1 page.title=Authentication 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 <h2 id=overview>Overview</h2> 28 29 <p>Android 6.0 introduces the concept of user-authentication-gated cryptographic 30 keys. To achieve this, two key components need to work together. 31 First is the cryptographic key storage and service provider, which stores 32 cryptographic keys and provides standard crypto routines on top of them. Second 33 is any number of user authenticators that may attest to the user's presence 34 and/or successful authentication.</p> 35 36 <p>The cryptographic key storage in Android is provided by the keystore service and Keymaster. 37 (Also see information about 38 the <a href="https://developer.android.com/training/articles/keystore.html">Android Keystore system</a>, 39 at the framework level, which is backed by the keystore service.) For Android 6.0, 40 the two supported authentication components are Gatekeeper (for 41 PIN/pattern/password authentication) and Fingerprint (for fingerprint 42 authentication). These components communicate their authentication 43 state with the keystore service via an authenticated channel.</p> 44 45 <ul> 46 <li><strong>The <a href="../keystore/index.html">hardware-backed Keystore</a>.</strong> 47 Cryptographic services, including hardware-backed cryptography for key storage, 48 which might include a Trusted Execution Environment (TEE).</li> 49 <li><strong><a href="gatekeeper.html">Gatekeeper</a>.</strong> Components for PIN, pattern, and password authentication.</li> 50 <li><strong><a href="fingerprint-hal.html">Fingerprint</a>.</strong> Components for fingerprint authentication.</li> 51 </ul> 52 53 <h2 id=architecture>Architecture</h2> 54 55 <p>The Gatekeeper and Fingerprint components work with Keystore and other 56 components to support the use of hardware-backed <a href="#authentication_token_format">authentication tokens</a> (referred to below as "AuthTokens").</p> 57 58 <h3 id=enrollment>Enrollment</h3> 59 60 <p>Upon first boot of the device after a factory reset, all authenticators are prepared to receive 61 credential enrollments from the user.</p> 62 63 <p>The user must initially enroll a PIN/pattern/password with Gatekeeper. This 64 initial enrollment creates a randomly generated, 64-bit User SID (user secure 65 identifier, described further below) that serves as an identifier for the user 66 and as a binding token for the user's cryptographic material. 67 This User SID is cryptographically bound to the user's password. 68 As detailed below, successful authentications to Gatekeeper result in AuthTokens that contain the User SID 69 for that password.</p> 70 71 <p>When a user wants to change their credential, they must present their existing 72 credential. If the existing credential is verified successfully, the User SID 73 associated with the existing credential is transferred to the new credential. 74 This allows the user to keep accessing their keys after changing their 75 credential. If a user does not present their existing credential, the new one 76 is enrolled with a fully random User SID. The user can access the device but 77 keys created under the old User SID are permanently lost. This is known as an 78 "untrusted enroll."</p> 79 80 <p>Note that an untrusted enroll will not be allowed under normal circumstances by 81 the Android framework, so most users won't ever see this functionality. 82 However, forcible password resets either by a device administrator or an 83 attacker may cause this to occur.</p> 84 85 <h3 id=authentication>Authentication</h3> 86 87 <p>Now that the user has set up a credential and received a User SID, they may 88 proceed to start authentication.</p> 89 90 <p>In the diagram below, authentication starts when a user provides a PIN, 91 pattern, password, or fingerprint. All TEE components share a secret key which 92 they use to authenticate each other's messages.</p> 93 94 <img src="../images/authentication-flow.png" alt="Authentication flow" id="figure1" /> 95 <p class="img-caption"><strong>Figure 1.</strong> Authentication flow</p> 96 97 <p>The numbers in the following steps correspond to the numbers in the diagram 98 above, and include reference to both the Android OS and the TEE OS: </p> 99 100 <ol> 101 <li>A user provides a PIN, pattern, password, or fingerprint. The 102 <code>LockSettingsService</code> or <code>FingerprintService</code> make a request via Binder to the 103 Gatekeeperd or fingerprintd daemon in the Android OS. Note that fingerprint 104 authentication occurs asynchronously after the fingerprint request is sent. 105 <li>This step involves <strong>either</strong> Gatekeeperd (option 1 below) 106 <strong>or</strong> fingerprintd (option 2 below), 107 depending on whether a pin/pattern/password, or fingerprint, is provided. 108 <ul> 109 <li>The Gatekeeperd daemon sends a pin, pattern, or password hash (received in step 110 1) to its counterpart (Gatekeeper) in the TEE. If authentication in the TEE is 111 successful, Gatekeeper in the TEE sends an AuthToken containing the 112 corresponding User SID, signed with the AuthToken HMAC key, to its 113 counterpart in the Android OS. 114 <li>Alternatively, the fingerprintd daemon, which listens for fingerprint events, 115 sends the data (received in step 1) to its counterpart (Fingerprint) in the 116 TEE. If authentication in the TEE is successful, Fingerprint in the TEE sends 117 an AuthToken, signed with the AuthToken HMAC key, to its counterpart in the Android OS. 118 </ul> 119 <li>The Gatekeeperd or fingerprintd daemon receives a signed AuthToken and passes 120 the AuthToken to the keystore service via an extension to 121 the keystore service's Binder interface. Additionally, Gatekeeperd notifies the keystore service when 122 the device is re-locked and when the device password changes. 