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+# Remote Provisioning HAL
+
+## Objective
+
+Design a HAL to support over-the-air provisioning of certificates for asymmetric
+keys. The HAL must interact effectively with Keystore (and other daemons) and
+protect device privacy and security.
+
+Note that this API is designed for KeyMint, but with the intention that it
+should be usable for other HALs that require certificate provisioning.
+Throughout this document we'll refer to the Keystore and KeyMint (formerly
+called Keymaster) components, but only for concreteness and convenience; those
+labels could be replaced with the names of any system and secure area
+components, respectively, that need certificates provisioned.
+
+## Key design decisions
+
+### General approach
+
+To more securely and reliably get keys and certificates to Android devices, we
+need to create a system where no party outside of the device's secure components
+is responsible for managing private keys. The strategy we've chosen is to
+deliver certificates over the air, using an asymmetric key pair created
+on-device in the factory as a root of trust to create an authenticated, secure
+channel. In this document we refer to this device-unique asymmetric key pair as
+Device Key (DK), its public half DK\_pub, its private half DK\_priv and a Device
+Key Certificate containing DK\_pub is denoted DKC.
+
+In order for the provisioning service to use DK (or a key authenticated by DK),
+it must know whether a given DK\_pub is known and trusted. To prove trust, we
+ask device OEMs to use one of two mechanisms:
+
+1.  (Preferred, recommended) The device OEM extracts DK\_pub from each device it
+    manufactures and uploads the public keys to a backend server.
+
+1.  The device OEM signs the DK\_pub to produce DKC and stores it on the device.
+    This has the advantage that they don't need to upload a DK\_pub for every
+    device immediately, but the disadvantage that they have to manage their
+    private signing keys, which means they have to have HSMs, configure and
+    secure them correctly, etc. Some backend providers may also require that the
+    OEM passes a factory security audit, and additionally promises to upload the
+    keys eventually as well.
+
+Note that in the full elaboration of this plan, DK\_pub is not the key used to
+establish a secure channel. Instead, DK\_pub is just the first public key in a
+chain of public keys which ends with the KeyMint public key, KM\_pub. All keys
+in the chain are device-unique and are joined in a certificate chain called the
+_Boot Certificate Chain_ (BCC), because in phases 2 and 3 of the remote
+provisioning project it is a chain of certificates corresponding to boot phases.
+We speak of the BCC even for phase 1, though in phase 1 it contains only a
+single self-signed DKC. This is described in more depth in the Phases section
+below.
+
+The BCC is authenticated by DK\_pub. To authenticate DK\_pub, we may have
+additional DKCs, from the SoC vendor, the device OEM, or both. Those are not
+part of the BCC but included as optional fields in the certificate request
+structure.
+
+The format of the the DK and BCC is specified within [Open Profile for DICE]
+(https://pigweed.googlesource.com/open-dice/+/HEAD/docs/specification.md).  To
+map phrases within this document to their equivalent terminology in the DICE
+specification, read the terms as follows: the DK corresponds to the UDS-derived
+key pair, DKC corresponds to the UDS certificate, and the BCC entries between
+DK\_pub and KM\_pub correspond to a chain of CDI certificates.
+
+Note: In addition to allowing 32 byte hash values for fields in the BCC payload,
+this spec additionally constrains some of the choices allowed in open-DICE.
+Specifically, these include which entries are required and which are optional in
+the BCC payload, and which algorithms are acceptable for use.
+
+### Phases
+
+RKP will be deployed in three phases, in terms of managing the root of trust
+binding between the device and the backend. To briefly describe them:
+
+* Phase 1: In phase 1 there is only one entry in the BCC; DK_pub and KM_pub are
+  the same key and the certificate is self-signed.
+* Phase 2: This is identical to phase 1, except it leverages the hardware root
+  of trust process described by DICE. Instead of trust being rooted in the TEE,
+  it is now rooted in the ROM by key material blown into fuses which are only
+  accessible to the ROM code.
+* Phase 3: This is identical to Phase 2, except the SoC vendor also does the
+  public key extraction or certification in their facilities, along with the OEM
+  doing it in the factory. This tightens up the "supply chain" and aims to make
+  key upload management more secure.
