I recently moved from SmartThings to HomeAssistant. One of the things I didn’t have to think about too much with SmartThings was how to authenticate all of my connected devices (laptops, phones, tablets, etc.) with my HA platform. I wanted to find a good balance between security and convenience with HomeAssistant.

HomeAssistant makes it easy to secure your install with a password. Coupled with TLS, this is pretty solid. But there’s just something about the idea of a publically facing page that anyone on the Internet can get to, protected with nothing but a password that made me feel uneasy.

Client certificates are a very robust authentication mechanism that involves installing a digital certificate on each device you wish to grant access to. Each certificate is signed by the server certificate, which is how the server knows that the client is valid.

This feels nicer than HomeAssistant’s built-in security measures to me for a few reasons:

  1. Individual client certificates can be revoked. You don’t have to configure authentication on every device you own if someone loses their phone.
  2. While I highly doubt there are any issues with HomeAssistant, I feel more confident in nginx and openssl.
  3. Unless you add a passphrase to the client certificates (I didn’t), the whole thing is passwordless and still manages to be pretty darn secure.
  4. If I ever became truly paranoid, I could turn on HomeAssistant’s password protection and my HA dashboard would essentially need two authentication factors (the SSL cert + the password).

While I did find this approach more appealing, there are several drawbacks:

  1. It’s way harder to set up. You need to run a bunch of openssl commands, and install a certificate on each device you want to grant access to.
  2. The HomeAssistant web UI requires WebSockets, which seem to not play nicely in combination with client certificates on Safari or iOS devices. My household has iOS users, so this was something I needed to figure out.

I think I managed to get this working. The only disadvantage is that clients are granted access for an hour after successfully authenticating once. The basic approach is to tag authenticated browsers with an access token that’s good for a short period of time, long enough for them to establish a WebSocket connection. I’ll go through the steps in setting this up.

What you need

  1. Install packages:  sudo apt-get install nginx nginx-extras lua5.1 liblua5.1-dev
  2. openssl
  3. luacrypto module, which exposes openssl bindings in lua.

luacrypto was kind of a pain to install. Here’s what I did to get it working with my nginx install. It involved patching (thanks to this very helpful StackOverflow post for the tip):

Setting up a Certificate Authority

There are already good guides on doing this. I recommend this one. In this guide, I’m using the default_CA parameters pre-filled by openssl on my system.

Generate client certificates

I put a script in /usr/local/bin  to make this easier:

You then run this for each device you want to grant access to:

Make sure to supply an export password. Generated certificate files will be placed in /etc/ssl/ca/certs/users .

Get the certificates on the devices

The .p12 file is the one you want. Make sure to not compromise the certificates in the process.

I rsynced the files to my laptop and attached them to a LastPass note, which I could access on my devices. On most devices, you should be able to just open the .p12 file and it’ll do what you want.

On iOS devices, I needed to serve the certificates over HTTPS on a trusted network because they needed to be “opened” by Safari in order to be recognized.

Configure nginx

Here’s my nginx config. You’ll need to substitute your domain and SSL certificate parameters:

If this all worked, you should be able to access from a device with a client certificate installed, but not otherwise. Unfortunately if you’re using iOS or Safari, you’ll probably notice that the page loads, but you get a forever spinny wheel. If you look in the debugger console, you might see messages that look like this:

This is because the browser isn’t sending the client certificate information when trying to connect to the WebSocket and is therefore failing.

Fixing compatibility with iOS/Safari

Safari does actually send client cert info along with the initial request. Nginx has a really cool module that allows you to insert all sorts of fancy logic with lua scripts. I added one that tags browsers supplying a valid client certificate with a cookie granting access for about an hour. This worked really well. Since this is all over HTTPS, and the access tokens are short-lived, I felt pretty comfortable.

The easiest way I could think of to create a cookie that was valid for a limited time was to use an HMAC. Basically I “sign” a hash of the client’s certificate along with an expiry timestamp. The certificate hash, expiration timestamp, and HMAC are all stored in cookies. Nginx can then validate that the expiration timestamp is in the future, and that the HMAC signature matches what’s expected.

You’ll notice the commented-out line in the nginx config above. Uncomment it:

And add the script:


Reverse engineering the new Milight/LimitlessLED 2.4 GHz Protocol

My last post went over an ESP8266-based wifi gateway for Milight/LimitlessLED bulbs. This supports a few kinds of bulbs that have been around for a couple of years (e.g., this one).

