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<title>Attestation compatibility guide | Articles | GrapheneOS</title>
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<main id="attestation-compatibility-guide">
<h1><a href="#attestation-compatibility-guide">Attestation compatibility guide</a></h1>
<p>Apps using the Play Integrity API or
<a href="https://developer.android.com/training/safetynet/deprecation-timeline">obsolete</a>
SafetyNet Attestation API to check the authenticity/integrity of the OS can support
GrapheneOS by using the standard Android hardware attestation API instead and
permitting our official release signing keys. Android's
<a href="https://developer.android.com/training/articles/security-key-attestation">hardware
attestation API</a> provides a much stronger form of attestation than the Play
Integrity API with the ability to whitelist the keys of alternate operating systems.
It also avoids an unnecessary dependency on Google Play services and Google's
Play Integrity servers.</p>
<p>Devices have been required to ship with hardware attestation support since Android
8. You can use hardware attestation on devices running Android 8 or later when the
<code>ro.product.first_api_level</code> system property isn't set to 25 or below,
which indicates they launched with Android 8 or later with hardware attestation
support as a mandatory feature. On older devices, you can continue using the Play
Integrity API. Some low quality devices shipped broken implementations of hardware
attestation despite the requirement to have it working for CDD/CTS certification and
the Play Integrity API currently still passes on those devices wrongly claiming them
to be CTS certified. If you don't want to fail on those devices, then you can start
with hardware attestation and fall back to the Play Integrity API or do both and
accept either passing as success.</p>
<p>Our <a href="https://github.com/GrapheneOS/Auditor">MIT / Apache 2 licensed Auditor
app</a> can be used a reference implementation for verifying hardware-based
attestations. There are some subtleties in the verification process such as making
sure only the 2nd certificate in the chain (the one signing the certificate for the
key generated by your app) has an attestation extension to prevent making a fake
attestation by extending the chain. You can reuse our code and simply omit support for
an app generated attestation signing key (attest key) and the other pinning
support.</p>
<p>After verifying the signature of the attestation certificate chain and extracting
the attestation metadata, you can enforce that <code>verifiedBootState</code> is
either <code>Verified</code> or <code>SelfSigned</code>. For the
<code>SelfSigned</code> case, you can check that <code>verifiedBootKey</code> matches
one of the official GrapheneOS verified boot keys. These are the base16-encoded
verified boot key fingerprints for the official GrapheneOS releases:</p>
<ul>
<li><code>ee0c9dfef6f55a878538b0dbf7e78e3bc3f1a13c8c44839b095fe26dd5fe2842</code>: Pixel Fold</li>
<li><code>94df136e6c6aa08dc26580af46f36419b5f9baf46039db076f5295b91aaff230</code>: Pixel Tablet</li>
<li><code>508d75dea10c5cbc3e7632260fc0b59f6055a8a49dd84e693b6d8899edbb01e4</code>: Pixel 7a</li>
<li><code>bc1c0dd95664604382bb888412026422742eb333071ea0b2d19036217d49182f</code>: Pixel 7 Pro</li>
<li><code>3efe5392be3ac38afb894d13de639e521675e62571a8a9b3ef9fc8c44fd17fa1</code>: Pixel 7</li>
<li><code>08c860350a9600692d10c8512f7b8e80707757468e8fbfeea2a870c0a83d6031</code>: Pixel 6a</li>
<li><code>439b76524d94c40652ce1bf0d8243773c634d2f99ba3160d8d02aa5e29ff925c</code>: Pixel 6 Pro</li>
<li><code>f0a890375d1405e62ebfd87e8d3f475f948ef031bbf9ddd516d5f600a23677e8</code>: Pixel 6</li>
<li><code>0abddeda03b6ce10548c95e0bea196faa539866f929bcdf7eca84b4203952514</code>: Pixel 5a</li>
<li><code>36a99eab7907e4fb12a70e3c41c456bcbe46c13413fbfe2436adee2b2b61120f</code>: Pixel 5</li>
<li><code>dcec2d053d3ec4f1c9be414aa07e4d7d7cbd12040ad2f8831c994a83a0536866</code>: Pixel 4a (5G)</li>
<li><code>9f2454a1657b1b5ad7f2336b39a2611f7a40b2e0ddfd0d6553a359605928df29</code>: Pixel 4a</li>
<li><code>3f15fdcb82847fed97427ce00563b8f9ff34627070de5fdb17aca7849ab98cc8</code>: Pixel 4 XL</li>
<li><code>80ef268700ee42686f779a47b4a155fe1ffc2eedf836b4803caab8fa61439746</code>: Pixel 4</li>
</ul>
<p>The <code>verifiedBootKey</code> field is binary data so you either need to encode
it as base16 to compare with these or convert these to binary. An easy approach is
storing the permitted key fingerprints in a set and enforcing that the verified boot
key is in the permitted set when <code>verifiedBootState</code> is
<code>SelfSigned</code>.</p>
<p>GrapheneOS regularly adds support for new devices so you should have a process for
regularly adding the new verified boot key fingerprints from this page.</p>
<p>The hardware attestation API also provides other useful information signed by the
hardware including the OS patch level, in a way that even an attacker exploiting the
OS after boot to gain root cannot trivially bypass. It's a better feature than the
Play Integrity API which has to be designed for the lowest common denominator.</p>
<p>GrapheneOS users are strongly encouraged to share this documentation with app
developers enforcing only being able to use the stock OS. Send an email to the
developers and leave a review of the app with a link to this information. Share it
with other users and create pressure to support GrapheneOS rather than locking users
into the stock OS without a valid security reason. GrapheneOS not only upholds the
app security model but substantially reinforces it, so it cannot be justified with
reasoning based on security, anti-fraud, etc.</p>
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