+
+### Privacy considerations
+
+Because DK and the DKCs are unique, immutable, unspoofable hardware-bound
+identifiers for the device, we must limit access to them to the absolute minimum
+possible. We do this in two ways:
+
+1.  We require KeyMint (which knows the BCC and either knows or at least has the
+ability to use KM\_priv) to refuse to ever divulge the BCC or additional
+signatures in plaintext. Instead, KeyMint requires the caller to provide an
+_Endpoint Encryption Key_ (EEK), with which it will encrypt the data before
+returning it. When provisioning production keys, the EEK must be signed by an
+approved authority whose public key is embedded in KeyMint. When certifying test
+keys, KeyMint will accept any EEK without checking the signature, but will
+encrypt and return a test BCC, rather than the real one.  The result is that
+only an entity in possession of an Trusted EEK (TEEK) private key can discover
+the plaintext of the production BCC.
+1.  Having thus limited access to the public keys to the trusted party only, we
+need to prevent the entity from abusing this unique device identifier.  The
+approach and mechanisms for doing that are beyond the scope of this document
+(they must be addressed in the server design), but generally involve taking care
+to ensure that we do not create any links between user IDs, IP addresses or
+issued certificates and the device pubkey.
+
+Although the details of the mechanisms for preventing the entity from abusing
+the BCC are, as stated, beyond the scope of this document, there is a subtle
+design decision here made specifically to enable abuse prevention. Specifically
+the `CertificateRequest` message sent to the server is (in
+[CDDL](https://tools.ietf.org/html/rfc8610)):
+
+```
+cddl
+CertificateRequest = [
+    DeviceInfo,
+    challenge : bstr,
+    ProtectedData,
+    MacedKeysToSign
+]
+```
+
+The public keys to be attested by the server are in `MacedKeysToSign`, which is
+a COSE\_Mac0 structure, MACed with a key that is found in `ProtectedData`. The
+MAC key is signed by DK\_pub.
+
+This structure allows the backend component that has access to EEK\_priv to
+decrypt `ProtectedData`, validate that the request is from an authorized device,
+check that the request is fresh and verify and extract the MAC key. That backend
+component never sees any data related to the keys to be signed, but can provide
+the MAC key to another backend component that can verify `MacedKeysToSign` and
+proceed to generate the certificates.
+
+In this way, we can partition the provisioning server into one component that
+knows the device identity, as represented by DK\_pub, but never sees the keys to
+be certified or certificates generated, and another component that sees the keys
+to be certified and certificates generated but does not know the device
+identity.
+
+### Key and cryptographic message formatting
+
+For simplicity of generation and parsing, compactness of wire representation,
+and flexibility and standardization, we've settled on using the CBOR Object
+Signing and Encryption (COSE) standard, defined in [RFC
+8152](https://tools.ietf.org/html/rfc8152). COSE provides compact and reasonably
+simple, yet easily-extensible, wire formats for:
+
+*   Keys,
+*   MACed messages,
+*   Signed messages, and
+*   Encrypted messages
+
+COSE enables easy layering of these message formats, such as using a COSE\_Sign
+structure to contain a COSE\_Key with a public key in it. We call this a
+"certificate".
+
+Due to the complexity of the standard, we'll spell out the COSE structures
+completely in this document and in the HAL and other documentation, so that
+although implementors will need to understand CBOR and the CBOR Data Definition
+Language ([CDDL, defined in RFC 8610](https://tools.ietf.org/html/rfc8610)),
+they shouldn't need to understand COSE.
+
+Note, however, that the certificate chains returned from the provisioning server
+are standard X.509 certificates.
+
+### Algorithm choices
+
+This document uses:
+
+*   ECDSA P-256 for attestation signing keys;
+*   Remote provisioning protocol signing keys:
+  *  Ed25519 / P-256
+*   ECDH keys:
+  *  X25519 / P-256
+*   AES-GCM for all encryption;
+*   SHA-256 for all message digesting;
+*   HMAC-SHA-256 for all MACing; and
+*   HKDF-SHA-256 for all key derivation.
+
+We believe that Curve25519 offers the best tradeoff in terms of security,
+efficiency and global trustworthiness, and that it is now sufficiently
+widely-used and widely-implemented to make it a practical choice.
+
+However, since Secure Elements (SE) do not currently offer support for curve
+25519, we are allowing implementations to instead make use of EC P-256 for
+signing and ECDH. To put it simply, the device unique key pair will be a P-256
+key pair for ECDSA instead of Ed25519, and the ProtectedData COSE\_Encrypt
+message will have its payload encrypted with P-256 ECDH key exchange instead of
+X25519.