About a year ago, newer bulbs and controllers started showing up that used a different 2.4 GHz protocol. This introduced some scrambling that made it difficult to emulate many devices. This was presumably done intentionally to prevent exactly the sort of thing that my last project accomplished (boo!).

The new bulbs actually have some really cool features that none of the old ones do, so there’s some incentive to figure this out. In particular, they support saturation, which allows for ~65k (2**16) colors with variable brightness instead of the 256 colors that the old one does. They also combine RGB and CCT (adjustable white temperature) in one bulb, which is super cool.

A few others have dug into this a little, but as far as I’ve been able to tell, no one has figured out (or at least shared) how to de-scramble the protocol. I think I’ve managed to do so. I should mention that I don’t have much experience doing this kind of thing, so it’s entirely possible the structure I’m imposing is a lot more complicated than what’s actually going on. But as far as I’ve been able to tell, it does work. I’ve tested with five devices – four remotes and one wifi box.

I’m going to start by detailing the structure, and I’ll follow up with some of the methodology I used to reverse the protocol.

Differences from old protocol

From a quick glance, there are a few superficial differences between the new and old protocols:

  1. Listens on a different channelset (this was true of different bulb types among the old bulbs too). The new bulbs use channels 8, 39, and 70.
  2. Different PL1167 syncword. It uses 0x7236 , 0x1809 .
  3. Packets have 9 bytes instead of 7.
  4. Packets are scrambled. The same command can look completely different.

The scrambling is the tricky part. As others who have stared at packet captures noticed, when the first byte of packets for the same command is held fixed, most of the other bytes stay fixed too. This suggests that the first byte is some kind of scramble key. Turns out this is the case.

Example packets for turning group 1 on with one of my remotes:


I’ll for a packet p, I’ll use pi to refer to the 0-indexed ith byte in the packet. For example, p0 refers to the 0th byte.

I’ll use p’ to refer to the scrambled packet for a packet p.


The 9 bytes of the packet are:

p0 p1 p2 p3 p4 p5 p6 p7 p8
Key 0x20 ID1 ID2 Command Argument Sequence Group Checksum

Packet scrambling

The designer of this protocol added in quite a few things to complicate reversing it. None of them are particularly hard on their own, but with them all added together it makes it pretty tough.

The scrambling algorithm is basically:

  1. A scramble key k is computed from p0
  2. Each byte position i has a different set of four 1-byte integers A[i]. Integer A[i][j] is used when p0 ≡ j mod 4.
  3. A[i][j] is up-shifted by 0x80 when p0 is in the range [0x54, 0xD3]. This does not apply to the checksum byte.
  4. p’i = ((pik) + A[i][p0 mod 4]) mod 256, where ⊕ is a bitwise exclusive or.

The algorithm to compute k is as follows (in ruby):

A values:

Position 0 1 2 3
p1 0x45 0x1F 0x14 0x5C
p2 0x2B 0xC9 0xE3 0x11
p3 0x6D 0x5F 0x8A 0x2B
p4 0xAF 0x03 0x1D 0xF3
p5 0x5A 0x22 0x30 0x11
p6 0x04 0xD8 0x71 0x42
p7 0xAF 0x04 0xDD 0x07
p8 0x61 0x13 0x38 0x64

There are probably actually several possible values for some of these. It really only matters that they line up in a particular way because of the checksum.

In addition to all of this, command arguments have different offsets from 0, and some commands (i.e., saturation and brightness) have the same p4 value with arguments spanning different ranges. For example, arguments for brightness start at 0x4F. Arguments for color start at 0x15.


The checksum byte is calculated by summing all bytes in the unscrambled packet except for the first (scramble key) and last (checksum), and k + 2.


Further detail is probably easier to communicate in code, so here is a ruby library that can encode/decode packets. The project on github also has a ton of packets I scraped from my devices.


I scraped a ton of packets with this script.

Figuring this out was mostly making assumptions, pattern recognition, and trial and error. The most helpful assumption was that sequential values for arguments in the UDP protocol had sequential values in the 2.4 GHz protocol (this turned out to be true).

I noticed that packets that had p0 values with the same remainder mod 4 followed a nearly sequential pattern. It often looked something like 0xC, 0xD, 0xA, 0xB, etc. A sequence follows this pattern when it’s xored with a constant. I wrote some cruddy ruby methods to brute-force search for constants that yielded the sequence [0, 1, …, N]. This also allowed me to find the values for the As.

