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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>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>

 <p>In Android 6.0, Keystore added <a href="/security/keystore/features.html">symmetric cryptographic
 primitives</a>, AES and HMAC, and 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, see the <a href="/security/keystore/tags">Authorization
 Tags</a> and <a href="/security/keystore/implementer-ref">Functions</a> pages.</p>

 <p>
 In Android 7.0, Keymaster 2 added support for key attestation and version binding.
 <a href="/security/keystore/attestation">Key attestation</a>
 provides public key certificates that contain a detailed
 description of the key and its access controls, to make the key's existence in
 secure hardware and its configuration remotely verifiable.
 </p>
 <p>
 <a href="/security/keystore/version-binding">Version binding</a>
 binds keys to operating system and patch
 level version. This ensures that an attacker who discovers a weakness in an old
 version of system or TEE software cannot roll a device back to the vulnerable
 version and use keys created with the newer version. In addition, when a key
 with a given version and patch level is used on a device that has been upgraded
 to a newer version or patch level, the key is upgraded before it can be used,
 and the previous version of the key invalidated. As the device is upgraded, the
 keys "ratchet" forward along with the device, but any reversion of the device to
 a previous release causes the keys to be unusable.
 </p>

 <p>
 In Android 8.0, Keymaster 3 transitioned from the old-style C-structure Hardware
 Abstraction Layer (HAL) to the C++ HAL interface generated from a definition
 in the new Hardware Interface Definition Language (HIDL). As part of the change,
 many of the argument types changed, though types and methods have a one-to-one
 correspondence with the old types and the HAL struct methods. See the
 <a href="/security/keystore/implementer-ref">Functions</a> page for more details.
 </p>
 <p>
 In addition to this interface revision, Android 8.0 extends Keymaster 2's
 attestation feature to support
 <a href="/security/keystore/attestation#id-attestation">ID attestation</a>.
 ID attestation provides a limited and optional mechanism for strongly attesting
 to hardware identifiers, such as device serial number, product name, and phone
 ID (IMEI / MEID). To implement this addition, change the ASN.1 attestation
 schema to add ID attestation. Keymaster implementations need to find some secure
 way to retrieve the relevant data items, as well as to define a mechanism for
 securely and permanently disabling the feature.
 </p>

 <h2 id="goals">Goals</h2>

 <p>The Android Keystore API and the underlying Keymaster HAL
 provides 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>
   <li>An access control scheme to enable restriction of keys to specified users,
 clients, and a defined time range</li>
 </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. To keep
 things secure, HAL implementations don't 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 this:</p>

 <div align="center">
   <img src="/security/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 1 HAL is completely incompatible with the
 previously-released HALs, e.g. Keymaster 0.2 and 0.3. To facilitate
 interoperability on devices running Android 5.0 and earlier that launched with
 the older Keymaster HALs, Keystore provides an adapter that implements the
 Keymaster 1 HAL with calls to the existing hardware library. The result cannot
 provide the full range of functionality in the Keymaster 1 HAL. In particular,
 it only supports RSA and ECDSA algorithms, and all of the key authorization
 enforcement is performed by the adapter, in the non-secure world.</p>

 <p>
 Keymaster 2 further simplified the HAL interface by removing the <code>get_supported_*</code>
 methods and allowing the <code>finish()</code> method to accept input. This reduces the
 number of round trips to the TEE in cases where the input is available all at
 once, and simplifies implementation of AEAD decryption.
 </p>

 <p>In Android 8.0, Keymaster 3 transitioned from the old-style C-structure
 HAL to the C++ HAL interface generated from a definition in the new
 Hardware Interface Definition Language (HIDL). A new-style HAL
 implementation is created by subclassing the generated
 <code>IKeymasterDevice</code> class and implementing the pure virtual
 methods. As part of the change, many of the argument types have changed,
 though types and methods have a one-to-one correspondence with the old
 types and the HAL struct methods.</p>


 <h3 id="hidl-overview">HIDL overview</h3>

 <p>
 The Hardware Interface Definition Language (HIDL) provides an implementation
 language-independent mechanism for specifying hardware interfaces. The HIDL
 tooling currently supports generation of C++ and Java interfaces. It's expected
 that most Trusted Execution Environment (TEE) implementers will find the C++
 tooling more convenient, so this document discusses only the C++ representation.
 </p>
 <p>
 HIDL interfaces consist of a set of methods, expressed as:
 </p>
 <pre class="devsite-click-to-copy">
 	methodName(<var>INPUT ARGUMENTS</var>) generates (<var>RESULT ARGUMENTS</var>);
 </pre>
 <p>
 There are various pre-defined types, and HALs can define new enumerated and
 structure types. For more details on HIDL, see the <a
      href="/reference/">Reference section</a>.
 </p>
 <p>
 An example method from the Keymaster 3 <code>IKeymasterDevice.hal</code> is:
 </p>


