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<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>The standard hardware attestation API can be used to verify the authenticity/integrity
of the hardware, firmware, OS and the app running on it. It provides a verified boot key
fingerprint for the OS for permitting secure aftermarket operating systems. The app id,
signing key fingerprint(s) and version code of the app enabling hardware attestation are
included in the signed public key certificate for the generated key. This enables the
app's service to make sure the app is genuine and unmodified along with chaining trust
through the OS to the app which can sign messages with the attested hardware keystore
key to prove they come from their app running on top of a verified OS, firmware and
hardware. The only practical way to bypass hardware attestation is through exploiting
the hardware keystore to obtain attestation signing keys, which is protected against by
the ability to revoke keys that are being misused. Play Integrity API strong integrity
level is directly based on the hardware key attestation API, but apps using it directly
can support aftermarket operating systems, check the hardware attested OS patch level
and other provided information. The hardware attestation API also supports pinning-based
security instead of only root-based security where keys can be leaked and used to fake
attestations. Apps can use pinning to establish a much higher security pairing with a
specific device to obtain fresh attestations with a very high level security based on
the security of the device's own hardware keystore rather than the overall ecosystem.
Hardware attestation also doesn't require using any Google service beyond regularly
fetching the list of revoked keys for root-based attestation. The app's service doesn't
have to go down or start permitting anything if the Google services becomes unavailable
or blocks the app from using it for one reason or another. Using hardware attestation is
therefore more reliable and lower risk for apps.</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>Google provides a <a href="https://github.com/google/android-key-attestation">key
attestation library</a> with examples. Our <a href="https://github.com/GrapheneOS/Auditor">MIT
/ Apache 2 licensed Auditor app</a> can be used as 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>0508de44ee00bfb49ece32c418af1896391abde0f05b64f41bc9a2dfb589445b</code>: Pixel 9a</li>
<li><code>af4d2c6e62be0fec54f0271b9776ff061dd8392d9f51cf6ab1551d346679e24c</code>: Pixel 9 Pro Fold</li>
<li><code>55d3c2323db91bb91f20d38d015e85112d038f6b6b5738fe352c1a80dba57023</code>: Pixel 9 Pro XL</li>
<li><code>f729cab861da1b83fdfab402fc9480758f2ae78ee0b61c1f2137dd1ab7076e86</code>: Pixel 9 Pro</li>
<li><code>9e6a8f3e0d761a780179f93acd5721ba1ab7c8c537c7761073c0a754b0e932de</code>: Pixel 9</li>
<li><code>096b8bd6d44527a24ac1564b308839f67e78202185cbff9cfdcb10e63250bc5e</code>: Pixel 8a</li>
<li><code>896db2d09d84e1d6bb747002b8a114950b946e5825772a9d48ba7eb01d118c1c</code>: Pixel 8 Pro</li>
<li><code>cd7479653aa88208f9f03034810ef9b7b0af8a9d41e2000e458ac403a2acb233</code>: Pixel 8</li>
<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>
</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>
<article id="apps-banning-grapheneos">
<h2><a href="#apps-banning-grapheneos">Apps banning GrapheneOS</a></h2>
<p>This is a list of the apps banning GrapheneOS with the Play Integrity API with
links to their Play Store pages for leaving feedback:</p>
<ul>
<li><a href="https://play.google.com/store/apps/details?id=au.gov.mygov.mygovapp" rel="nofollow">myGov</a> (Australian government app)</li>
<li><a href="https://play.google.com/store/apps/details?id=br.gov.meugovbr" rel="nofollow">gov.br</a> (Brazilian government app)</li>
<li><a href="https://play.google.com/store/apps/details?id=ch.ticketcorner.mobile.app.Android" rel="nofollow">Ticketcorner</a></li>
<li><a href="https://play.google.com/store/apps/details?id=com.authy.authy" rel="nofollow">Authy</a></li>
<li><a href="https://play.google.com/store/apps/details?id=com.ebay.mobile" rel="nofollow">eBay</a></li>
<li><a href="https://play.google.com/store/apps/details?id=com.mcdonalds.mobileapp" rel="nofollow">McDonald's</a> (International app used for many but not all countries not including the US)</li>
<li><a href="https://play.google.com/store/apps/details?id=com.ridedott.rider" rel="nofollow">Dott</a></li>
<li><a href="https://play.google.com/store/apps/details?id=com.swissquote.android" rel="nofollow">Swissquote</a></li>
<li><a href="https://play.google.com/store/apps/details?id=com.swisssign.swissid.mobile" rel="nofollow">SwissID</a></li>
<li><a href="https://play.google.com/store/apps/details?id=de.tk.tkapp" rel="nofollow">TK-App</a> (German health insurance app which uses it for fingerprint login)</li>
<li><a href="https://play.google.com/store/apps/details?id=it.pagopa.io.app" rel="nofollow">IO</a> (Italian government app which uses it for the digital wallet feature)</li>
</ul>
<p>In addition to leaving feedback for these apps on the Play Store, file support
requests and leave feedback on third party review sites. Ask them to stop banning
GrapheneOS and explain that it's a much more secure OS than what they permit which
does not lose any of the standard security model. Explain that they can use the
hardware key attestation API to verify that a device is running GrapheneOS to permit
it alongside an OS licensing Google apps as they do with the Play Integrity API
already. Make sure to push back against false claims that it has something to do
with compatibility or security issues. The only reason they aren't permitting it is
because we do not license Google Mobile Services (GMS) and these apps are enforcing
Google's business interests rather than security.</p>
</article>
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