Roughly the same process was repeated for each byte. Bytes that are constants were trickier because they didn’t follow a sequence. I instead brute-forced values for A given a sequence of xor keys.

Next Steps

I’ll be porting over the scrambling code to my ESP8266 milight hub to add support for the new bulbs.

UPDATE 2017-03-28: A few kind volunteers sent me packet captures from their devices, and the ID bytes were not staying fixed under decoding. Assuming my methodology is right, these should be the right values for all parameters, with the possible exception of p1 and the checksum byte.

UPDATE 2017-03-20: I found that the wifi box I was testing with supported older protocols, which transmits the unscrambled device ID. There were several possible values for the ID byte offsets, and I chose a few of them arbitrarily. The decoded ID in the scrambled protocol was not matching the ID in the unscrambled protocol. Updating the additive offset values fixed this.

Milight WiFi Gateway Emulator on an ESP8266

Milight bulbs* are cheap smart bulbs that are controllable with an undocumented 2.4 GHz protocol. In order to control them, you either need a remote* (~$13), which allows you to control them directly, or a WiFi gateway* (~$30), which allows you to control them with a mobile app or a UDP protocol.

A few days ago, I posted my Arduino code to emulate a Milight WiFi gateway on an ESP8266 (link). This allows you to use an NRF24L01+ 2.4 GHz tranceiver module* and an ESP8266* to emulate a WiFi gateway, which provides the following benefits:

  1. Virtually unlimited groups. The OTS gateways are limited to four groups.
  2. Exposes a nice REST API as opposed to the clunky UDP protocol.
  3. Secure the gateway with a username/password (note that the 2.4 GHz protocol used by the bulbs is inherently insecure, so this only does so much good).

I wanted to follow up with a blog post that details how to use this. I’m going to cover:

  1. How to setup the hardware.
  2. How to install and configure the firmware.
  3. How to use the web UI and REST API to pair/unpair and control bulbs.

Shopping List

This should run you approximately ~$10, depending on where you shop, and how long you’re willing to wait for shipping. Items from Chinese sellers on ebay usually come at significant discounts, but it often takes 3-4 weeks to receive items you order.

  1. An ESP8266 module that supports SPI. I highly recommend a NodeMCU v2*.
  2. An NRF24L01+ module. You can get a pack of 10* on Amazon for $11. You can also get one that supports an external antenna if range is a concern (link*).
  3. Dupont female-to-female jumper cables (at least 7). You’ll need these to connect the ESP8266 and the NRF24L01+.
  4. Micro USB cable.

If you get a bare ESP8266 module, you’ll need to figure out how to power it (you’ll likely need a voltage regulator), and you’ll probably have to be mildly handy with soldering.

Setting up the Hardware

The only thing to do here is to connect the ESP8266 to the NRF24L01+ using the jumper cables. I found this guide pretty handy, but I’ve included some primitive instructions and photos below.

NodeMCU Pinout

NRF24L01+ Pinout


NodeMCU Pin NRF24L01+ Pin
3V (NOT Vin) VCC

Installing drivers

There are a couple of different versions of NodeMCUs (I’m not convinced they’re all actually from the same manufacturer). Depending on which one you got, you’ll need to install the corresponding USB driver in order to flash its firmware.

The two versions I’m aware of are the v2 and the v3. The v2 is smaller and has a CP2102 USB to UART module. You can identify it as the small square chip near the micro USB port:

NodeMCU v2 with CP2102 circled

Install drivers for the v2 here.

The v3 is larger and has a CH34* UART module, which thin and rectangular:

NodeMCU v3 with CH34* circled

The CH34* drivers seem more community-supported. This blog post goes over different options.

I’ve been able to use both the v2 and v3 with OS X Yosemite.

Installing Firmware

If you’re comfortable with PlatformIO, you can check out the source from Github. You should be able to build and upload the project from the PlatformIO editor.

Update – Mar 26, 2017: I highly recommend using PlatformIO to install the firmware. The below instructions are finicky and unless you get the arguments exactly right, the filesystem on your ESP will not work correctly. Using PlatformIO is a more robust way to get a fresh ESP set up. Further instructions are in the README.

Update – Feb 16, 2017: if you’re not using a NodeMCU (e.g., Wemos), the pre-compiled firmware will probably not work. You’ll need to compile your own. Check out the source, and update platformio.ini  to look like this, and build. Thanks to u/tamu_nerd on r/homeautomation for pointing this out.

