src/security/authentication/index.jd

page.title=Authentication
@jd:body

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    <h2>In this document</h2>
    <ol id="auto-toc">
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<h2 id=overview>Overview</h2>

<p>Android 6.0 introduces the concept of user-authentication-gated cryptographic
keys. To achieve this, two key components need to work together.
First is the cryptographic key storage and service provider, which stores
cryptographic keys and provides standard crypto routines on top of them. Second
is any number of user authenticators that may attest to the user's presence
and/or successful authentication.</p>

<p>The cryptographic key storage in Android is provided by the keystore service and Keymaster.
(Also see information about
the <a href="https://developer.android.com/training/articles/keystore.html">Android Keystore system</a>,
at the framework level, which is backed by the keystore service.) For Android 6.0,
the two supported authentication components are Gatekeeper (for
PIN/pattern/password authentication) and Fingerprint (for fingerprint
authentication). These components communicate their authentication
state with the keystore service via an authenticated channel.</p>

<ul>
  <li><strong>The keystore service and <a href="keymaster.html">Keymaster</a>.</strong> Cryptographic services,
   including hardware-backed cryptography for key storage,
   which might include a Trusted Execution Environment (TEE).</li>
  <li><strong><a href="gatekeeper.html">Gatekeeper</a>.</strong> Components for PIN, pattern, and password authentication.</li>
  <li><strong><a href="fingerprint-hal.html">Fingerprint</a>.</strong> Components for fingerprint authentication.</li>
</ul>

<h2 id=architecture>Architecture</h2>

<p>The Gatekeeper and Fingerprint components work with Keymaster and other
components to support the use of hardware-backed <a href="#authentication_token_format">authentication tokens</a> (referred to below as "AuthTokens").</p>

<h3 id=enrollment>Enrollment</h3>

<p>Upon first boot of the device after a factory reset, all authenticators are prepared to receive
credential enrollments from the user.</p>

<p>The user must initially enroll a PIN/pattern/password with Gatekeeper. This
initial enrollment creates a randomly generated, 64-bit User SID (user secure
identifier, described further below) that serves as an identifier for the user
and as a binding token for the user's cryptographic material.
This User SID is cryptographically bound to the user's password.
As detailed below, successful authentications to Gatekeeper result in AuthTokens that contain the User SID
for that password.</p>

<p>When a user wants to change their credential, they must present their existing
credential. If the existing credential is verified successfully, the User SID
associated with the existing credential is transferred to the new credential.
This allows the user to keep accessing their keys after changing their
credential. If a user does not present their existing credential, the new one
is enrolled with a fully random User SID. The user can access the device but
keys created under the old User SID are permanently lost. This is known as an
"untrusted enroll."</p>

<p>Note that an untrusted enroll will not be allowed under normal circumstances by
the Android framework, so most users won't ever see this functionality.
However, forcible password resets either by a device administrator or an
attacker may cause this to occur.</p>

<h3 id=authentication>Authentication</h3>

<p>Now that the user has set up a credential and received a User SID, they may
proceed to start authentication.</p>

<p>In the diagram below, authentication starts when a user provides a PIN,
pattern, password, or fingerprint. All TEE components share a secret key which
they use to authenticate each other's messages.</p>

<img src="../images/authentication-flow.png" alt="Authentication flow" id="figure1" />
<p class="img-caption"><strong>Figure 1.</strong> Authentication flow</p>

<p>The numbers in the following steps correspond to the numbers in the diagram
above, and include reference to both the Android OS and the TEE OS: </p>

<ol>
  <li>A user provides a PIN, pattern, password, or fingerprint. The
<code>LockSettingsService</code> or <code>FingerprintService</code> make a request via Binder to the
Gatekeeperd or fingerprintd daemon in the Android OS. Note that fingerprint
authentication occurs asynchronously after the fingerprint request is sent.
  <li>This step involves <strong>either</strong> Gatekeeperd (option 1 below)
  <strong>or</strong> fingerprintd (option 2 below),
  depending on whether a pin/pattern/password, or fingerprint, is provided.
  <ul>
    <li>The Gatekeeperd daemon sends a pin, pattern, or password hash (received in step
1) to its counterpart (Gatekeeper) in the TEE. If authentication in the TEE is
successful, Gatekeeper in the TEE sends an AuthToken containing the
corresponding User SID, signed with the AuthToken HMAC key, to its
counterpart in the Android OS.
    <li>Alternatively, the fingerprintd daemon, which listens for fingerprint events,
sends the data (received in step 1) to its counterpart (Fingerprint) in the
TEE. If authentication in the TEE is successful, Fingerprint in the TEE sends
an AuthToken, signed with the AuthToken HMAC key, to its counterpart in the Android OS.
  </ul>
  <li>The Gatekeeperd or fingerprintd daemon receives a signed AuthToken and passes
the AuthToken to the keystore service via an extension to
the keystore service's Binder interface. Additionally, Gatekeeperd notifies the keystore service when
the device is re-locked and when the device password changes.
  <li>The keystore service passes to Keymaster the AuthTokens received from Gatekeeperd and
fingerprintd, verifying the AuthTokens with the key shared with the Gatekeeper
and Fingerprint trustlets. Keymaster trusts the timestamp in the token as the
last authentication time and bases a key release decision (to allow an app to
use the key) on the timestamp.
</ol>

