oak

Oak is a software platform for building distributed systems providing externally verifiable (or falsifiable) claims about system behaviors in a transparent way. Oak provides building blocks that are used to build Enclave Applications, i.e. applications that are isolated from the host on which they run, and are remotely attestable. In this doc we focus on distributed systems composed of heterogeneous "nodes"; a node may be an Enclave Application running on a server (e.g. private data center, public cloud provider, etc.), a regular application running on a client device (e.g. phone, laptop), a regular HTTP or gRPC service, a storage system, etc. When two nodes talk to each other, either or both of them may be Enclave Applications. For each interaction between two nodes, we refer to them as "initiator" and "responder". We avoid the terms "client" and "server" for those roles, because in some cases the "initiator" node may actually be running on a "server" - all combinations are possible. An Enclave Application in an Oak distributed system can remotely attest itself to other nodes: this consists in providing evidence of its own software and hardware identity, bound to local cryptographic keys (stored in tamper-proof hardware, guaranteeing that keys cannot be exfiltrated or copied), ultimately signed by one or more software or hardware roots of trust available on the host of that node. The software identity of an Enclave Application consists of measurements (i.e. digests / hashes) of the executable binaries that contribute to it, as well as additional data (e.g. configuration). ## Root of Trust Oak focuses on VM-based [Trusted Execution Environments (TEEs)](https://en.wikipedia.org/wiki/Trusted_execution_environment) (e.g. AMD SEV-SNP, Intel TDX) as hardware root of trust for Enclave Applications. This allows an Enclave Application to provide a remote attestation report signed by hardware-bound keys in the TEE, which are not accessible by the service provider. These keys are ultimately rooted in the TEE manufacturer (e.g. AMD, Intel); this reduces (ideally, removes) the need to trust the service provider of that node in order to validate its evidence, by distributing trust across multiple non colluding actors. ## Encrypted Communication At startup, each Enclave Application generates a local key pair, and binds the public key to its own evidence. When a node intends to communicate with an Enclave Application (as either initiator or responder), an end-to-end encrypted channel is established between the two nodes. As part of the channel establishment, each node also provides its own evidence (if any) to the other node, and may use the evidence provided by the other node to establish its trustworthiness, before deciding whether to exchange data with it. This encrypted and attested channel provides: - Confidentiality: each node knows that data sent over it may only be read by the other node - Integrity: each node knows that data received over it must have been written by the other node - Authenticity: each node knows the identity of the binary in any attested node processing the data it sends over the channel. ## Split Architecture In order to minimize the attack surface and the Trusted Computing Base (TCB) of an Enclave Application, Oak encourages a split architecture: - Enclave Application: a binary built (reproducibly) from open source code, running inside the TEE. The Enclave Application has access to the decryption keys bound to the attestation evidence; it decrypts incoming requests from other nodes, processes them, encrypts responses and sends them back over the communication channel. This is the binary that is associated with externally verifiable claims. No other binary has access to incoming requests in plaintext. - Host Application: a binary that runs on the host (untrusted) server environment that acts as the front-end for the Enclave Application; it exposes a gRPC endpoint which is externally reachable by other nodes. This binary only ever handles encrypted data; its main job is to create and maintain an instance of the Enclave Application in the TEE, and forward encrypted data to and from the Enclave Application. It may also contain proprietary logic that is needed to interact with the control plane or other internal systems necessary to run on the service provider infrastructure. This architecture means the service provider would not be able to swap the Enclave Application binary with a different (potentially malicious) one without being detected. If the service provider were to introduce a backdoor in the Enclave Application, it would have to sign the binary and publish it in a verifiable log via the Transparent Release process, which would make the backdoor irreversibly visible to external observers. ## Sealed Computing A canonical use of Oak is to build privacy-preserving sealed computing applications. In a sealed computing application, a node (usually a client device) sends data to an enclave application (usually a server), which processes data without the service provider hosting the enclave application being able to see the inputs, outputs, or side effects of the computation. The following is a simplified architectural diagram. ## Operating System Oak supports two flavors of Operating Systems within the TEE, on which Enclave Applications may be deployed: - Oak Restricted Kernel: a very minimal custom written microkernel that only supports a single CPU and a single application. It places very severe restrictions on what the application can do by only exposing a very limited set of syscalls to the application. Oak Restricted Kernel applications are suitable for cases where review of the entire trusted code base inside the TEE is critical, and the limited features and reduced performance is an acceptable