Update – Feb 26, 2017: if you’ve used your ESP for other things before, it’s probably a good idea to clear the flash with --port /dev/ttyUSB0 erase_flash . Thanks to Richard for pointing this out in the comments.

If not, you can download a pre-compiled firmware binary here. If you’re on Windows, the NodeMCU flasher tool is probably the easiest way to get it installed.

On OS X (maybe Linux?), following the NodeMCU guide, you should:

  1. Connect the NodeMCU to your computer using a micro USB cable.
  2. Install esptool
  3. Flash the firmware:

    Note that  /dev/cu.SLAB_USBtoUART should be substituted for  /dev/cu.wchusbserial1410 if you’re using a v3 NodeMCU. Be sure to specify the real path to the firmware file.
  4. Restart the device. To be safe, just unplug it from USB and plug it back in.

Setup firmware

Note that you’ll have to do all of these things before you can use the UI, even if you used the pre-compiled firmware:

  1. Connect the device to your WiFi. Once it’s booted, you should be able to see an unsecured WiFi network named “ESPXXXXXX”, where XXXXXX is a random identifier. Connect to this network and follow the configuration wizard that should come up.
  2. Find the IP address of the device. There are a bunch of ways to do this. I usually just look in my router’s client list. It should be listening on port 80, so you could use nmap  or something.

If you installed the firmware with PlatformIO, you can skip this. If you installed the precompiled firmware image, you’ll need to upload the UI with a command like this:

curl -vvv -X POST -F 'image=@web/index.html' http://<ip_of_esp>/data/web

You should now be able to navigate to http://<ip_of_isp>.

Using the Web UI

The UI is useful for a couple of things.

If you have Milight bulbs already, you probably have them paired with an existing device. Rather than unpairing them and re-pairing with the ESP8266 gateway, you can just have the ESP8266 gateway spoof the ID of your existing gateway or remote. Just click on the “Start Sniffing” button near the bottom and push buttons in the app or on the remote. You should see packets start to appear:

The “Device ID” field shows the unique identifier assigned to that device. To have the ESP8266 gateway spoof it, scroll up to the top and enter it:

The controls should not work as expected. You can click on the “Save” button below if you want to save the identifier in the dropdown for next time.

The UI is also useful for pairing/unpairing bulbs. Just enter the gateway ID, click on the group corresponding to the bulb you wish to pair/unpair, screw in the bulb, and quickly (within ~3-5s) press the appropriate button. The bulb should flash on and off if it was successful.

Using the REST API

The UI is great for poking around and setting things up, but if you want to tie this into a home automation setup, you’ll probably want a programmatic interface. The API is fully documented in the Github readme, but here’s a quick example:

This will turn bulbs paired with device 0xCD86, group 2 on and set the color to red (hue = 0).

UPDATE – Feb 12, 2016

I realized this project would be a lot more immediately useful to people if it just supported the existing Milight UDP protocol. This would allow people to use the existing integrations others have built for OpenHab, Home Assistant, SmartThings, etc.

The Web UI has a section to manage gateway servers. Each server will need a device ID and a port.

* Amazon affiliate link.

In-depth how-to: Integrating Dash Buttons with SmartThings

After quite a bit of iteration, I’m mostly happy with the way I’ve integrated Dash buttons into my home automation setup. Here’s a demo:

My goals were:

  1. Make the buttons as responsive as possible.
  2. Make it robust.
  3. Setup should survive reboots and power outages without manual intervention.
  4. Integrate with SmartThings.

In a previous post, I outlined two different approaches. I went with the approach that had the lowest latency (<1s). This one is quite a bit more work — mostly because it requires a dedicated wireless card.

Here’s the equipment I used:

  1. Raspberry Pi 2 (I used the CanaKit starter kit. If you’re buying now, you’d probably want the Pi 3 edition).
  2. Edimax EW-7811Un USB WiFi dongle.

Important! to use this approach, you need at least one WiFi dongle that supports monitor mode. The Edimax dongle I suggested doesn’t support monitor mode, but the one that comes with the CanaKit 2 does. Note that the Pi 3’s onboard WiFi device does not support monitor mode, so you’ll want to buy a dongle that does (you can buy the CanaKit dongle separately for $9).

Set up dash buttons

This approach will work with the normal setup process, but with a slight modification, you can ensure that the dash buttons don’t contact Amazon when pressed.