<p class="note"><strong>Note:</strong> AuthTokens are invalidated whenever a device reboots.</p>

<h2 id=authentication_token_format>Authentication token format</h2>

<p>The AuthToken format described in the
<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 
necessary for token sharing and compatibility across languages and
components. See the following file:</p>
<pre>
hardware/libhardware/include/hardware/hw_auth_token.h
</pre>

<p>A simple serialization protocol with the required fields is defined in the
table below. The fields are fixed size.</p>

<p>Field descriptions are below the table.</p>
<table>
 <tr>
    <th><strong>Field</strong></th>
    <th><strong>Type</strong></th>
    <th><strong>Required or Optional</strong></th>
 </tr>
 <tr>
    <td>AuthToken Version</td>
    <td>1 byte</td>
    <td>Required</td>
 </tr>
 <tr>
    <td>Challenge</td>
    <td>64-bit unsigned integer</td>
    <td>Optional</td>
 </tr>
 <tr>
    <td>User SID</td>
    <td>64-bit unsigned integer</td>
    <td>Required</td>
 </tr>
 <tr>
    <td>Authenticator ID</td>
    <td>64-bit unsigned integer in network order</td>
    <td>Optional</td>
 </tr>
 <tr>
    <td>Authenticator type</td>
    <td>32-bit unsigned integer in network order</td>
    <td>Required</td>
 </tr>
 <tr>
    <td>Timestamp</td>
    <td>64-bit unsigned integer in network order</td>
    <td>Required</td>
 </tr>
 <tr>
    <td>AuthToken HMAC key (SHA-256)</td>
    <td>256-bit blob</td>
    <td>Required</td>
 </tr>
</table>

<h3 id=field_descriptions>Field descriptions </h3>

<p>This section describes the fields of the AuthToken table above.</p>

<p><strong>AuthToken Version:</strong> Group tag for all fields below.</p>

<p><strong>Challenge:</strong> A random integer to prevent replay attacks. Usually the ID of a requested
crypto operation. Currently used by transactional fingerprint authorizations.
If present, the AuthToken is valid only for crypto operations containing the
same challenge.</p>

<p><strong>User SID</strong>: Non-repeating user identifier tied cryptographically to all keys associated
with device authentication. For more information, see the Gatekeeper page.</p>

<p><strong>Authenticator ID (ASID)</strong>: Identifier used to bind to a specific authenticator policy. All
authenticators have their own value of ASID that they can change according to
their own requirements.</p>

<p><strong>Authenticator Type</strong>: Either Gatekeeper or Fingerprint, as follows:</p>
<table>
 <tr>
    <th><strong>Authenticator Type</strong></th>
    <th><strong>Authenticator Name</strong></th>
 </tr>
 <tr>
    <td>0x00</td>
    <td>Gatekeeper</td>
 </tr>
 <tr>
    <td>0x01</td>
    <td>Fingerprint</td>
 </tr>
</table>

<p><strong>Timestamp</strong>: Time (in milliseconds) since the most recent system boot.</p>

<p><strong>AuthToken HMAC key</strong>: Keyed SHA-256 MAC of all fields except the HMAC field.</p>

<h2 id=device_boot_flow>Device boot flow</h2>

<p>On every boot of a device, the AuthToken HMAC key must be generated and shared
with all TEE components (Gatekeeper, Fingerprint, and Keymaster). Thus, the HMAC key
must be randomly generated every time the device reboots, for added protection against replay attacks.</p>

<p>The protocol for sharing this HMAC key with all components is a
platform-dependent implementation feature. The key must <strong>never</strong>
be made available outside the TEE. Thus, if a TEE OS lacks an
internal inter-process communication (IPC) mechanism,
and the TEE needs to transfer the data through the untrusted OS, the transfer
must be done via a secure key exchange protocol.</p>

<p>The Trusty operating system, which runs next to Android, is an example of a
TEE, but other TEEs can be used instead. Trusty uses an internal IPC system to
communicate directly between Keymaster and Fingerprint or Gatekeeper. The HMAC
key is kept solely in Keymaster. Fingerprint and Gatekeeper request the key
from Keymaster for each use, and do not persist or cache the value.</p>

<p>Note that no communication happens between applets in the TEE because some TEEs
are lacking in IPC infrastructure. This also
permits the keystore service to quickly deny requests that are bound to fail as it has
knowledge of the authentication table in the system, saving a potentially
costly IPC into the TEE.</p>