page.title=Hardware-backed Keystore
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    <h2>In this document</h2>
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<p>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 <a
href="https://developer.android.com/reference/android/security/keystore/KeyGenParameterSpec.html">android.security.keystore</a>.</p>

<p>Keystore has been <a href="features.html">significantly enhanced</a> 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 <a
href="implementer-ref.html">Implementer's Reference</a>.</p>

<p>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.</p>

<h2 id=goals>Goals</h2>

<p>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.</p>

<p>In addition to expanding the range of cryptographic primitives, Keystore in
Android 6.0 adds the following:</p>

<ul>
  <li>A usage control scheme to allow key usage to be limited, to mitigate the risk
of security compromise due to misuse of keys
  <li>An access control scheme to enable restriction of keys to specified users,
clients, and a defined time range
</ul>

<h2 id=architecture>Architecture</h2>

<p>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:</p>

<div align="center">
  <img src="../images/access-to-keymaster.png" alt="Access to Keymaster" id="figure1" />
</div>
<p class="img-caption"><strong>Figure 1.</strong> Access to Keymaster</p>

<p>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 <a
href="https://developer.android.com/reference/android/security/keystore/KeyGenParameterSpec.html">Android
Developer site</a>.</p>

<p>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.</p>

<h2 id=compatibility_with_previous_versions>Compatibility with previous versions</h2>

<p>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.</p>