cobratbq

The comment is meant to separate the hardware from the protections possible in software. TKey provides a unique secret per device, that transforms into a unique secret per (device + program-binary + user-secret). So, protection against swapped device is possible, just not if you simply check if the device is genuine.

Programs are loaded every time the device is connected by transmitting it to the device. So, you could swap the program binary on the computer, and have it send a malicious binary. Again, a unique secret is assigned to a byte-exact program-binary, so this is detected in the process.

(edit: I'm assuming the device contains the original firmware. I'm referring to programs loaded onto the TKey once programmed. Just to check/avoid confusion.)

You're right. I misremembered; read up on a lot of things in last months. Doesn't really matter, because we're discussing a protocol anyways.

Yeah, sorry, I realized that later. I forgot I posted the comment already.

> No protocol exists completely separate from its implementation.

That's a fair point. I am well aware of this.

I'd love to respond to this, but your comment "... that explicitly provides no security guarantees when someone has physical access to it, .." is too abstract for me. I'll make a few guesses.

- Is the device hackable? AFAIK not at this moment. The firmware is minimal. It is a relatively new device, so maybe I am not fully informed.

- Is the device stealable/swappable? Yes. However, it isn't possible/easy to access the internal device-secret (UDS) therefore, swapping it out leads to different secret for the program, cascading into the identity, therefore authentication would fail. (Also, if you steal it, then it's gone. :-P)

- There are protections against opening it up. I'm not an expert on this, so I cannot reliably reproduce from memory the ways it is resistant to this. However, it already means your destroying hardware in the process.

I'm not sure if we're talking about the same things. I am not confident that I understand your comment well enough to confirm/reject, so I'm going to clarify for that reason.

> I understand it that the above quoted text represents the device's long term private key?

The device owns an internal, inaccessible long-term 32-byte key called UDS. (I called it "device secret" in other comments.) The device contains fairly minimal firmware, primarily there to initialize the device and accept the bytes that are the program-binary.

When a program-binary is sent, it is written to memory. Then a secret is derived for the program, called CDI. Then device-firmware passes over control to the firmware-binary by jumping to the entry-point (IIRC).

`CDI = blake2s(UDS, blake2s(program-binary), 32-byte-user-secret)`. (Note: this is done in device-firmware and only the CDI is exposed to the program.

> You can do this:

I think here we're on the right track, but I'm guessing you're suggesting an rough idea. So, yeah, from the CDI one can derive a key for identity.

So the current protocol describes using that deterministically generated identity for authentication, and generate random bytes for deriving an ephemeral keypair for the key-exchange.

The client only needs an ephemeral keypair so relies on proper randomness only. (And it keeps a record of the identity public key, to verify its proof.)

> Next problems are replay attack prevention and forward secrecy. The two-birds-with-one-stone for that is for each side to:

I think these are not possible. Assuming (for this discussion) that your client is honest, you start off every session with proper randomness in your (unpredictable) ephemeral key. So from the first message on, replays are not feasible. Forward secrecy should be covered given that the whole session key, as it is established, is based off fresh input from randomness.

This, of course, leaves open the discussion on long-running sessions that keep using the same key without rotation. This is not addressed.

> I don't know much about the TKey, but it looks like they have some kind of remote attestation protocol available? (https://github.com/tillitis/tkey-verification/tree/main/cmd/...). That's usually how you avoid TOFU.

There is a tool to verify if the device is genuine by mechanism of a signature. You're outlining most of the process. The question is whether avoiding TOFU is the goal, right? I'm thinking, with the physical device in your hands and during first use, it's quite reasonable to establish the identity for your 'program' + 'user-secret'.

> You now have a chain of trust from (1) the tillitis CA -> (3) the TKey device platform pubkey @ some specific firmware version -> (2) your custom software pubkey @ some specific version.

This does mean you make this a global + centralized effort, right? (Also, it creates a dependency.)

> (3) Somehow your custom software's pubkey gets locally certified by the platform's pubkey from (1). (not sure what this looks like w/ the TKey)

With the specific firmware version: this requires a (possibly centralized) certification-process if only for a keypair, or qualification effort (if any) for the program?

To conclude: I am not convinced yet that TOFU is necessarily a bad thing. However, I do appreciate some ability to authenticate over many uses / longer stretches of time. (Hence the key-exchange + authn.) It seems there is a trade-off here, TOFU can be eradicated but requires other properties/effort. OTOH, the program-specific secret makes for a very strict form of trust. I'll take your comments into consideration, but it seems whichever way one chooses, there is a trade-off to be made.

That's fine. It is indeed possible to perform a check for genuine hardware. (I'm not sure it qualifies as "attestation".) It does not protect you from malicious program-binaries and swapped devices.

