The availability of a trusted execution environment in a system on a chip (SoC) offers an opportunity for Android devices to provide hardware-backed, strong security services to the Android OS, to platform services, and even to third-party apps. Developers seeking the Android-specific extensions should go to android.security.keystore.
Keystore has been significantly enhanced in Android 6.0 with the addition of symmetric cryptographic primitives, AES and HMAC, and the addition of an access control system for hardware-backed keys. Access controls are specified during key generation and enforced for the lifetime of the key. Keys can be restricted to be usable only after the user has authenticated, and only for specified purposes or with specified cryptographic parameters. For more information, please see the Implementer's Reference.
Before Android 6.0, Android already had a simple, hardware-backed crypto services API, provided by versions 0.2 and 0.3 of the Keymaster Hardware Abstraction Layer (HAL). Keystore provided digital signing and verification operations, plus generation and import of asymmetric signing key pairs. This is already implemented on many devices, but there are many security goals that cannot easily be achieved with only a signature API. Keystore in Android 6.0 extends the Keystore API to provide a broader range of capabilities.
The goal of the Android 6.0 Keystore API and the underlying Keymaster 1.0 HAL is to provide a basic but adequate set of cryptographic primitives to allow the implementation of protocols using access-controlled, hardware-backed keys.
In addition to expanding the range of cryptographic primitives, Keystore in Android 6.0 adds the following:
- A usage control scheme to allow key usage to be limited, to mitigate the risk of security compromise due to misuse of keys
- An access control scheme to enable restriction of keys to specified users, clients, and a defined time range
The Keymaster HAL is an OEM-provided, dynamically-loadable library used by the Keystore service to provide hardware-backed cryptographic services. HAL implementations must not perform any sensitive operations in user space, or even in kernel space. Sensitive operations are delegated to a secure processor reached through some kernel interface. The resulting architecture looks like the following:
Within an Android device, the "client" of the Keymaster HAL consists of multiple layers (e.g. app, framework, Keystore daemon), but that can be ignored for the purposes of this document. This means that the described Keymaster HAL API is low-level, used by platform-internal components, and not exposed to app developers. The higher-level API, for API level 23, is described on the Android Developer site.
The purpose of the Keymaster HAL is not to implement the security-sensitive algorithms but only to marshal and unmarshal requests to the secure world. The wire format is implementation-defined.
Compatibility with previous versions
The Keymaster v1.0 HAL is completely incompatible with the previously-released HALs, e.g. Keymaster v0.2 and v0.3. To facilitate interoperability on pre-Marshmallow devices that launched with the older Keymaster HALs, Keystore provides an adapter that implements the 1.0 HAL with calls to the existing hardware library. The result cannot provide the full range of functionality in the 1.0 HAL. In particular, it will only support RSA and ECDSA algorithms, and all of the key authorization enforcement will be performed by the adapter, in the non-secure world.