+
+The CDDL in the rest of the document will use the '/' operator to show areas
+where either curve 25519 or P-256 may be used. Since there is no easy way to
+bind choices across different CDDL groups, it is important that the implementor
+stays consistent in which type is chosen. E.g. taking ES256 as the choice for
+algorithm implies the implementor should also choose the P256 public key group
+further down in the COSE structure.
+
+### Testability
+
+It's critical that the remote provisioning implementation be testable, to
+minimize the probability that broken devices are sold to end users. To support
+testing, the remote provisioning HAL methods take a `testMode` argument. Keys
+created in test mode are tagged to indicate this. The provisioning server will
+check for the test mode tag and issue test certificates that do not chain back
+to a trusted public key. In test mode, any EEK will be accepted, enabling
+testing tools to use EEKs for which they have the private key so they can
+validate the content of certificate requests. The BCC included in the
+`CertificateRequest` must contain freshly-generated keys, not the real BCC keys.
+
+Keystore (or similar) will need to be able to handle both testMode keys and
+production keys and keep them distinct, generating test certificate requests
+when asked with a test EEK and production certificate requests when asked with a
+production EEK. Likewise, the interface used to instruct Keystore to create keys
+will need to be able to specify whether test or production keys are desired.
+
+## Design
+
+### Certificate provisioning flow
+
+TODO(jbires): Replace this with a `.png` containing a sequence diagram.  The
+provisioning flow looks something like this:
+
+Provisioner -> Keystore: Prepare N keys
+Keystore -> KeyMint: generateKeyPair
+KeyMint -> KeyMint: Generate  key pair
+KeyMint --> Keystore: key\_blob,pubkey
+Keystore -> Keystore: Store key\_blob,pubkey
+Provisioner -> Server: Get TEEK
+Server --> Provisioner: TEEK
+Provisioner -> Keystore: genCertReq(N, TEEK)
+Keystore -> KeyMint: genCertReq(pubkeys, TEEK)
+KeyMint -> KeyMint: Sign pubkeys & encrypt BCC
+KeyMint --> Keystore: signature, encrypted BCC
+Keystore -> Keystore: Construct cert\_request
+Keystore --> Provisioner: cert\_request
+Provisioner --> Server: cert\_request
+Server -> Server: Validate cert\_request
+Server -> Server: Generate certificates
+Server --> Provisioner: certificates
+Provisioner -> Keystore: certificates
+Keystore -> Keystore: Store certificates
+
+The actors in the above diagram are:
+
+*   **Server** is the backend certificate provisioning server. It has access to
+    the uploaded device public keys and is responsible for providing encryption
+    keys, decrypting and validating requests, and generating certificates in
+    response to requests.
+*   **Provisioner** is an application that is responsible for communicating with
+    the server and all of the system components that require key certificates
+    from the server. It also implements the policy that defines how many key
+    pairs each client should keep in their pool.
+*   **Keystore** is the [Android keystore
+    daemon](https://developer.android.com/training/articles/keystore) (or, more
+    generally, whatever system component manages communications with a
+    particular secure aread component).
+*   **KeyMint** is the secure area component that manages cryptographic keys and
+    performs attestations (or perhaps some other secure area component).
+
+### `BCC`
+
+The _Boot Certificate Chain_ (BCC) is the chain of certificates that contains
+DK\_pub as well as other often device-unique certificates. The BCC is
+represented as a COSE\_Key containing DK\_pub followed by an array of
+COSE\_Sign1 "certificates" containing public keys and optional additional
+information, ordered from root to leaf, with each certificate signing the next.
+The first certificate in the array is signed by DK\_pub, the last certificate
+has the KeyMint (or whatever) signing key's public key, KM\_pub. In phase 1
+there is only one entry; DK\_pub and KM\_pub are the same key and the
+certificate is self-signed.
+
+Each COSE\_Sign1 certificate is a CBOR Web Token (CWT) as described in [RFC
+8392](https://tools.ietf.org/html/rfc8392) with additional fields as described
+in the Open Profile for DICE. Of these additional fields, only the
+_subjectPublicKey_ and _keyUsage_ fields are expected to be present for the
+KM\_pub entry (that is, the last entry) in a BCC, but all fields required by the
+Open Profile for DICE are expected for other entries (each of which corresponds
+to a particular firmware component or boot stage). The CWT fields _iss_ and
+_sub_ identify the issuer and subject of the certificate and are consistent
+along the BCC entries; the issuer of a given entry matches the subject of the
+previous entry.