 <pre class="devsite-click-to-copy">generateKey(vec&lt;KeyParameter&gt; keyParams)
         generates(ErrorCode error, vec&lt;uint8_t&gt; keyBlob,
                   KeyCharacteristics keyCharacteristics);</pre>
 <p>
 This is the equivalent of the following from the keymaster2 HAL.:
 </p>

 <pre class="devsite-click-to-copy">keymaster_error_t (*generate_key)(
         const struct keymaster2_device* dev,
         const keymaster_key_param_set_t* params,
         keymaster_key_blob_t* key_blob,
         keymaster_key_characteristics_t* characteristics);
 </pre>
 <p>
 In the HIDL version, the <code>dev</code> argument is removed, because it's
 implicit. The <code>params</code> argument is no longer a struct containing a
 pointer referencing an array of <code>key_parameter_t</code> objects, but a
 <code>vec</code> (vector) containing <code>KeyParameter</code> objects. The
 return values are listed in the "<code>generates</code>" clause, including a
 vector of <code>uint8_t</code> values for the key blob.
 </p>
 <p>
 The C++ virtual method generated by the HIDL compiler is:
 </p>

 <pre class="devsite-click-to-copy">
 Return&lt;void&gt; generateKey(const hidl_vec&lt;KeyParameter&gt;& keyParams,
                          generateKey_cb _hidl_cb) override;
 </pre>
 <p>
 Where <code>generate_cb</code> is a function pointer defined as:
 </p>

 <pre class="devsite-click-to-copy">std::function&lt;void(ErrorCode error, const hidl_vec&lt;uint8_t&gt;& keyBlob,
                    const KeyCharacteristics& keyCharacteristics)&gt;
 </pre>
 <p>
 That is, <code>generate_cb</code> is a function that takes the return values
 listed in the generate clause. The HAL implementation class overrides this
 <code>generateKey</code> method and calls the <code>generate_cb</code> function
 pointer to return the result of the operation to the caller. Note the function
 pointer call is <strong>synchronous</strong>. The caller calls
 <code>generateKey</code> and <code>generateKey</code> calls the supplied
 function pointer, which executes to completion, returning control to the
 <code>generateKey</code> implementation, which then returns to the caller.
 </p>
 <p>
 For a detailed example, see the default implementation in
 <code>hardware/interfaces/keymaster/3.0/default/KeymasterDevice.cpp</code>.
 The default implementation provides backward compatibility for devices with
 old-style keymaster0, keymaster1, or keymaster2 HALS.</p>


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 </html>
	<html devsite>
	<head>
	<title>Hardware-backed Keystore</title>
	<meta name="project_path" value="/_project.yaml" />
	<meta name="book_path" value="/_book.yaml" />
	</head>
	<body>
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	Copyright 2017 The Android Open Source Project

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->



	<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>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>

	<p>In Android 6.0, Keystore added <a href="/security/keystore/features.html">symmetric cryptographic
	primitives</a>, AES and HMAC, and 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, see the <a href="/security/keystore/tags">Authorization
	Tags</a> and <a href="/security/keystore/implementer-ref">Functions</a> pages.</p>

	<p>
	In Android 7.0, Keymaster 2 added support for key attestation and version binding.
	<a href="/security/keystore/attestation">Key attestation</a>
	provides public key certificates that contain a detailed
	description of the key and its access controls, to make the key's existence in
	secure hardware and its configuration remotely verifiable.
	</p>
	<p>
	<a href="/security/keystore/version-binding">Version binding</a>
	binds keys to operating system and patch
	level version. This ensures that an attacker who discovers a weakness in an old
	version of system or TEE software cannot roll a device back to the vulnerable
	version and use keys created with the newer version. In addition, when a key
	with a given version and patch level is used on a device that has been upgraded
	to a newer version or patch level, the key is upgraded before it can be used,
	and the previous version of the key invalidated. As the device is upgraded, the
	keys "ratchet" forward along with the device, but any reversion of the device to
	a previous release causes the keys to be unusable.
	</p>

	<p>
	In Android 8.0, Keymaster 3 transitioned from the old-style C-structure Hardware
	Abstraction Layer (HAL) to the C++ HAL interface generated from a definition
	in the new Hardware Interface Definition Language (HIDL). As part of the change,
	many of the argument types changed, though types and methods have a one-to-one
	correspondence with the old types and the HAL struct methods. See the
	<a href="/security/keystore/implementer-ref">Functions</a> page for more details.
	</p>
	<p>
	In addition to this interface revision, Android 8.0 extends Keymaster 2's
	attestation feature to support
	<a href="/security/keystore/attestation#id-attestation">ID attestation</a>.
	ID attestation provides a limited and optional mechanism for strongly attesting
	to hardware identifiers, such as device serial number, product name, and phone
	ID (IMEI / MEID). To implement this addition, change the ASN.1 attestation
	schema to add ID attestation. Keymaster implementations need to find some secure
	way to retrieve the relevant data items, as well as to define a mechanism for
	securely and permanently disabling the feature.
	</p>

