1 page.title=Hardware-backed Keystore 2 @jd:body 3 4 <!-- 5 Copyright 2015 The Android Open Source Project 6 7 Licensed under the Apache License, Version 2.0 (the "License"); 8 you may not use this file except in compliance with the License. 9 You may obtain a copy of the License at 10 11 http://www.apache.org/licenses/LICENSE-2.0 12 13 Unless required by applicable law or agreed to in writing, software 14 distributed under the License is distributed on an "AS IS" BASIS, 15 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 16 See the License for the specific language governing permissions and 17 limitations under the License. 18 --> 19 <div id="qv-wrapper"> 20 <div id="qv"> 21 <h2>In this document</h2> 22 <ol id="auto-toc"> 23 </ol> 24 </div> 25 </div> 26 27 <p>The availability of a trusted execution environment in a system on a chip (SoC) 28 offers an opportunity for Android devices to provide hardware-backed, strong 29 security services to the Android OS, to platform services, and even to 30 third-party apps.</p> 31 32 <p>Keystore has been <a href="features.html">significantly enhanced</a> in 33 Android 6.0 with the addition of symmetric cryptographic primitives, AES and 34 HMAC, and the addition of an access control system for hardware-backed 35 keys. Access controls are specified during key generation and enforced for the 36 lifetime of the key. Keys can be restricted to be usable only after the user has 37 authenticated, and only for specified purposes or with specified cryptographic 38 parameters. For more information, please see the <a 39 href="implementer-ref.html">Implementer's Reference</a>.</p> 40 41 <p>Before Android 6.0, Android already had a simple, hardware-backed crypto 42 services API, provided by versions 0.2 and 0.3 of the Keymaster Hardware 43 Abstraction Layer (HAL). Keystore provided digital signing and verification 44 operations, plus generation and import of asymmetric signing key pairs. This is 45 already implemented on many devices, but there are many security goals that 46 cannot easily be achieved with only a signature API. Keystore in Android 6.0 47 extends the Keystore API to provide a broader range of capabilities.</p> 48 49 <h2 id=goals>Goals</h2> 50 51 <p>The goal of the Android 6.0 Keystore API and the underlying Keymaster 1.0 HAL 52 is to provide a basic but adequate set of cryptographic primitives to allow the 53 implementation of protocols using access-controlled, hardware-backed keys.</p> 54 55 <p>In addition to expanding the range of cryptographic primitives, Keystore in 56 Android 6.0 adds the following:</p> 57 58 <ul> 59 <li>A usage control scheme to allow key usage to be limited, to mitigate the risk 60 of security compromise due to misuse of keys 61 <li>An access control scheme to enable restriction of keys to specified users, 62 clients, and a defined time range 63 </ul> 64 65 <h2 id=architecture>Architecture</h2> 66 67 <p>The Keymaster HAL is an OEM-provided, dynamically-loadable library used by the 68 Keystore service to provide hardware-backed cryptographic services. HAL 69 implementations must not perform any sensitive operations in user space, or even 70 in kernel space. Sensitive operations are delegated to a secure processor 71 reached through some kernel interface. The resulting architecture looks 72 like the following:</p> 73 74 <div align="center"> 75 <img src="../images/access-to-keymaster.png" alt="Access to Keymaster" id="figure1" /> 76 </div> 77 <p class="img-caption"><strong>Figure 1.</strong> Access to Keymaster</p> 78 79 <p>Within an Android device, the "client" of the Keymaster HAL consists of 80 multiple layers (e.g. app, framework, Keystore daemon), but that can be ignored 81 for the purposes of this document. This means that the described Keymaster HAL 82 API is low-level, used by platform-internal components, and not exposed to app 83 developers. The higher-level API, for API level 23, is described on the <a 84 href="http://developer.android.com/reference/java/security/KeyStore.html">Android 85 Developer site</a>.</p> 86 87 <p>The purpose of the Keymaster HAL is not to implement the security-sensitive 88 algorithms but only to marshal and unmarshal requests to the secure world. The 89 wire format is implementation-defined.</p> 90 91 <h2 id=compatibility_with_previous_versions>Compatibility with previous versions</h2> 92 93 <p>The Keymaster v1.0 HAL is completely incompatible with the 94 previously-released HALs, e.g. Keymaster v0.2 and v0.3. To facilitate 95 interoperability on pre-Marshmallow devices that launched with the older 96 Keymaster HALs, Keystore provides an adapter that implements the 1.0 HAL with 97 calls to the existing hardware library. The result cannot provide the full range 98 of functionality in the 1.0 HAL. In particular, it will only support RSA and 99 ECDSA algorithms, and all of the key authorization enforcement will be performed 100 by the adapter, in the non-secure world.</p>
