[security] Introducing signing support to MySensors

[Anticimex]

@axillent I know, and I dont even think you can get ATSHA204 anymore. In any case it does not matter as the main difference is that ATSHA204A also allows users to do SHA256 calculations, something we don't need so we are compatible with both variants.

[Anticimex]

I should add that I have used ATSHA204A in all my testing (it is the only variant I have) so we should be covered on that part.

[gigi]

@Anticimex very nice work.

[kalle]

Wow, great work!

[therik]

Awesome, when I order new boards I will include this feature. One question, does your implementation require the use of both SDA and SCL pins or just SDA? I think I prefer the larger SOT-23 package that only has SDA pin. I guess simply, one-wire or I2C?

Thanks.

[Anticimex]

@therik then you're in luck :) only single wire option is supported. I decided to do this because I think it is the most attractive HW option.

[rvendrame]

@Anticimex , fantastic take. One question so far --- Is (or will be) possible to a gateway working with both signed and non-signed messages? I don't have OTA, and to re-work all my existing sensors it will demand a considerable effort.

Having security on further sensors is anyway amazing.

Thanks a lot!

[Anticimex]

@rvendrame Thanks.
Yes. You can mix everything. You can have some nodes which require signed messages and some who does not in the same network at the same time. This should also work though relays (sign messages can pass relay nodes that are incapable of signing).
The gateway will keep track on which nodes that require signing and which who don't, and do signing accordingly.
So you can leisurely introduce signing only on newly added nodes or only activate it for the existing nodes that you want it on. It should even be possible to keep old nodes without reflashing their firmware (although that is a usecase I have not tested).

[Anticimex]

EDIT: Moved the location of the SHA204 personalizer in the first post since it has moved in the tree.
Also, I have noticed that recent activity on development branch seem to have broken the signing support. I am investigating the issue.
Life on the development branch is 'hard'. I am sorry about the outage, but these things are to be expected when many developers work on the same code piece and are not fully in-sync with each others activities. I will post here a SHA to a known "good" state once I have sorted the issue. I encourage all to test this out once I do, so we can get more test-coverage on this quite big feature in the library. Once we have it stabilized, it might get included in a "v1.5" of the library and then live a happy stable life on 'master' :)

[Anticimex]

Now a patch is merged to 'development' for the issue. Please give it a go. I need help in testing. I have tested it successfully with the SecureActuator sketch and the ATSHA204 signing backend.

[Dirk_H]

@Anticimex Wow this is really good work AND explanations.

I'm thinking about one issue: Lets assume I use a remote as garage opener. Now on a bad day I loose this remote. Is it possible to block the commands of the remote. I.e. have a blacklist like thing? Is it even possible to (in theory with the used components and in practise with the current code) to identify each SHA device? I.e. does Bob know "the message comes from someone I turst" or does he know "the messages comes from allice (which I trust)".

Obviously I'd be unable to change the PSKs because I need to lock the PSK Memroy area... So I'd have to throw away all devices for this approach - right?

Thanks for your great work again.

[Anticimex]

@Dirk_H Thanks!
Well, the ATSHA itself does support this kind of thing. But it is difficult to make it generic enough to open up for other backends to share the same functionality. It IS possible to skip locking the data area and still have signing working though. But that obviously opens the possibility to physically attack the node and read or replace the key. However, if someone got physical access to your node, they already have physical access to your garage. So the door would already be open.
So if we assume you skipped locking the data section of the ATSHA204 you could bring down the node and revoke the PSK by writing a new one. That would invalidate any "rouge" nodes with the old PSK.
Personally, I do not think I will ever lock down the data section since it is anyway only by physical access to the device you can rewrite or read the PSK and with physical access to the node..well...you are pretty much owned then in any case.

[Dirk_H]

Ok thanks for clarification Anticimex. I meant if you have a remote to open your garage and leave it in your car which e.g. gets stolen. However it good that you pointed out that locking the device is no "must". I think in many applications it might be even contraproductive.

locking the device would only prevent the thief from copying but not from using - Which would have higher importanance in this case. Maybe this lokcing feature is generelly more used for dongle like applications.

[Anticimex]

@Dirk_H yes if you have a handheld device that moves about, I recommend locking it to prevent key tampering in case it is lost. That makes it a black box.
I do understand what you mean. My point is, you have a node in the garage that is the actuator. You do not want your actuator to be tampered to accept different keys (or having the firmware replaced entirely). So you need to protect you actuator in any case. Looking the chip data in the actuator then has little benefit since if someone had access to it, they could just flash a firmware that ignores signatures.
But your keyfob is "public" so it is the typical part you want to lock. If you loose it nobody would be able to exploit it to transmit valid signatures if you replace the key in your actuator. I will consider updating the post with a number of use cases and highlight when data locking is advisable and when it is not.