	<h2 id="goals">Goals</h2>

	<p>The Android Keystore API and the underlying Keymaster HAL
	provides 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>
	<li>An access control scheme to enable restriction of keys to specified users,
	clients, and a defined time range</li>
	</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. To keep
	things secure, HAL implementations don't 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 this:</p>

	<div align="center">
	<img src="/security/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 1 HAL is completely incompatible with the
	previously-released HALs, e.g. Keymaster 0.2 and 0.3. To facilitate
	interoperability on devices running Android 5.0 and earlier that launched with
	the older Keymaster HALs, Keystore provides an adapter that implements the
	Keymaster 1 HAL with calls to the existing hardware library. The result cannot
	provide the full range of functionality in the Keymaster 1 HAL. In particular,
	it only supports RSA and ECDSA algorithms, and all of the key authorization
	enforcement is performed by the adapter, in the non-secure world.</p>

	<p>
	Keymaster 2 further simplified the HAL interface by removing the <code>get_supported_*</code>
	methods and allowing the <code>finish()</code> method to accept input. This reduces the
	number of round trips to the TEE in cases where the input is available all at
	once, and simplifies implementation of AEAD decryption.
	</p>

	<p>In Android 8.0, Keymaster 3 transitioned from the old-style C-structure
	HAL to the C++ HAL interface generated from a definition in the new
	Hardware Interface Definition Language (HIDL). A new-style HAL
	implementation is created by subclassing the generated
	<code>IKeymasterDevice</code> class and implementing the pure virtual
	methods. As part of the change, many of the argument types have changed,
	though types and methods have a one-to-one correspondence with the old
	types and the HAL struct methods.</p>


	<h3 id="hidl-overview">HIDL overview</h3>

	<p>
	The Hardware Interface Definition Language (HIDL) provides an implementation
	language-independent mechanism for specifying hardware interfaces. The HIDL
	tooling currently supports generation of C++ and Java interfaces. It's expected
	that most Trusted Execution Environment (TEE) implementers will find the C++
	tooling more convenient, so this document discusses only the C++ representation.
	</p>
	<p>
	HIDL interfaces consist of a set of methods, expressed as:
	</p>
	<pre class="devsite-click-to-copy">
	methodName(<var>INPUT ARGUMENTS</var>) generates (<var>RESULT ARGUMENTS</var>);
	</pre>
	<p>
	There are various pre-defined types, and HALs can define new enumerated and
	structure types. For more details on HIDL, see the <a
	href="/reference/">Reference section</a>.
	</p>
	<p>
	An example method from the Keymaster 3 <code>IKeymasterDevice.hal</code> is:
	</p>


	<pre class="devsite-click-to-copy">generateKey(vec<KeyParameter> keyParams)
	generates(ErrorCode error, vec<uint8_t> keyBlob,
	KeyCharacteristics keyCharacteristics);</pre>
	<p>
	This is the equivalent of the following from the keymaster2 HAL.:
	</p>

	<pre class="devsite-click-to-copy">keymaster_error_t (*generate_key)(
	const struct keymaster2_device* dev,
	const keymaster_key_param_set_t* params,
	keymaster_key_blob_t* key_blob,
	keymaster_key_characteristics_t* characteristics);
	</pre>
	<p>
	In the HIDL version, the <code>dev</code> argument is removed, because it's
	implicit. The <code>params</code> argument is no longer a struct containing a
	pointer referencing an array of <code>key_parameter_t</code> objects, but a
	<code>vec</code> (vector) containing <code>KeyParameter</code> objects. The
	return values are listed in the "<code>generates</code>" clause, including a
	vector of <code>uint8_t</code> values for the key blob.
	</p>
	<p>
	The C++ virtual method generated by the HIDL compiler is:
	</p>

	<pre class="devsite-click-to-copy">
	Return<void> generateKey(const hidl_vec<KeyParameter>& keyParams,
	generateKey_cb _hidl_cb) override;
	</pre>
	<p>
	Where <code>generate_cb</code> is a function pointer defined as:
	</p>

	<pre class="devsite-click-to-copy">std::function<void(ErrorCode error, const hidl_vec<uint8_t>& keyBlob,
	const KeyCharacteristics& keyCharacteristics)>
	</pre>
	<p>
	That is, <code>generate_cb</code> is a function that takes the return values
	listed in the generate clause. The HAL implementation class overrides this
	<code>generateKey</code> method and calls the <code>generate_cb</code> function
	pointer to return the result of the operation to the caller. Note the function
	pointer call is <strong>synchronous</strong>. The caller calls
	<code>generateKey</code> and <code>generateKey</code> calls the supplied
	function pointer, which executes to completion, returning control to the
	<code>generateKey</code> implementation, which then returns to the caller.
	</p>
	<p>
	For a detailed example, see the default implementation in
	<code>hardware/interfaces/keymaster/3.0/default/KeymasterDevice.cpp</code>.
	The default implementation provides backward compatibility for devices with
	old-style keymaster0, keymaster1, or keymaster2 HALS.</p>



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