Merge "Summarize security HALs" into main am: 06717b5e am: d8fecae7 am: 6728e640

# Security-Related HALs

The `security/` subdirectory holds various security-related HALs.  (The final two sections of this

document also describe security-related HALs that are in other places under `hardware/interfaces/`.)

The most significant HAL is KeyMint (**`IKeyMintDevice`** in the

`hardware/interfaces/security/keymint/` directory), which allows access to cryptographic

functionality where the key material is restricted to a secure environment.  This functionality is

used by Android system services, and is also made available to apps via Android Keystore.

A KeyMint implementation (or an implementation of its predecessor, Keymaster) that runs in an

isolated execution environment (e.g. ARM TrustZone) is required for most Android devices; see [CDD

9.11](https://source.android.com/docs/compatibility/13/android-13-cdd#911_keys_and_credentials).

A device may optionally also support a second KeyMint instance, running in a dedicated secure

processor; this is known as StrongBox ([CDD

9.11.2](https://source.android.com/docs/compatibility/13/android-13-cdd#9112_strongbox)).

Two specific features of KeyMint are worth highlighting, as they have an impact on the other

security-related HALs:

- KeyMint supports keys that can only be used when the operation is authenticated by the user,

  either by their lock screen knowledge factor (LSKF, e.g. PIN or pattern) or by a strong biometric

  (e.g. fingerprint).

- KeyMint supports *attestation* of public keys: when an asymmetric keypair is created, the secure

  environment produces a chain of signed certificates:

  - starting from a trusted root certificate

  - terminating in a leaf certificate that holds the public key; this leaf certificate may also

    describe the state of the device and the policies attached to the key.

## Authentication Verification

User authentication must also take place in a secure environment (see the final section below), but

the results of that authentication are communicated to KeyMint via Android.  As such, the

authentication result (a *hardware auth token*) is signed with a per-boot shared HMAC key known only

to the secure components, so that it's authenticity can be verified.

If an authenticator, for example GateKeeper (described by the **`IGatekeeper`** HAL in

`hardware/interfaces/gatekeeper/`), is co-located in the same secure environment as KeyMint, it can

use a local, vendor-specific, method to communicate the shared HMAC key.

However, if the authenticator is in a different environment than the KeyMint instance then a local

communication mechanism may not be possible.  For example, a StrongBox KeyMint instance running in a

separate secure processor may not have a communication channel with a TEE on the main processor.

To allow for this, the **`ISharedSecret`** HAL (in `hardware/interfaces/security/sharedsecret`)

describes an N-party shared key agreement protocol for per-boot derivation of the shared HMAC key,

based on a pre-provisioned shared secret.  This HAL can be implemented by any security component

– whether KeyMint instance or authenticator – that needs access to the shared HMAC key.

User authentication operations are also timestamped, but a StrongBox KeyMint instance may not have

access to a secure time source that is aligned with the authenticator's time source.

To allow for this, the **`ISecureClock`** HAL (in `hardware/interfaces/secureclock`) describes a

challenge-based timestamp authentication protocol.  This HAL is optional; it need only be

implemented if there is a KeyMint instance without a secure source of time.

## Attestation Key Provisioning

As noted above, key generation may also generate an attestation certificate chain, which requires

that the secure environment have access to a signing key which in turn chains back to the Google

root.

Historically these signing keys were created by Google and provided to vendors for installation in

batches of devices (to prevent their use as unique device identifiers).  However, this mechanism had

significant disadvantages, as it required secure handling of key material and only allowed for

coarse-grained revocation.

The remote key provisioning HAL (**`IRemotelyProvisionedComponent`** in

`hardware/interfaces/security/rkp/`) provides a mechanism whereby signing certificates for

attestation can be retrieved at runtime from Google servers based on pre-registered device identity

information.  This mechanism is used to provision certificates for KeyMint's signing keys, but is

not restricted to that purpose; it can also be used in other scenarios where keys need to be

provisioned (for example, for [Widevine](https://developers.google.com/widevine/drm/overview)).

## Keymaster

The Keymaster HAL (**`IKeymasterDevice`** in `hardware/interfaces/keymaster/`) is the historical

ancestor of many of the HALs here (and may still be present on older devices).  Its functionality is

effectively the union of the following current HALs:

- **`IKeyMintDevice`**

- **`ISharedSecret`**

- **`ISecureClock`**

## Related Authentication HALs

Authentication of users needs to happen in a secure environment, using vendor-specific

functionality, and so involves the use of one of the following HALs (all of which are outside the

`security/` subdirectory).

- The **`IGatekeeper`** HAL (in `hardware/interfaces/gatekeeper/`) provides user authentication

  functionality based on the user's lock-screen knowledge factor (LSKF), including throttling

  behaviour to prevent attacks.  Authentication tokens produced by this HAL are consumed by KeyMint,

  validated using the shared HMAC key described above.

  - The optional **`IWeaver`** HAL (in `hardware/interfaces/weaver`) improves the security of LSKF

    authentication by converting the user's LSKF into a *synthetic password* via hashing and

    stretching. This is required to be implemented on a separate secure element, which prevents

    offline attacks on Gatekeeper storage. Note that Weaver does not directly interact with KeyMint;

    the synthetic password is fed into Gatekeeper in place of the plain user password, and then

    Gatekeeper interacts with KeyMint as normal.

- The **`IFingerprint`** and **`IFace`** HAL definitions (under `hardware/interfaces/biometrics/`)

  allow access to biometric authentication functionality that is implemented in a secure

  environment.  Authentication tokens produced by these HALs are consumed by KeyMint, validated

  using the shared HMAC key described above.

- The optional **`IConfirmationUI`** HAL (in `hardware/interfaces/confirmationui`) supports

  functionality where the user confirms that they have seen a specific message in a secure manner.

  Confirmation tokens produced by this HAL are consumed by KeyMint, validated using the shared HMAC

  key described above.