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

[Eurbaix]

@Anticimex You raise a good point regarding cyphertext size. I was thinking initially AES 128 would be a good choice but, if routing is in clear, the encrypted part of the packet is now reduced to 16 bytes. Something to look at... or may be 16 bytes is acceptable in the use case scenario of MySensors since not all information is necessarily sensitive.

Also, what are you referring to when you say "security personalizer"? I don't remember something with this name in the current security in MySensors?

P.S.: No rush for the document sharing. It will take a while anyway to come up with something that covers all the basics so a few days more or less won't make a real difference.

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[Anticimex]

@Eurbaix security personalization is the corner stone of the current signing solution :) you can't use it without personalization (except for the simplified security flag on the development branch).

[Eurbaix]

@Anticimex Well, it seems I missed something important in the current signing implementation. I'll have a second look.

[d00616]

I had similar ideas with two firmwares. One for initialization and one running MySensors.

If asymetric encryption is used, there is no need for an PSK for encryption. A key is calculated on both sides this can be used as a node key. A library supporting 8 bit and 32 bit MCU is micro-ecc. The problem is, the key must be compared on both sides to detect man in the middle attacks. This could be by synchronous LED blinking the hash of the calculated key and accepting the key on the gateway. This allows to integrate sensors without the need of the SecurityPersonalizer.

The 32 byte package size can be extended by an nRF24/ECB protocol extension. First packet is send without ACK and when a second packet is received the ACK payload reports if the first packet is received. Then we have 64 byte packages, compatible with classic hardware.

I'm unsure if routing encrypted packages is a good idea. An attacker can force the relay to route invalid packages. On the other side the attacker can block the complete channel by sending invalid data.

When the motivation of the new security scheme is to reduce the cost. There are nRF51822 boards starting at less than 2.50€ and nRF52832 boards for less than 4.50€.

Compared the Arduino Pro Mini + nRF24 solution with the nRF51 you get ~100 times more performance without the 2k RAM limit and a lot of Flash for 0.50€ more costs. The nRF51 has an better range compared with the nrf24, allows to implement better radio protocols or switching to BLE (actually without MySensors) and it's possible to connect RFM modules. I think in six month the pricing is eqal.

With an bootloader like mcuboot, which needs porting to MySensors the key can be protected by disallowing flashing unsigned firmware and disable debugging.

I think we shouldn't do compromises to support 8 bit MCUs.

[Sergio Rius]

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

There are, in fact, three different keys in my proposal

Here you're basically proposing a handshake system, but, what you talk about in the first section of points, are you referring to the GW or nodes?
I don't see the need of this first key. Anything that identifies the sensor, like it's name would serve the same purpose. Or I understand it wrong?

I think that if you have two networks/channels, as I have, sensors from the two networks would have different keys. Then if we wanted to add a new sensor to one of the networks, what prevents the GW1 to see a sensor from the second network (that may have connectivity issues, alias it may be asking for reconnection) as a new sensor?

[d00616]

For an mixed network with classic hardware and new hardware we can work with two address ranges. This is possible for nRF networks.

[Sergio Rius]

I see, but I always try to address security and script burning in general trying to arrive an scenario where programming knowledge isn't necessary. Like choosing from a web page the sensor I want, connecting it to the computer via usb and letting the page/program/whatever to do it all.

WPS was considered insecure because there where ways of wirelessly force the router to start the WPS procedure. Then added a password to the WPS process and now they're removing this support, whats I think it's an error.

[Sergio Rius]

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

As for OTA, the initialization I proposed never send a clear key OTA. The initialization PSK is written on the sensor when you flash it (so, wired comms) and not part of the firmware. I was thinking something along the line of:

Flash and run a small firmware (which has the initialization PSK hardcoded) that will just store the initialization PSK in the EEPROM.
Flash and run the full MySensor firmware (which doesn't have the PSK hardcoded), then run the initialization using the PSK in the EEPROM.
This way, the full MySensor firmware, that will be updated OTA, never have to include a PSK so it can be sent in clear. Also, since comms are now encrypted, OTA updates could be encrypted although I don't see a need to encrypt firmware updates since they do not contain sensitive information.

Why not having mysensors framework generating this key on the first start-up? If you're concerned about some-one getting through the algorithm, it could require some seed from the user that may be common for him. That's still compatible with my approach of a code generator if the generator asks for the seed.