The only thing you need to do differently is set up the buttons on a network you can delete later. I have dd-wrt on my router, so I used a virtual interface. If your router supports a “guest network” or something to that effect, it’s the same thing.

Create the network, set up the dash buttons on it, delete the network. The buttons will still attempt to connect when pressed, but won’t be able to because it doesn’t exist.

Install required packages

Set up the network

If you’re using ethernet + a WiFi dongle, you shouldn’t need to do much of anything. If you’re using two WiFi devices, it’s a little trickier. In order for this to work consistently across reboots, you’ll have to:

  1. Make sure that the interfaces (wlan0, etc.) are named consistently. They seemed to randomly swap by default, which obviously caused some problems.
  2. Tell the OS which device should be connecting to the network.

(1) is easy enough with ifrename. There’s probably a way to do it with udev, but this is way easier. It allows you to assign names to interfaces based on hardware (MAC) addresses. Open up /etc/iftab  in your favorite editor (just create it if it doesn’t exist). Mine looks like this:

After a reboot, you should see that the devices are named appropriately:

Notice you can name the interfaces whatever you want. monitorwan and mainwan seemed more informative than wlan0 and wlan1. 🙂

(2) is also pretty straightforward. There might be an easier way to do this, but I just did it by editing /etc/network/interfaces to my liking:

The wpa-psk field is a pre-shared key generated from your network SSID and passphrase. You can generate it with the  wpa_passphrase tool (from the wpasupplicant package):

You can apply these settings with a  sudo service networking restart . Probably good to reboot to make sure it works as expected.

Download ha_gateway

This setup uses ha_gateway, which is a small REST gateway I use to bridge a bunch of custom hackery with the rest of my home automation setup (mostly SmartThings). To install it, just check out the project from Github:

While I haven’t tested ha_gateway with anything but ruby 2.3.1, it probably works with 1.9+. If you’re getting errors when running bundle install , post a comment and I’ll help debug.

Create the monitor interface

In order to use monitor mode, we create a virtual monitor interface. We can do this with the iw tool, but I stuffed all of the setup into a script shipped with ha_gateway. It takes two arguments: the interface you’re using for monitor mode, and what you want to name the virtual interface

This should create an interface called DashMonitor :

To test if it’s working, you can try a tcpdump:

If you have basically any WiFi traffic around you, you should see packets pretty much immediately. If you don’t, it either means the monitor device isn’t working, or you’re legitimately not seeing traffic on whatever channel the NIC is tuned to.

To make sure the monitor device survives reboots, you can invoke the same script from /etc/rc.local :

Figure out MAC addresses of your dash button(s)

The easiest way I’ve found to do this is to use the monitor mode NIC and search for packets associated with the network you set them up on. I set my dash buttons up on a network called CMDashButton:

The hardware address is shown after “SA:” prefix.

Configure ha_gateway

ha_gateway is configured using a central YAML config file. You can just copy from the example:

Ignore the stuff at the beginning and skip down to the  listeners: key. You’ll create a listener for each dash button you want to use:

This will fire an HTTP PUT request to with the specified params every time the button is pressed. We can worry about making it do something useful later. First, let’s verify the button presses are getting picked up.

Use the script to fire up the ha_gateway listener process. Note you’ll have to run it with sudo — it won’t be able to listen on the monitor interface otherwise:

After waiting 10-20 seconds, press your dash button. You should see a log message that looks like this:

This means ha_gateway is successfully detecting dash button presses! Now let’s make it do something useful.

Integrating with Smart Things

ha_gateway integrates with SmartThings. We’ll be able to control your existing ST devices and routines with the dash button. Getting this working is a little complicated because SmartThings requires clients to oauth with it. Let’s get that out of the way first.

First, you’ll have to install ha_gateway’s SmartApp. Log into your ST account ( and click on “My Smart Apps”. Click on the green “New SmartApp” button on the right near the top. Click on the “From Code” tab and paste in this code:

This should take you to an editor page. Couple of things to do to finalize setup:

  1. Publish the newly created app – click on “Publish”, then “For Me”
  2. Click on the “App Settings” button, then click on the “OAuth” section.
  3. Click on the “Enable OAuth for this SmartApp” button.
  4. You should see two text fields containing a  “Client ID” and a “Client Secret”. Make note of ’em.
  5. Click on “Update” near the bottom. OAuth settings won’t persist if you skip this!

