Better security without the need of a cryptoprocessor: out-of-band authentication

[Anticimex]

@Eurbaix you are welcome to submit a pull request and we can have a look on your proposal.

[d00616]

@Eurbaix said in Better security without the need of a cryptoprocessor: out-of-band authentication:

A good alternative, and I'm surprised its not more popular in MySensor, is STM32F103, basically the same price as an arduino mini pro, no hardware encryption or secure memory but plenty of CPU and memory for software encryption, which should be OK as long as blacklisting a sensor is easy.

I think in the near future you can see more Sensors based on nRF5 (Cortex-M0/4 and integrated nRF24 compatible radio) or SAMD platform.

Both SAMD and STM32 platform are not ready for battery powered sensors. Sleep() is not implemented. I think SAMD and STM32 are no good choices for asymmetric cryptographic operations because they don't have a hardware random number generator. This requires additional hardware like the ATSHA204.

There are another interesting MCU, the TI CC1350. Arduino and MySensors support is missing at the moment, but this MCU is an dual band wireless controller.

Also, what about assymetric encryption, which would perfect here?

The nRF5 and CC1350 MCU are supporting AES-CCM in hardware. Other MCU with integrated radio seems to supporting AES-CCM or AES-CCM* In my opinion the symmetric encryption should be AES-CCM. Hashing should be based on AES-CCM.

The asymmetric encryption is not so easy to answer. Hardware support is rare. The upcoming MCU nRF52840 has a CryptoCell 310 hardware included. This hardware supports RSA and some ECC algorithm. In my opinion we should use ECC to keep the RAM requirement low.

For ECC there are some implementations like micro-ecc or curve25519-donna. I think the curve should be selected carefully.

[Anticimex]

@Eurbaix regarding asymmetric encryption, do you have something special in mind?
And I also have a few questions on your original proposal;

GW generate random PSK for every new node? How? What prevents the GW from accepting "rogue" nodes and adding them? Inclusion mode?

After the node has authenticated with the GW and received a communication token, is it not possible for a different node to use that token (since the PSK can be assumed to be known as it is distributed OTA from the GW, right?) during the time window he token is valid?

Your proposal is interesting but let's discuss it some more :)

Edit: I also find the term PSK a bit misleading if the GW calculate and distribute them (or I misunderstood you). PSK suggests it is pre-shared and hence not distributed OTA (and therefore not subject to eavesdropping).

[Sergio Rius]

I was from two days ago thinking on a system for making the same proposal, and arrived to the same conclusion.

@Anticimex I would recommend to take just one step in the direction we decide is correct. We are forgiving here the controller. The controller can calculate keys during the pairing process. Expecting the controllers to adapt to this would be presumptuous, but we have community handled controllers that could make this job if we sacrifice the "temporary" nature of the key/token for a while.

The key should be doing things compatible with the path we want to take.
All of this, thinking on not changing supported mcus platforms.

[Anticimex]

@Sergio-Rius I disagree. Controllers should NEVER be involved in the OTA security protocol. Then the OTA security is dependent on the controller, and we have a LOT of different controllers with various levels of support.

[Sergio Rius]

I'm very interested on this and I would want to contribute. I think I could be of some help if you tell me where to follow this subject ;)

[Anticimex]

@Sergio-Rius This thread is as good as anyone. But just leave the controller out of this.

Also, generically from a key distributing and encryption point of view, remember that we are dealing with a mesh network.
If we use node specific keys and encrypt the messages based on this key, any repeater-nodes have to be able to convey the messages as well.

[Sergio Rius]

@Anticimex you're probably right, but could a "security protocol and messaging" be defined. One that could be selectively implemented by controllers. I mean as a way of decoupling the key/nonce generator like a radius server.
You could implement a security node Independent from the radio or gateway chosen.

[Anticimex]

@Sergio-Rius I don't like it. I don't think any trust should be placed on the controllers. And how would it work if you have a multi-controller setup if one controller starts jabbering about things other controllers have no clue about?

