Merge "Add a README for Remote Key Provisioning." into tm-dev am: 8c1586bc am: 66824101

# 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)