Copy the client ID and client secret into ha_gateway’s config YAML:

Setting site_location: is important so that the OAuth redirect ends up hitting the Pi again. For now, also make sure that require_hmac_signatures:  is set to false. It’ll make it easier to go through the OAuth process.

Now fire up the ha_gateway web server by running  bin/ . Now navigate to:


This should direct you to an OAuth page on ST’s site. Select a hub, check the switches you want to allow control of, and click “Authorize”. You’ll be redirected to and endpoint that outputs a JSON blob containing information about the devices you authorized, which might look something like this:

You can control each device via RESTful PUT requests. For example:

This would send the “toggle” command to “Xmas Tree”, which would turn it off since its previous status was on.

If you wanted to configure a dash button to switch on and off your Christmas Tree, you’d edit the listener config like so:

Notice we don’t need to provide the full URL, just the path. ha_gateway will assume we want to send the request to its REST server. It’ll fill in the URL specified in the site_location:  key.

You can start both the REST server and the listener process with the included start script. It’ll run the listener process as root, so make sure you’ve got an active sudo session (i.e., make sure it’s not prompting for a password):

Logs are in logs/ha_gateway.log  and logs/listeners.log .

You can also run routines. You can access /smartthings/routines to get a list of routines. To run a routine, send a GET request to /smartthings/routines/<routine_name>. Normalize routine_name to be all lowercase, remove non-alphanumeric characters, and replace spaces with underscores (e.g., “Good Night!” -> “good_night”).

Starting ha_gateway at boot

Obviously we want the REST server and the listener process to survive a reboot. This is pretty easy. I use monit because I already had it set up, but it’s probably more straightforward to just add this line to /etc/rc.local :

Make sure it appears above the exit 0  at the end of the script.

Securing it

If you don’t mind anyone on your network being able to access ha_gateway (and therefore turn off your Christmas Cheer), you can enable HMAC signatures. This will require anyone making a request to sign the request with a shared secret. Just edit the config file:


This works really well for me. It was way more work than I expected when I decided to look into hacking the dash buttons. I have five dash buttons for various uses, and they work very reliably. Adding new buttons is really straightforward.

Thoughts on Amazon Dash Button Hacks

Excited by the prospect of $5 hackable IoT buttons, I ordered a couple to toy around with and use in my home automation setup. I knew some folks had already figured out how to hack them, but hadn’t looked closely at how they were doing it until my buttons arrived.

Not surprisingly, the hacks are pretty hacky. There were two approaches I found:

  1. Listen for ARP packets sent from the dash button. Devices send ARP probes after associating with a network in order to determine whether their hardware (MAC) address is already in use. My casual experimentation showed these packets showed up 3-5 seconds after the button was pressed. This is the standard approach first outlined in Ted Benson’s medium post.
  2. Listen for probe request packets sent from the dash button. Devices wishing to connect with a particular network will send these out to kick off the process. These show up much sooner after pressing the button — usually within 1 second. Outlined by ridiculousfish here.

There are some not awesome things about both of these. Probably the worst is that you need to run something like tcpdump as root, and your network device has to be put in promiscuous mode (in the case of #1), or monitor mode (in case of #2). Setting promiscuous mode on a NIC instructs it to not ignore traffic that isn’t addressed to it (it normally does). Monitor mode is a different beast. It’s more like sniffing the waves out of the air. The card sits there without being associated with a network and can report any packets it hears on whatever channel it’s set to.

#2 has some particularly nasty downsides:

  • Need a dedicated wireless device that supports monitor mode. Since you’ll presumably need whatever box you’re working with connected, you’ll need another network device (second wireless device or ethernet).
  • Not all wireless devices support monitor mode. Fortunately, the dongle that came with my Raspberry Pi does. This post lists a few chipsets that support monitor mode.
  • Requires some networking setup. Creation of virtual monitor device feels kludgy and does not persist across restarts.
  • Much easier to spoof. As we’re listening to packets sent before the device is associated with a network, an attacker would only need the MAC address of your button. Probably not a good idea to use the buttons for much beyond switching your lights on and off. : )

Even with these, I’m pursuing #2 because:

  • Latency for #1 is just too high for most uses.
  • Since we’re listening to probe requests, we detect a button press before it associated with a network. This means we can set the buttons up on a fake network (I used a virtual interface in dd-wrt), and delete it when we’re done. Net effect is that buttons won’t be able to connect when they’re pressed, and won’t try to order Clorox Wipes when you try to turn on the kitchen lights.
  • This last benefit is mostly nice because Amazon was sending me a push notification every time I pressed a button to the effect of “Select an item to order to finish setup!”. Had to turn push notifications for Amazon off.

