From 82ed7a42c095d254b11d390b631c051e3055bdee Mon Sep 17 00:00:00 2001 From: Max Bires Date: Fri, 27 May 2022 12:05:39 -0700 Subject: [PATCH] Add a README for Remote Key Provisioning. This document goes a little more in depth on the motivating factors and background mechanisms that occur with RKP, that are not appropriate for direct inclusion in the HAL docs in the .aidl files. Fixes: 234159998 Test: Readable Merged-In: I141fb098c536a5468b1113af64dcf6185ea7ae9f Change-Id: I141fb098c536a5468b1113af64dcf6185ea7ae9f --- security/keymint/RKP_README.md | 374 +++++++++++++++++++++++++++++++++ 1 file changed, 374 insertions(+) create mode 100644 security/keymint/RKP_README.md diff --git a/security/keymint/RKP_README.md b/security/keymint/RKP_README.md new file mode 100644 index 0000000000..89a2598f66 --- /dev/null +++ b/security/keymint/RKP_README.md @@ -0,0 +1,374 @@ +# 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) +