> I think you're over-describing your use case, to the point that it's unclear what you're really saying. I read your "Introduction" section several times, and I don't understand if you're just saying "the use case is an authenticated key exchange" or something different. That makes it hard to judge the protocol.

Okay. I indeed failed to abstract away from the device properly. I'm guessing that the confusion comes from the fact that I had a specific device in mind, but failed to describe it.

> These two requirements are contradictory. How do you "authenticate" a server that has a different identity each time you interact with it?

Not necessarily. So how TKey works: the hardware contains a preprogrammed device secret. The hardware does not have storage. Upon each power-up it expects a program to be loaded. Upon loading that program, a secret is computed for that specific program: `blake2s(device-secret, program-binary, user-secret)` (user-secret is optional). The secret is generated deterministically, but unpredictable because we don't know the device-secret.

> Why not just use Diffie-Hellman? What else is this offering?

Given that there exists an "identity", which is the same every time the program loads, this identity can be used for authentication. The identity is different for every program, but after acquiring it once, a client application can perform a key-exchange that finishes with the signature proving the authenticity.

If 'device' or 'program-binary' or 'user-secret' changes, the secret changes. So if the secret is the same, you have quite strong guarantees that nobody screwed around with your device or program.

I will have a look. I checked quickly already, so if I understand the notation, I also leave out the last transaction. (2 messages vs 3 messages) Presumably because the authentication is one-sided. Will investigate further.

You're right. I wanted to abstract away from specific hardware and express that in the requirements. I definitely failed at that. See other comments for specifics and details on the device.

Yeah, thanks for reminding me. That is a nice suggestion.

You're right, mostly. I am not sure if TKey is officially considered an enclave. See more details here: <https://news.ycombinator.com/item?id=39834820>

I understand the "roll-your-own-crypto" comment. Note that I am taking the perspective of the protocol here, right? So, sure, vulnerabilities in the program are definitely a possibility. (See also other comments explaining more details about the device.) This is part of an experiment to see what can be accomplished with the TKey, which offers an interesting combo of security features.

Feel free to pour me an avalanche of missed attacks. I would be interested to know if your attacks are on the protocol level or in other ways. C programming risks are obviously there. RNG risks are obviously there. You are right that there are plenty of considerations in the program too. They are just not part of this post. (I also commented on the RNG remarks in other replies.)

To check: did you realize that you plug this device in your USB port, then send a program to it, then start using the device with that program loaded? (This is at run-time, every time, right?) Because the secret is generated for this specific combination, different programs will also have different secrets.

I get that you would want to authn the hardware itself. If that is your point, sure, you're right. However, that aims to address a slightly different problem, because then the certificate chain is tied to the hardware only. Note that part of the charm of the _identity_ generated in the program, is that the identity changes if only a single byte of program-binary is different. So it protects from malicious binaries too. (But not bugs in the program itself.)

Thanks, much appreciated. I'm not claiming to know everything, far from it. However, given this simple but interesting device (see other comments for details) I prefer to keep things simple. This is my attempt at simple-but-correct. :-)

I get that Verifpal is not perfect, doesn't do everything. I have considered switching. However, a significant part of proving the mechanism is having correct definitions for all of the needed primitives. If Verifpal tackles the right, correct definitions of the primitives, that is already a significant step, right? Considering the protocol is not complicated, or trying to be, one might try a less complex solution to start with. I am not planning to "gloss over" and there is a bigger pictures. But, feel free to make any missed dimensions concrete. (Note, you might want to check some other comments, as I explain a bit more about the device.)

See also my other comments. The device is tillitis TKey. There is a True RNG (source of entropy) but not recommended, i.e. not cryptographically-secure. However, together with Blake2s (in firmware) or another hash-function, you can at least approximate a source of reasonably secure randomness. Needs to be part of the your user program (to be loaded onto TKey), so you can make this as complicated as you like. You can indeed use the input-data as entropy/seed-bytes and hash it with other data and entropy from TRNG.

I haven't tested yet how fast the TRNG refreshes. Right now, I take 4 bytes (1 collection) of TRNG entropy to be hashed with a static byte-buffer.

The RNG would be used for (sk_device,pk_device). So 'identity' keypair for authn, 'device' (ephemeral) keypair for key-exchange.

Thanks for the input. See <https://news.ycombinator.com/item?id=39834820> for more details. The device is the TKey, so essentially only 32-byte secret value that is determined at program-load-time. (See more details in other comment.) The RNG is not really there. There is a true RNG (i.e. source of entropy) but not cryptographically-secure. However, you can use this and other data with the Blake2s (from firmware) or your own hash-functcion, to attempt to approximate randomness. So you essentially need to initialize it yourself and can go wild if you want to.

