platform_hardware_interfaces/security/README.md

# 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.