trade-off. Ideally it should be possible for a single expert human reviewer to review the code of the Oak Restricted Kernel in its entirety in a couple of weeks. - Oak Containers: The Enclave Application is provided as part of an OCI runtime bundle. This includes a full Linux kernel and a Linux distribution with a container runtime engine inside the TEE. Using the OCI runtime bundle as a compatibility layer provides a more familiar development experience and more flexibility. Oak Containers also supports multiple CPUs in the TEE and is therefore suitable for cases with high performance requirements. The drawback of this approach is that the size of the entire trusted code base running in the TEE is significantly larger, as it is not feasible for a single human reviewer to review the code for the entire Linux kernel and Linux distribution. The trust model for this approach relies on the security of the broader Linux ecosystem. The additional features and flexibility that the Linux kernel provides to the application also makes it more difficult to reason about the behavior of an application, which in turn makes the review process more complicated. ## Remote Attestation Remote Attestation is a process by which a TEE provides evidence to a remote party about its own state and the identity of the workload running inside the TEE (the Enclave Application). Oak uses a multi-stage protocol in order to measure the identity of each layer in the boot process (beyond just the bootloader) and cryptographically bind it together with the TEE evidence. The [Oak Stage 0 virtual firmware](stage0) is pre-loaded into the VM memory, and it is covered by the TEE attestation report. To cover the later boot stages, Oak relies on the [Device Identifier Composition Engine (DICE)](https://trustedcomputinggroup.org/work-groups/dice-architectures/) approach, where each boot stage is responsible for measuring the next stage. Oak augments the attestation report with this additional evidence so that the identity of all the components of the workload is covered (e.g. kernel and application) by the evidence. When a node connects to an Enclave Application, it first requests the attestation evidence and associated endorsements. The endorsements include a certificate chain from AMD to prove that the attestation report was signed by a legitimate AMD CPU and optionally signed statements from the developers of the various components running inside the enclave. The other node then verifies the evidence using these endorsements to make sure it matches its expectations. The client node should ensure that the enclave application is running in an up-to-date and correctly configured TEE (e.g. the CPU is running up to date versions of the microcode and SNP firmware and that debugging is disabled) and that the identity of the various components running inside the enclave matches expectations. Once the node is satisfied that the enclave application meets all its expectations, it uses a public key that is bound to the evidence to set up an encrypted channel with the enclave application. To be sure that the enclave application will handle data in accordance with expectations, external reviewers must review the code running in the TEE. Oak enables this by publishing provenance information of most Oak components. These act as a reverse index that allows a reviewer to look up the exact source code commit that was used to build each component given the measurement digest of the binary from the attestation evidence. All Oak components are reproducibly buildable (see Transparent Release below), which means that a reviewer can fetch the specific source code commit and rebuild the component to confirm that it produces the same exact measurement digest. This gives the reviewer confidence that the code version they are reviewing is the exact same code version that was loaded into the enclave during startup. ## Transparent Release Transparent Release is a set of formats and tools that allow developers to publish and consume claims about binary artifacts in a transparent and non-repudiable way. As part of the remote attestation process, a node obtains the identity of the Enclave Application in the form of one or more binary digests (e.g. usually at least one per each layer of the boot chain); it then needs to establish whether these measurements are trustworthy. As a first approximation, we could assume that the client knows exactly which digests to expect by hard-coding them, and can verify the actual values against the expected ones locally. This works, but it is very limiting with respect to the evolution of the server application: as soon as any of the Enclave Application binaries changes as part of a new release, the node will stop trusting the Enclave Application, and the two will not be able to interact any more. We could solve this by hard-coding in the node a public key used to sign the binary digests, instead of the digests themselves. This way, when the Enclave Application binaries change, the developer can sign them with the corresponding private key in their possession, and distribute these signatures alongside the remote attestation report to the node, which can then verify the validity of these signatures. This works, but it has the problem of allowing the Enclave Application to present different signatures to different nodes, which would be very hard for users to detect. The solution implemented is to sign not only the digests when a new release is created, but also to log this signature to an external append-only verifiable log. Oak relies on [Rekor](https://docs.sigstore.dev/logging/overview/) (by Sigstore) as the external verifiable log for this purpose. As part of the Transparent Release process the signature is logged to Rekor, and an inclusion proof (or inclusio