Also, I don't understand why the repeaters are a problem. I don't see why they have to read all the messages, hence being deciphered. Why they don't just "relay" them?

[Anticimex]

@Sergio-Rius if the message is fully encrypted, how would the repeaters know how to route?

[Sergio Rius]

@Anticimex It may not be fully encrypted, but having description packets. Other protocols do like this.
I understand that the repeaters should only care for messages directly sent to them. Like a remote-button to a garage door node. Other messages should then be relayed to the network. I think that is more productive and increase performance.

Perhaps I miss a more complex scenario.

[Anticimex]

@Sergio-Rius no that is correct. But repeaters to understand if a message is directed for them or to be repeated, part of the message need to be in plain text and then we need to mind the crypto algorithm vs payload limitations.
There are many solutions to this, I am just saying that it has to be taken into account as well.
This is not an issue when encryption is handled by the physical layer (the radio itself) but then security is specific to the radio. And we don't want that.

[Eurbaix]

@d00616 asymetric encryption has some advantages but ECC is more computing intensive and asymetric encryption needs double decryption compared to single decryption with a symetric algorithm. I went with symetric because it's usually lighter and faster. Also, you would still need an initializing PSK to be able to differentiate a rogue sensor trying to register on your network vs. one of your own sensors. I don't thing a LED flashing a hash is really practical.

P.S.: I know that current cheap platforms, like NRF5, are much more powerful for not much more money, but we should also keep in mind that a lot of existing sensors have already been deployed. If we drop the Atmega328 support, anyone who want to benefit of the new security solution will have to throw away all its existing sensor and rebuild new ones. That's why I am a strong advocate of legacy support, I hate throwing away something that could still works just because there is something new and shinner available for the same price.

[Sergio Rius]

In fact I initially thought that more than one GW was possible in a MyS network. That messages where broadcast until a receiver ACK-ed for it.

@Anticimex I totally agree with you.
@Eurbaix Not only the hardware. Some people are doing a great job in NodeManager that could need re-adaptations (dunno).

I almost always work with async encryption. Well... I think a mixed, cascaded solution may be ideal on this scenario. Just use sha256 ciphered key, surrounded by more packets sync-ciphered and wrapped together.
I don't know if the hardware may support it, but could be like peeling an onion, if you understand. Only the GW knows the key and deciphers the message.

Payload is hardly inescapable.

[d00616]

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

P.S.: I know that current cheap platforms, like NRF5, are much more powerful for not much more money, but we should also keep in mind that a lot of existing sensors have already been deployed. If we drop the Atmega328 support, anyone who want to benefit of the new security solution will have to throw away all its existing sensor and rebuild new ones. That's why I am a strong advocate of legacy support, I hate throwing away something that could still works just because there is something new and shinner available for the same price.

I agree here. Atmega can be supported by switching back to the old scheme or by dropping features for this platform.

[Eurbaix]

@Sergio-Rius The reason for the initialization PSK is to be able to differentiate a sensor you flashed yourself from a sensor someone else flashed and try to register on your network.Of course, you could use the sensor name as the key, but it is a weak key and it would also require to store the new sensor name on the GW every time you want to initialize a new sensor. The initialization PSK being common to all nodes, you don't need to change anything on the GW when you build a new sensor.

You can't use an initial key generated by the GW because it would then need to be sent OTA to the sensor. Since the sensor do not have a PSK yet, this generated key would have to be send in clear. A rogue sensor listening at this moment could read this key and register instead of your own sensor. Although limiting the time during which a GW would accept a new sensor is limited by the "WPS" button, you have to think about a rogue sensor constantly listening and trying to register on your network until the GW is in "WPS" mode. (Note that I only refer to the basic concept of "WPS", which is limiting the time when a new sensor can register by using a button to initiate a 5-10 seconds window where the GW would accept a new sensor. The idea is simply that, since the initialization is using a common PSK which is a security risk if it is stored in easily accessible EEPROM, you want to limit as much as possible when this initialization PSK can be used, so only once during the initialization then it is deleted, and only if a button has been pressed on the GW within the last 5-10 seconds).

Also, when using several separate networks (i.e. different GW linked to the same controllers, but each GW talking to its own sensor network), by using a different initialization PSK on each gateway (and on the sensors that must register on a specific gateway), you make sure that a sensor can only register on the right gateway. Then, each GW has only the list of sensor PSK for his own network so there is no risk a sensor attempt to talk to a GW it is not supposed to (if that's a concern, like having a low security sensor network and a high security sensor network).