Thanks for the feedback. See also comment https://news.ycombinator.com/item?id=39834820 Note that this device is general purpose security device with no persistence. So the requirements were really specific for that reason.

The device I have in mind, primarily, is tillitis' TKey. The TKey does not have persistence, and offers a 32-byte secret value that is deterministically unpredictable (Blake2s) depending on hardware + program-binary + user-supplied-secret. Because of this, you will need TOFU, because you can only know its identity at first run, or its unique secret for that matter. (Of course, if you rely on the device+program, you would need to store the identity public key for subsequent connects.)

Do you still see an opening for a MitM if you assume TOFU works? There is authentication, just not on first connect. I'd be interested in your view on what I missed.

The exfiltration is of course a concern. Part of the prototype I'm doing, is looking into how much we can protect against this. (See also feedback I left to tillitis regarding access to the assigned secret, <https://github.com/tillitis/tillitis-key1/issues/186>.)

Concerning rotations, absolutely correct. Consider though that any change of program-binary or user-supplied secret or device brings its own secret. Note that because of the lack of persistence on the device, a client would store a data, even if only for use within the device.

Note that the secret is determined at every execution as the program-binary loads. The device, TKey, itself is a general RiscV processing unit with device-firmware that initializes a little bit of device and memory, takes the program sent to the device, then starts executing at the program-binary's entry-point.

The device I primarily had in mind is tillitis' TKey. Essentially a general purpose (slow) processing unit. The secret is 32-bytes long and given no storage, that's essentially all you work with. However, the secret is dependent on device + program-binary + 32-byte-user-secret.

A certificate is also a secret + certified public key, right? So, if you cannot store the certification on the TKey (no persistence) than you're left with the same construction. Right now, I'm skipping the complexity of certs (PKCS11, IIRC) etc. The identity being persistent is in order to authenticate the device.

1. Be thorough. Be sure you fully understand the issue, instead of jumping to conclusions.

2. Keep asking 'why'. Make sure you understand what the problem (or rather the requested feature) is, instead of being able to parrot what someone else says the problem is. (Even if the conclusion ends up being the same, you need the gained comprehension to determine the right solution.)

3. Separation of concerns. Keep practicing on separating orthogonal concerns that are all seemingly relevant to a problem. Evaluate each concern separately. Drop any of the concerns that are irrelevant in solving the solution. This can save you huge amounts of time, enough to make a deadline you would otherwise never make. (This will also help you to reduce the amount of added complexity.)

Updates on his situation (more recent on top):

- 2019-04-18: Ola Bini's statement, from arbitrary detention, statement discussed in an article. [10]

- 2019-04-16: Ola's parents are able to visit him in prison, Ola has been getting threats in prison. [11]

- 2019-04-16: Press conference organized for parent and lawyer to give updates concerning Ola's situation.

- 2019-04-16: Ola's detention is full of irregularities, as concluded by his lawyers. [12]

- 2019-04-15: Ola's parents on their way now to Quito, address press in Quito.

- 2019-04-15: Charges against Ola Bini are known, judge orders preventitive detention, Ola's lawyers file appeal.

- 2019-04-14: many news sites, communities and research group find out about Ola's detention. Opinions and statements get published in support. (See FreeOlaBini.org)

- 2019-04-12: Ola has finally been able to speak with lawyers, after being detained for 17 hours.

- 2019-04-12: Friends, colleagues start discovering about his detainment. The statement issued by his employer. First news reports. [13]

- 2019-04-11, 15:20h local time: Ola Bini arrested at Quito airport in Ecuador. (Martin Fowler [14])

- 2019-04-11: First signs of trouble, already identified by Ola himself. [15]

- 2019-04-10: Ola announcing his flight to Japan, which was already known 2 weeks prior. This trip will later be explained as "Ola fleeing Ecuador" after Ola has been detained.

[10] https://twitter.com/freeolabini/status/1118911539707248640 https://apnews.com/fdd20f1e9ea04eb18e11374647b59dd8 [11] https://twitter.com/freeolabini/status/1118211351971811328 https://twitter.com/aosita/status/1118228547213103107 [12] https://www.eff.org/deeplinks/2019/04/free-ola-bini [13] https://autonomia.digital/cad/2019/04/12/ola-bini-arrest.htm... https://gizmodo.com/ecuador-arrests-digital-privacy-activist... [14] https://twitter.com/martinfowler/status/1116520916383621121 [15] https://twitter.com/olabini/status/1116341908371062794

Please take any (unintended) violation of the guidelines as my personal mistake. This is one of my first times posting at HN.