platform_hardware_interfaces/security

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.

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).

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).

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.