[staticassertion]

Is a user necessary? I feel like one thing I'd use an enclave for is as a signing oracle for service to service communications.

Like I have service A and service B. A is going to talk to B, and has some secret that identifies it (maybe a private key for mTLS). I'd like for A to be able to talk to B without having access to that secret - so it would pass a message into the enclave, get a signed message out of it, and then proceed as normal.

Would that not be reasonable? Or I guess maybe I'd want to attest that the signing service is what I expect?

[jacobr1]

> Or I guess maybe I'd want to attest that the signing service is what I expect?

Exactly. If you have a threat-model where you want to limit access to your secrets from a limited code path, you need to attest that only specific, signed code is running within the enclave that can access the secrets. You might only need this to satisfy your own curiosity, but in practice it probably is something you need to prove to your internal security team, third-party auditor, or even direct to a customer.

[staticassertion]

Got it, ok. Yeah, I think that's reasonable, though I do also think that it's "extra". I would consider moving the key to the enclave without attestation to be a win, though I very much like the idea of having that level of authenticity as well.

Thanks for clearing that up.

[gendal]

I may not be fully understanding your scenario but it sounds like A needs to prove to B that it knows a secret but doesn't want to actually reveal/send the secret to B?

And the idea, therefore, is that A sends the secret to an enclave, which inspects the secret and, if correct, signs a message to say "I, the enclave, have verified that A does indeed know the secret". (Apologies if I've oversimplified or got this wrong).

But assuming the above is roughly correct then, without remote attestation, you have a problem, and it comes down to the question of who's running the verification code, I think.

If A is running the checker, why should B believe what it says? If A is running the code, they can just change it so that it signs the statement irrespective of whether it's true.

But If B is running the checker, then A will have just sent their secret to a service run by B, violating the requirement that A doesn't send the secret to B!

You could ask a third party to run it of course. But if you don't want to introduce that third party then this is where remote attestation comes in:

If A is running the checker in an enclave then RA allows B to verify that the "A knows the secret" message really did come from a codepath that has actually done the right thing. In this scenario, B is the "user" of the enclave from the perspective of reliance on Remote Attestation. (Aside: I know it's weird to think of an actor that doesn't interact with a system to be a user of it, so I'm probably using poor terminology when I say 'user'... it's more that, in this scenario, B is _relying_ on properties of the enclave such as its attestation)

And if B is running the checker then RA allows A to verify that it won't just turn around and reveal the secret to B.

[staticassertion]

Almost but not quite. A needs to prove to B that it's allowed to talk to B. So there's a signing service in an Enclave that A can access. It passes a message in, the enclave signs the message, and A sends the message to B.

The secret never leaves the enclave.

The goal here is that if an attacker can execute code within A's operating system that they can not exfiltrate the password. They might be able to get the enclave to sign on their behalf, but that's significantly better than an exposed secret - simply removing the attacker from the box would be sufficient to remediate, vs having to rotate the secret.

To mitigate impersonation, I suppose one could do a number of things involving a second key, but I think that this simple version demonstrates the value of having a signing oracle. This is actually not an atypical approach, just not using sgx - I know companies that keep their signing keys in separate processes, which are mutually seccomp'd such that they can only pipe messages to each other for signing purposes of apps before publishing. But in the sgx case you have a much stronger guarantee than just seccomp.

So to me the only problem that attestation solves here is if the attacker is somehow in the SGX enclave, but actually the much more likely scenario is that they aren't, and that they just ask the oracle to sign on their behalf - because B can't verify that A is the one asking to sign. Given that there is a single entity deploying both the software in the enclave and the service that interacts with it at least, that seems to be the case to me - like, in this scenario A, B, and the software in the enclave are all deployed by me, barring malicious action to interfere with that - but again, the most likely scenario is the attacker just owned the box and has a regular user on there.

But... also also, A can prevent impersonation via tricking the oracle by having another keypair shared between it and the enclave, and then it becomes a matter of protecting that memory from an attacker who can almost certainly scrape your memory - a hard thing to do.

So you end up with:

System 0, A: Key 0 System 0, Enclave: Key 1, Key 0 System 1, B: Key 1

Key 0 is used to 'auth' A to Enclave. Key 1 is used to auth A to B (via enclave oracle).

This is just my perspective on it.

[gendal]

That's helpful - thanks.

We don't presently support it in Conclave but SGX (which we use) does, I believe, support the idea of, in effect, packaging up a secret in a program and then encrypting it so it can only run in an enclave and hence keep the secret safe even when running on malicious hosts. I'd need to check but I suspect you're right that there are situations there where RA isn't required.

But to take your specific example (and maybe I'm still misunderstanding), does your scheme actually work in practice? Let's assume a simple model where the enclave runs on A's machine. So we can assume that requests to sign something come from A. This avoids us having to worry about A having to authenticate to the enclave, which just leads us to a circularity (how does A protect the key it uses for authentication, etc)?

And now we introduce the attacker, as in your scenario. As you say, eliminating the attacker removes their ongoing ability to interact with the enclave, since it expects to communicate only with locally running processes.

Except... if the attacker is on your box, they could simply take a copy of the enclave! And simply run it on their own machine. It's possible SGX contains the ability to lock an enclave to a specific CPU, in which case your scheme seems like it should work (to my untutored eye... I lead the Conclave team but am by no means an expert)... but I'm not actually sure it works that way. I'll look in to it.

[gendal]

Just checked... yeah... you can arrange so that an encrypted enclave can only run on a specific machine through careful use of SGX primitives. So I think your idea would probably work.