Also, I assume that GW are always in a secure location (cannot be stolen and read). If a GW can be compromised, we're opening an entirely new can of worms.

And regarding your last comment: I, also, think async encryption is a better solution on non-constrained network (i.e. when memory and computing power is not a big concern). But, in this context, if we want to keep legacy support, we better look for the most lightweight encryption algorithms. Also, anything using a 256 key cannot be used with the current protocol since a 256 key or token would take the entire packet, with no room left for the routing information. Since "out-of-band" needs to send new tokens regularly, we have to stay with 128 bits. Going with 256 keys would require to also modify the protocol and make the entire thing much more complex to implement

[Anticimex]

No matter what path we choose it is not dropping support per se. Worst case is that atmega users have to stick with current solution. Best case is a sw equivalent that might or might not leverage features of the atsha204a to accomplish the things needed by a new security infrastructure capable of fitting into legacy devices with reasonable space to spare.

[Sergio Rius]

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

Of course, you could use the sensor name as the key, but it is a weak key and it would also require to store the new sensor name on the GW every time you want to initialize a new sensor. The initialization PSK being common to all nodes, you don't need to change anything on the GW when you build a new sensor.

We are seeing this from a different point of view. I can't get the controller out of this equation (by my own limitations). I would like to start the pairing process from the controller, see whats available and only authorize what it was meant to. That's why I was thinking more on a "identification key"

If I understand well:
A key is hardcoded (to say) in the GW.
Then you must write that key in the new sensor (other network will have a different key).
The pairing process is initiated, so you turn on the sensor.
The GW acknowledges with the sensor and sends to him a new key that is saved and then starts authentication and encryption as normal.

As for sha256, anything below that is crackeable. And so, yes, were're not designing the next pentagon network here. But you must think if 256+ size messages will be the next logical step or a question of time.
By now, I totally agree with you.

I personally think atmega is so charismatic. ESP32 had to be the panacea, but I'm still waiting for it.

[Eurbaix]

@Anticimex Totally agree. The initial idea of my proposal was a security scheme compatible with existing sensors where a compromised sensor without an atsha204a can be easily blacklisted, this preferably with a minimal impact on the firmware size compared to the purely software solution already implemented. Also, it would provide obfuscation in addition to authentication, since I am not really comfortable with someone knowing if my house is empty and/or locked.

Since the existing software solution already have AES, and "out-of-band" with token generated from the GW doesn't require a random number generator on the sensors, I think the "out-of-band" security could be implemented without a significant increase in the firmware size. The only compromise would be a 16 bytes encrypted payload if we stay with AES 128. Personnaly, I would accept this compromise so people using more modern platform can take advantage of the HW AES and free up some memory for the more complex sensors. After all, MySensors don't normally send tons of data, so it would only mean sending 2-4 packets instead of 1-2. Also, since the firmware updates do not contain any key, they don't need to be encrypted and can use the full payload.

[Eurbaix]

@Sergio-Rius Yes, your description is exactly what I was thinking about (except in much less words than I used). One important step you missed is that the initial "hardcoded" key must be erased during the "pairing" process so it cannot be compromised if a sensor is stolen. That's why it has to be in EEPROM on an atmega (other platforms, like the NRF5, have no EEPROM but can erase a flash page, which would do the same thing).

As for 128 keys, yes they are crackeable. You only need all the computers of the Cambridge university (or was it Oxford?) during the entire weekend to crack a single cypher block. After all, banks still use AES 128 and, last time I checked, there was still some money left on my bank account ;-). Sure, at one point, 128 encryption will be brakeable in minutes (or nanoseconds if the Chinese are not lying about their prototype Qubits computer) but we're not there yet.

P.S.: I don't know about the charisma of the atmega, but when it comes to I/O per dollar, it's hard to beat. And since I started programming on a portable TRS-80 PC-3 (1.4KB of RAM!) I still think there's lot we can do with the "huge" memory of the atmega. As long as we're not trying to implement edge-computing in MySensors, the atmega still has a future as a very affordable IoT platform to play with.

[Anticimex]

Actually, the block size is 16 bytes also for 256 bit AES keys.