+
+The BCC is designed to be constructed using the Open Profile for DICE. In this
+case the DK key pair is derived from the UDS as described by that profile and
+all BCC entries before the leaf are CBOR CDI certificates chained from DK\_pub.
+The KM key pair is not part of the derived DICE chain. It is generated (not
+derived) by the KeyMint module, certified by the last key in the DICE chain, and
+added as the leaf BCC entry. The key usage field in this leaf certificate must
+indicate the key is not used to sign certificates. If a UDS certificate is
+available on the device it should appear in the certificate request as the leaf
+of a DKCertChain in AdditionalDKSignatures (see
+[CertificateRequest](#certificaterequest)).
+
+The Open Profile for DICE allows for an arbitrary configuration descriptor. For
+BCC entries, this configuration descriptor is a CBOR map with the following
+optional fields. If no fields are relevant, an empty map should be encoded.
+Additional implementation-specific fields may be added using key values not in
+the range \[-70000, -70999\] (these are reserved for future additions here).
+
+```
+| Name              | Key    | Value type | Meaning                           |
+| ----------------- | ------ | ---------- | ----------------------------------|
+| Component name    | -70002 | tstr       | Name of firmware component / boot |
+:                   :        :            : stage                             :
+| Component version | -70003 | int        | Version of firmware component /   |
+:                   :        :            : boot stage                        :
+| Resettable        | -70004 | null       | If present, key changes on factory|
+:                   :        :            : reset                             :
+```
+
+Please see
+[ProtectedData.aidl](https://cs.android.com/android/platform/superproject/+/master:hardware/interfaces/security/keymint/aidl/android/hardware/security/keymint/ProtectedData.aidl)
+for a full CDDL definition of the BCC.
+
+### `CertificateRequest`
+
+The full CBOR message that will be sent to the server to request certificates
+is:
+
+```cddl
+CertificateRequest = [
+    DeviceInfo,
+    challenge : bstr,       // Provided by the server
+    ProtectedData,          // See ProtectedData.aidl
+    MacedKeysToSign         // See IRemotelyProvisionedComponent.aidl
+]
+
+DeviceInfo = [
+    VerifiedDeviceInfo,     // See DeviceInfo.aidl
+    UnverifiedDeviceInfo
+]
+
+// Unverified info is anything provided by the HLOS. Subject to change out of
+// step with the HAL.
+UnverifiedDeviceInfo = {
+    ? "fingerprint" : tstr,
+}
+
+```
+
+It will be the responsibility of Keystore and the Provisioner to construct the
+`CertificateRequest`. The HAL provides a method to generate the elements that
+need to be constructed on the secure side, which are the tag field of
+`MacedKeysToSign`, `VerifiedDeviceInfo`, and the ciphertext field of
+`ProtectedData`.
+
+### HAL
+
+The remote provisioning HAL provides a simple interface that can be implemented
+by multiple secure components that require remote provisioning. It would be
+slightly simpler to extend the KeyMint API, but that approach would only serve
+the needs of KeyMint, this is more general.
+
+NOTE the data structures defined in this HAL may look a little bloated and
+complex. This is because the COSE data structures are fully spelled-out; we
+could make it much more compact by not re-specifying the standardized elements
+and instead just referencing the standard, but it seems better to fully specify
+them. If the apparent complexity seems daunting, consider what the same would
+look like if traditional ASN.1 DER-based structures from X.509 and related
+standards were used and also fully elaborated.
+
+Please see the related HAL documentation directly in the source code at the
+following links:
+
+*   [IRemotelyProvisionedComponent
+    HAL](https://cs.android.com/android/platform/superproject/+/master:hardware/interfaces/security/keymint/aidl/android/hardware/security/keymint/IRemotelyProvisionedComponent.aidl)
+*   [ProtectedData](https://cs.android.com/android/platform/superproject/+/master:hardware/interfaces/security/keymint/aidl/android/hardware/security/keymint/ProtectedData.aidl)
+*   [MacedPublicKey](https://cs.android.com/android/platform/superproject/+/master:hardware/interfaces/security/keymint/aidl/android/hardware/security/keymint/MacedPublicKey.aidl)
+*   [RpcHardwareInfo](https://cs.android.com/android/platform/superproject/+/master:hardware/interfaces/security/keymint/aidl/android/hardware/security/keymint/RpcHardwareInfo.aidl)
+*   [DeviceInfo](https://cs.android.com/android/platform/superproject/+/master:hardware/interfaces/security/keymint/aidl/android/hardware/security/keymint/DeviceInfo.aidl)
+