We’ll see how it turns out!

Riddlegate: automating apartment intercoms

If you’ve ever lived in an apartment building or gated complex, you’ve probably seen one of these things:

Apartment Intercom

A person dials some code to reach you, you make sure it’s someone you’re expecting, and you press some digits on your phone to grant access.

However handy this is, there are situations where it’s not very helpful. I can’t count how many packages I’ve missed because I was away from my phone when the delivery driver arrived. I really wanted to be able to just give them a code that allowed them access.

Enter Twilio. It enables exactly this. You rent a phone number through them (very cheap — on the order of $1/month), which can be configured to call HTTP endpoints when it receives a call, SMS, etc.

I built a small application around this to automate my apartment’s intercom. The intercom is configured to call my Twilio number, which will interact with the application. I called it “Riddlegate” after that plotline in The Neverending Story with the sphinxes. It’s pretty simple and self-explanatory. Here’s a screenshot of the admin UI:


Now when I have a guest, I can give them instructions that don’t involve me being near my phone:

  1. Dial <number>
  2. Wait for tone
  3. Dial <passcode>

And Riddlegate will buzz them in!

A (hopefully-not-too-terse) setup guide is included in the Github README.


Given that this controls access to your building/complex, security is an important concern. There are a few things Riddlegate does to improve security:

  1. Twilio signs all requests it sends with your API key. Riddlegate validates these signatures and denies access when it detects an invalid signature. This prevents a would-be attacker from brute-forcing your access code if they were to discover your endpoint URL.
  2. Admin area is password-protected. This is obviously only as secure as the password you choose. Also obviously better if you serve over HTTPS.

Security cameras: automatically recording and uploading footage when a door is opened

My last post detailed how to integrate a cheap IP cam with SmartThings. I briefly mentioned a SmartApp that took a picture when the door opened. This was pretty straightforward. I wanted to take it a step further and trigger recording when my door was opened (but no one is home). Beyond the obvious, I had the following requirements:

  1. SmartThings needs to be able toggle recording. The SmartApp should notify my household when it begins recording.
  2. Tampering with the camera shouldn’t destroy already captured footage.
  3. Footage should start uploading somewhere offsite as soon as possible.
  4. Control over uploaded footage.

I already covered (1) in my last post. My integration with the IP camera enables a scheduled recording feature, and configures this feature to always record. Switching off recording clears the schedule.

Uploading footage to S3

The camera I’m using already has builtin support to upload footage to an FTP server, which leaves everything but uploading to offsite storage.

Since it’s easy and cheap, I decided to upload footage to Amazon S3. I then needed a tool that:

  1. Watched for newly created files to appear in the directory my internal FTP server is pointed at. When a new file is detected:
  2. Immediately begin a streaming upload to S3. Since the file size isn’t known ahead of time, I made use of the multipart upload API, which allows for the breaking down of uploads into smaller (5MB) chunks.
  3. When the file is done being written to, complete the upload (i.e., tell Amazon the file is done being uploaded). This makes the file available on S3.

This seemed like a good fit for inotify, which allows for the monitoring of filesystem events. It’s possible to set up notifications that are triggered when, for example, a new file within a directory is opened, closed, or modified.

I didn’t find a tool that did exactly what I wanted, so I made a ruby library that did. I called it “s3reamer“. My sincerest apologies for being an awful portmanteau-ist. I run this on my home server:

This will automatically begin uploading to S3 when recording on the device is triggered. Here’s a snippet from the log file:

Uploading starts within a few seconds of recording being switched on. Most of that delay is waiting for the camera to begin uploading.

SmartApp to trigger recording when door opens

This part was pretty straightforward. Code below. This also sends a push notification when recording is switched on so my household knows and can react accordingly.

Integrating Foscam FI9821P with SmartThings

Motivated mostly by curiosity, I was recently in the market for a cheap IP camera. After a little bit of research, I settled on a Foscam FI9821P (I got mine for ~$45 as an Amazon Warehouse Deal). The app provided by Foscam is pretty nice, but I wanted to integrate it with my home automation setup as well. In particular, I wanted to accomplish the following:

  1. Secure access. Any communication with the camera should require some secure authentication mechanism.
  2. SmartThings integration. I wanted a device in SmartThings I could play around with.
  3. REST endpoint. Although I could probably get most of what I want done with SmartThings alone, I didn’t want to be bound to it.