[Anticimex]

EDIT: I have added a "usecase" section outlining three typical usecases. In summary: keep data section unlocked on all nodes that you can guarantee will be tamper-free. Lock data area on all nodes that require (or do) signing and are located in a more "volatile" environment (where they can be stolen or tampered with).

[rvendrame]

@Anticimex, let me try to get a holistic view:

• I think commercial solutions like z-wave have even less security (as they are design to work across multiple vendors / solutions, so no pre-shared key). Have a look on this http://research.sensepost.com/cms/resources/conferences/2013/bh_zwave/Security Evaluation of Z-Wave_WP.pdf

• The use case discussed here (lost garage remote): If I remove the remote from my automation software, the remote will not be able to open my garage, right? What you are saying is that if someone finds the remote, by hacking it it should be possible to obtain the pre-shared key (but it involves a considerable effort , maybe even SMD de-soldering and so)... Correct?

• Ok, now that someone else have my pre-shared key (and assuming that I didn't change it): He/she needs to sniff into my MYS network, learn MYS protocols, monitor the traffic for some time, and with all that, it should be possible to emulate the GW telling the garage door to "open".

• I guess the same applies to my wi-fi network --- If I loose my iphone, someone can break it and retrieve my router wi-fi password, connect into it, and unlock the door via my door lock App, right?

Sorry if those questions sound too stupid --- I'm new to all of this things... But I think MYS is primarily targeting for home DIY users, so if we achieve the same level of a house alarm for example --- wouldn't that be enough for a first step?

[Anticimex]

@rvendrame In security, there are no stupid questions.
Regarding the lost keyfob case, if we assume that you use the ATSHA204 device (not the software emulation) and you have locked the device, the PSK is unobtainable. Even if the device is picked apart, there is no way to extract the PSK from the ATSHA204 device.
However, also assuming that you do not alter your PSK in the "receiving" end, you are still at risk, much in the same way as your car is at risk if you loose the keyfob to it.
Also, if you use the same PSK on other parts of your system (like your GW) an attacker could pick the stolen ATSHA device and put it in their own device to allow it to transmit signed messages to any node in your system until you revoke the PSK in your GW (and corresponding nodes).
This is unfortunate, but with the limited memory we have in the current design, it is unfeasible to have a GW (or a node) keep track on white and/or blacklisted devices. I have not been able to figure out a way of doing that with the ATSHA in a way that does not prevent a SW emulation of the feature that is acceptable enough from a security point of view.
Basically: Keep track on your transmitting nodes.
My ambition with the signing feature is to protect a network from external attackers. More sophisticated features are needed to also protect against attackers using your own devices against you. I would like to investigate the possibility of blacklisting further, but I only have so much time, and Arduinos only have so much memory so that is something either someone else need to investigate further (I really want to start focusing on sensors myself now) or something to be considered with more sophisticated hardware. The ATSHA204 can cover for this as well, but it will require more memory on the devices using it.

And yes, if your controller is accessible through internet or through wifi, an attacker will most likely come from that direction, and not directly on your nodes. But "MySensors" cannot cover security in those areas.

[rvendrame]

@Anticimex said:

Also, if you use the same PSK on other parts of your system (like your GW) an attacker could pick the stolen ATSHA device and put it in their own device to allow it to transmit signed messages to any node in your system until you revoke the PSK in your GW (and corresponding nodes).

But that requires SMD-desoldering right? I guy who knows SMD + network sniffing + MYS hacking certainly will target something much bigger than my house ;-)

[Anticimex]

@rvendrame Hehe, well, if they go for hacking you keyfob, they probably know enough to realize that desoldering is not necessary. Just tap into the programming interface of the Arduino and write your own sketch to it ;)
However, they DO need to figure out your network topology, understand the MySensors protocol and so on. But we have to assume that is the case since we are open source. So everybody knows (or has access to) everything. The essential security here is your PSK. Your PSK must never EVER fall into the wrong hands. Either by theft or by accidental reveal (publishing it here for instance).
As long as you keep your PSK private and make sure the nodes that has the PSK programmed in on way or another do not fall into the wrong hands, you should be OK.

[Anticimex]

@rvendrame Actually, I have been thinking and I think I have figured out a way to implement diversification in the signing protocol without revealing IDs over the air. I need some more time to think about it, and make an feasibility assessment on wether it is possible on the limited space available (perhaps to make it an optional feature).
I will post an update here if I figure it out (or if I deem it to much work or simply can't be bothered ;) )
Anyway, the end result (if I succeed) will make it possible to revoke nodes on an ID-basis in case a node/device is lost/compromised. So if someone steals your keyfob they will have to go through quite some trouble to be able to use the device to hijack into your network (if you update your network to not accept that particular keyfob anymore).
I'll think about it. But this weekend I have to be social, so don't expect the solution (if any) anytime soon.