123 <li>The keystore service passes to Keymaster the AuthTokens received from Gatekeeperd and 124 fingerprintd, verifying the AuthTokens with the key shared with the Gatekeeper 125 and Fingerprint trustlets. Keymaster trusts the timestamp in the token as the 126 last authentication time and bases a key release decision (to allow an app to 127 use the key) on the timestamp. 128 </ol> 129 130 <p class="note"><strong>Note:</strong> AuthTokens are invalidated whenever a device reboots.</p> 131 132 <h2 id=authentication_token_format>Authentication token format</h2> 133 134 <p>The AuthToken format described in the 135 <a href="https://android.googlesource.com/platform/hardware/libhardware/+/master/include/hardware/hw_auth_token.h"><code>hw_auth_token.h</code></a> file is 136 necessary for token sharing and compatibility across languages and 137 components. See the following file:</p> 138 <pre> 139 hardware/libhardware/include/hardware/hw_auth_token.h 140 </pre> 141 142 <p>A simple serialization protocol with the required fields is defined in the 143 table below. The fields are fixed size.</p> 144 145 <p>Field descriptions are below the table.</p> 146 <table> 147 <tr> 148 <th><strong>Field</strong></th> 149 <th><strong>Type</strong></th> 150 <th><strong>Required or Optional</strong></th> 151 </tr> 152 <tr> 153 <td>AuthToken Version</td> 154 <td>1 byte</td> 155 <td>Required</td> 156 </tr> 157 <tr> 158 <td>Challenge</td> 159 <td>64-bit unsigned integer</td> 160 <td>Optional</td> 161 </tr> 162 <tr> 163 <td>User SID</td> 164 <td>64-bit unsigned integer</td> 165 <td>Required</td> 166 </tr> 167 <tr> 168 <td>Authenticator ID</td> 169 <td>64-bit unsigned integer in network order</td> 170 <td>Optional</td> 171 </tr> 172 <tr> 173 <td>Authenticator type</td> 174 <td>32-bit unsigned integer in network order</td> 175 <td>Required</td> 176 </tr> 177 <tr> 178 <td>Timestamp</td> 179 <td>64-bit unsigned integer in network order</td> 180 <td>Required</td> 181 </tr> 182 <tr> 183 <td>AuthToken HMAC key (SHA-256)</td> 184 <td>256-bit blob</td> 185 <td>Required</td> 186 </tr> 187 </table> 188 189 <h3 id=field_descriptions>Field descriptions </h3> 190 191 <p>This section describes the fields of the AuthToken table above.</p> 192 193 <p><strong>AuthToken Version:</strong> Group tag for all fields below.</p> 194 195 <p><strong>Challenge:</strong> A random integer to prevent replay attacks. Usually the ID of a requested 196 crypto operation. Currently used by transactional fingerprint authorizations. 197 If present, the AuthToken is valid only for crypto operations containing the 198 same challenge.</p> 199 200 <p><strong>User SID</strong>: Non-repeating user identifier tied cryptographically to all keys associated 201 with device authentication. For more information, see the Gatekeeper page.</p> 202 203 <p><strong>Authenticator ID (ASID)</strong>: Identifier used to bind to a specific authenticator policy. All 204 authenticators have their own value of ASID that they can change according to 205 their own requirements.</p> 206 207 <p><strong>Authenticator Type</strong>: Either Gatekeeper or Fingerprint, as follows:</p> 208 <table> 209 <tr> 210 <th><strong>Authenticator Type</strong></th> 211 <th><strong>Authenticator Name</strong></th> 212 </tr> 213 <tr> 214 <td>0x00</td> 215 <td>Gatekeeper</td> 216 </tr> 217 <tr> 218 <td>0x01</td> 219 <td>Fingerprint</td> 220 </tr> 221 </table> 222 223 <p><strong>Timestamp</strong>: Time (in milliseconds) since the most recent system boot.</p> 224 225 <p><strong>AuthToken HMAC key</strong>: Keyed SHA-256 MAC of all fields except the HMAC field.</p> 226 227 <h2 id=device_boot_flow>Device boot flow</h2> 228 229 <p>On every boot of a device, the AuthToken HMAC key must be generated and shared 230 with all TEE components (Gatekeeper, Fingerprint, and Keymaster). Thus, the HMAC key 231 must be randomly generated every time the device reboots, for added protection against replay attacks.</p> 232 233 <p>The protocol for sharing this HMAC key with all components is a 234 platform-dependent implementation feature. The key must <strong>never</strong> 235 be made available outside the TEE. Thus, if a TEE OS lacks an 236 internal inter-process communication (IPC) mechanism, 237 and the TEE needs to transfer the data through the untrusted OS, the transfer 238 must be done via a secure key exchange protocol.</p> 239 240 <p>The Trusty operating system, which runs next to Android, is an example of a 241 TEE, but other TEEs can be used instead. Trusty uses an internal IPC system to 242 communicate directly between Keymaster and Fingerprint or Gatekeeper. The HMAC 243 key is kept solely in Keymaster. Fingerprint and Gatekeeper request the key 244 from Keymaster for each use, and do not persist or cache the value.</p> 245 246 <p>Note that no communication happens between applets in the TEE because some TEEs 247 are lacking in IPC infrastructure. This also 248 permits the keystore service to quickly deny requests that are bound to fail as it has 249 knowledge of the authentication table in the system, saving a potentially 250 costly IPC into the TEE.</p> 251