[Sergio Rius]

@Anticimex I didn't mean it to be in the controller. I just talked about a key generator (and perhaps storage) node, that could be on the network and recognized by the gateway or/and the repeaters.
That should be a way of only implementing one time the most expensive components and decoupling for compatibility.

Of course all of this are ideas and all need time to studying.

[Anticimex]

@Sergio-Rius Oh, I just read your "selectively implemented by controllers" and thought that it should be up to the controller to use that.
Well, a dedicated key storage node is a thought, although as things are with and without security currently, the use of atmega328p based GW:s is coming to an end.
So we should not base future security functionality on basis that the GW is performance limited.
Me and @d00616 have been discussing a scheme where the GW generates a root key and distribute node specific keys. But there are many aspects to cover still, one such is handling of repeater nodes, and we are also constrained in time for how much time and effort we can spend on this at the moment. There is already a security scheme in place and although it is not perfect, it is adequate from a security point of view.
And personally, I currently simply don't have the time to spare to design and implement something newer right now. Suggestions and PR:s are always welcome. That's how I started out, others are welcome to do the same ;)
Although since then I have been elevated to some form of security responsible, so I will for that sake, place some pointers to what a new security scheme need to cover:

• Authentication
• Obfuscation
• Handle node-jumping (repeater nodes)
• Support resource limited nodes (not necessarily GW:s)
• Not transmit sensitive data OTA
• Be simple to use
• Be well documented
• Either support key protection or simple key revocation (preferably on a per-node basis)

That's all I can think of right now :)

[d00616]

@Anticimex said in Better security without the need of a cryptoprocessor: out-of-band authentication:

That's all I can think of right now

That's most complete.

Looking at the most modern MCU's (nRF51, nRF52, CC1350, EFR32BG13P733) with integrated radio mostly AES-CCM is available as hardware component. With AES-CCM encryption and authentication is available. AES-CCM is part of the BLE specification. I think this type of MCU's are the future and the time of 8 Bit MCUs with external radio modules are mostly over. The nRF5 MCU's are compatible with MySensors starting with release 2.2.

In my personal opinion AES-CCM is a good choice for battery powered nodes, but I think it's no problem to accept other security schemes.

[Anticimex]

@d00616 I agree. It is good to base any scheme of something that at least some HW can provide already. As such, I don't think the actual algorithms is the key issue though. It is the routing I mostly worry about. If we do full encryption and expect the mesh network to be working, key needs to be shared at least with the repeaters and the endpoint.
But it could be that we settle for leaving the message header unencrypted, but then we need to also handle padding of the payload (algorithm specific). Not a huge issue but something to consider.
Also, the payload sizes do have an effect on how to do this. But there are huge changes planned for version 3 of the library when it comes to protocol and payloads, so we also need to decide if the security layer sits close to the transport (RF specific) or higher up in the stack.
I for one, feel that what we primarily want to protect is the payload. But perhaps also certain parts of the header (like sensor type, etc). We should probably align this with other core team members looking into protocol changes, so that we separate routing parts in the header from "semi-payload" data (sensor type, etc).

[Sergio Rius]

@Anticimex With that scheme, what happens when the GW fails?
Once keys are generated should they be saved on nodes and locked like now?
Could it be possible for a malicious user to hang the GW and supplant it, or take control of a far part of the network?

I really like the idea of those new platforms. But I wonder what problems would arise if they're tired to an specific radio. I almost can't use the 2.4ghz band, for example.

[Anticimex]

@Sergio-Rius if the GW fails your hub is out and your controller isolated from the sensor network.
If you have a different use case, just present a proposal and ideally a PR. Just leave the controller out of the equation.
What would be different in this situation from your key storage node failing?

[Anticimex]

@Sergio-Rius regarding tied to a specific radio, I don't think I have ever suggested such a thing?
When it talk transport, it is still in the library context. Not the physical layer which I consider the radio to be.