SmartThings has a device type for cameras, so as long as there’s some way to access the camera within SmartThings, (2) is easy. In a previous post, I outlined a setup that uses HMAC to secure communication with smart home devices. I leveraged it in this project as well.

I should mention that I stumbled across some existing attempts at this, but nothing that would’ve given (1) and (3).

I put together this route for my home automation gateway, which accomplishes (1) and (3). With it, I can capture a snapshot and control some rudimentary functionalities of the camera. I can, for example, request a snapshot of what the camera is currently seeing simply by accessing this URL (with the appropriate security headers in place):


You can see that there’s baked in support for multiple cameras (since the endpoint is scoped by a camera name). While I don’t anticipate buying more cameras, I figured adding support would make this project more generally useful.

To integrate with SmartThings, I created a virtual device (code embedded below). It allows me to request an image, shift the camera to one of three preset positions, and to start/stop recording. Here’s a demo of the interface:SmartThings Interface

This project was a lot of fun, and quite a bit easier than I was anticipating. My favorite thing this has enabled is a SmartThings SmartApp that signals the camera to take a picture when my front door opens. To avoid being too creepy, this only happens when no one is home. If I can muster the motivation, I’ll probably write a separate post about that.

Cheap alternative to Phillips Hue LED Strip

I have some RGB LED strips in my bedroom to light an area other lighting in the room doesn’t reach. The strips I bought were inexpensive, but they only interact with the included infrared remote. I wanted to be able to control these lights with SmartThings. There are a couple of ways you can do this:

Easy and spendy

Phillips has a bunch of products that integrate nicely with SmartThings. The obvious contender here is this guy. However, for this to work, you’d also need a Phillips Hue Bridge. In total, this is going to run you somewhere between $150 and $250, depending on how many feet of LED strip you want.

Partially because this seemed unreasonably expensive, but especially considering I’d already glued LED strips to my walls, this solution wasn’t appealing.

Cheap and complicated

Browsing around, I found a cheap ($30) LED controller advertising “WiFi” control (link):

It was exactly what I was hoping for. It has a tiny TCP server that allows network control. The official mobile app is actually quite good, but it doesn’t integrate with the rest of my SmartThings stuff. I toyed around a little bit and managed to reverse engineer the protocol. I put it in a rubygem, available here.

This allowed me to programmatically control the LEDs, but obviously still no integration with SmartThings. Fortunately, that wasn’t very hard either.


The overall design looks something like this:


I’ll elaborate on each of these components in the following sections.

REST server

The TCP API works nicely, but I wanted to wrap it in something that’d be easier to interface with. I wrote a really small REST gateway using sinatra. This serves two functions:

  1. Easy access. Obviously, integrating directly with a TCP server kind of sucks in comparison to making a REST call.
  2. Security. I added a before block in the sinatra to verify HMAC codes computed using a shared secret. This prevents unauthorized parties from using this server. Wouldn’t want randos turning my lights off and on!

This little server listens for requests on port 8000.


I use nginx as the externally facing endpoint because I have a bunch internal webservers, and nginx makes it easier to manage all of them. It also adds the ability to address the webserver using a subdomain instead of a custom port. The config looks like this:

Notice the server is listening on port 81. My router opens port 80, and forwards it to port 81 on my home server. I do this because internal services I don’t want to expose to the outside world run on port 80, but I’d prefer to use port 80 from the outside world. The request chain looks something like:

Integrating with SmartThings

SmartThings has what they call “Virtual Devices“, which is a way to define a custom device in terms of its capabilities. A Virtual Device can, for example, declare that it’s a switch (giving it an on/off toggle), a switch level (giving it a dimmer slider), or a “color control” (giving it a hue/saturation control). One can also insert code that’s called whenever one of the controls changes value. Perfect!

I created the following Virtual Device based on the Phillips Hue device template that interacts with the REST server mentioned previously.

All one needs to do at this point is create a new device within SmartThings that makes use of this Virtual Device.


This ended up working out way better than I expected. From what I can tell, it behaves exactly like a first-class citizen within SmartThings. I’m super happy with how easy (and fun!) it was.


  1. [Oct-21, 2015] — I noticed there was a bug in the SmartThings Virtual Device signature generation method. It wasn’t properly padding signatures beginning with 0. I fixed this by using the built in byte[].encodeHex().