[Sergio Rius]

I was only presenting possible situations for discussing.
I'm analyst (processes, reliability and security). Perhaps I'm too anal with my profession. Also I'm not too good expressing myself in English.
I'm so sorry if I'm a hassle.
I'm on holidays, I should better come back to work ;)

[Anticimex]

@Sergio-Rius you are not a hassle. And I am sure as a analyst you appreciate challenging and be challenged :)
I suggest if we should be a bit mot concrete, that you if you would like, describe your proposal or thoughts in the context of the bullet list I presented above.
For sure those bullets can all be addressed on various ways, but any security solution should adresse them all (and no, the proposal does not have to be fully documented from the start :))

[Eurbaix]

@Anticimex There are, in fact, three different keys in my proposal:

• A initialization PSK which should be stored in EEPROM, not the firmware, so it can be erased after the sensor registration. This PSK is common to all new sensors but only used once.
• A sensor PSK, which is sent over the air during the registration, encrypted with the initialization PSK.
• The sensor PSK is used to encrypt the token request and answer so another sensor cannot "read" the token since it doesn't have the same sensor PSK.
• The token is then used as an authentication and encryption key during the rest of the communication.

So here's a few more details about how it would work:

• When firmware is initially loaded on a new sensor, the initialization PSK is also loaded in the sensor EEPROM.
• The GW needs a "WPS" button (as modern routers have)
• Just before powering-up a sensor for the first time, press the GW's "WPS" button, the sensor will accept a connection with the initialization PSK for a few seconds (10-15s) and, along the normal sensor "registration", also send a new key to the sensor. The sensor will then erase the original PSK and replace it with this "sensor PSK", which is specific to this sensor. Since this original PSK is only used during the initial registration then deleted, it will only exist in the sensor memory while the sensor is in a secure location. In case this PSK is compromised, you only need to create a new key in the gateway, that will then be used when you load new sensors.
• This sensor PSK is then used to encrypt the token request since the GW knows each sensor key. The token is sent encrypted with this sensor PSK so another sensor cannot "read" this token. If the token is requested for a sensor-to-sensor communication, the GW will also send the token to the second sensor, encrypted with the second sensor PSK.
• For the rest of the communication (sensor to sensor, or sensor to GW), this token becomes the encryption key and authentication key.

This way, a common PSK is only used during the initialization and can only be used for initialization. This PSK is not accepted for regular communication with the GW (or other sensors). Also, a rogue sensor who have this initialization PSK can only request a "sensor PSK" when the WPS button is pressed. The chance of a rogue sensor sending a initialization request just after the WPS button has been pressed is minimal (except if you routinely initialized dozens of sensors everyday). If fact, the chances are low enough that the initialization PSK is even optional, in my opinion. After all, router use WPS without initialization PSK and this approach is considered reasonnably secure.

Last, if a "sensor PSK" is compromised, you only have to erase this entry in the sensor PSK table on the GW and the rest of your sensor network will be secure. If we are not using a cryptoprocessor to store the sensor PSK, the risk of the sensor PSK to be read in a stolen sensor exist, but the fact that sensor PSK are individuals make this risk easier to mitigate (as long as you notice that a sensor has been stolen of course).

[Anticimex]

@Eurbaix sounds good. Just a few comments.

• WPS is considered broken (but I have not read up on the details, I am not sure it is due to the theory of operation as you outlined)
• if all communications are encrypted, is it the full payload or partial? Which algorithm do you suggest? Currently, we are limited to a full payload length of 32 bytes. Some bytes of these are for routing info so that repeater nodes can relay the message. I suppose these repeaters will be unable to decrypt messages not ment for them so at least the routing bytes need to be sent in plain text. And the algorithm block size might then be an issue given the current limitations of the payload size.

All in all I think the suggestion is sound and much in line with what we have already been theorizing around in the core team.