128 lines
8.5 KiB
HTML
128 lines
8.5 KiB
HTML
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<h1><a href="#attestation-compatibility-guide">Attestation compatibility guide</a></h1>
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<p>Apps using SafetyNet attestation to check the authenticity/integrity of the OS can
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support GrapheneOS by using the standard Android hardware attestation API and
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permitting our official release signing keys. Android's
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<a href="https://developer.android.com/training/articles/security-key-attestation">hardware
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attestation API</a> provides a much stronger form of attestation than SafetyNet with
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the ability to whitelist the keys of alternate operating systems. It also avoids an
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unnecessary dependency on Google Play services and Google's SafetyNet servers.</p>
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<p>Devices have been required to ship with hardware attestation support since Android
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8. You can use hardware attestation on devices running Android 8 or later when the
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<code>ro.product.first_api_level</code> system property isn't set to 25 or below,
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which indicates they launched with Android 8 or later with hardware attestation
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support as a mandatory feature. On older devices, you can continue using SafetyNet
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attestation. Some low quality devices shipped broken implementations of hardware
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attestation despite the requirement to have it working for CDD/CTS certification and
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SafetyNet currently still passes on those devices wrongly claiming them to be CTS
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certified. If you don't want to fail on those devices, then you can start with
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hardware attestation and fall back to SafetyNet attestation or do both and accept
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either passing as success.</p>
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<p>After verifying the signature of the attestation certificate chain and extracting
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the attestation metadata, you can enforce that <code>verifiedBootState</code> is
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either <code>Verified</code> or <code>SelfSigned</code>. For the
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<code>SelfSigned</code> case, you can check that <code>verifiedBootKey</code> matches
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one of the official GrapheneOS verified boot keys. These are the base16-encoded
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verified boot key fingerprints for the official GrapheneOS releases:</p>
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<ul>
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<li><code>439b76524d94c40652ce1bf0d8243773c634d2f99ba3160d8d02aa5e29ff925c</code>: Pixel 6 Pro</li>
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<li><code>f0a890375d1405e62ebfd87e8d3f475f948ef031bbf9ddd516d5f600a23677e8</code>: Pixel 6</li>
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<li><code>0abddeda03b6ce10548c95e0bea196faa539866f929bcdf7eca84b4203952514</code>: Pixel 5a</li>
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<li><code>36a99eab7907e4fb12a70e3c41c456bcbe46c13413fbfe2436adee2b2b61120f</code>: Pixel 5</li>
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<li><code>dcec2d053d3ec4f1c9be414aa07e4d7d7cbd12040ad2f8831c994a83a0536866</code>: Pixel 4a (5G)</li>
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<li><code>3f15fdcb82847fed97427ce00563b8f9ff34627070de5fdb17aca7849ab98cc8</code>: Pixel 4 XL</li>
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<li><code>80ef268700ee42686f779a47b4a155fe1ffc2eedf836b4803caab8fa61439746</code>: Pixel 4</li>
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<li><code>9f2454a1657b1b5ad7f2336b39a2611f7a40b2e0ddfd0d6553a359605928df29</code>: Pixel 4a</li>
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<li><code>3f36e3482e1ff82986576552cb4fd08af09f8b09d3832314341e04c42d2919a4</code>: Pixel 3a XL</li>
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<li><code>73d6c63a07610404fe16a4e07dd24e41a70d331e9d3ef7bba2d087e4761eb63a</code>: Pixel 3a</li>
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<li><code>06dd526ee9b1cb92aa19d9835b68b4ff1a48a3ad31d813f27c9a7d6c271e9451</code>: Pixel 3 XL</li>
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<li><code>0f9a9cc8ade73064a54a35c5509e77994e3aa37b6fb889dd53af82c3c570c5cf</code>: Pixel 3</li>
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</ul>
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<p>The <code>verifiedBootKey</code> field is binary data so you either need to encode
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it as base16 to compare with these or convert these to binary. An easy approach is
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storing the permitted key fingerprints in a set and enforcing that the verified boot
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key is in the permitted set when <code>verifiedBootState</code> is
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<code>SelfSigned</code>.</p>
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<p>GrapheneOS regularly adds support for new devices so you should have a process for
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regularly adding the new verified boot key fingerprints from this page.</p>
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<p>The hardware attestation API also provides other useful information signed by the
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hardware including the OS patch level, in a way that even an attacker exploiting the
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OS after boot to gain root cannot trivially bypass. It's a better feature than the
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SafetyNet API designed for the lowest common denominator.</p>
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<p>GrapheneOS users are strongly encouraged to share this documentation with app
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developers enforcing only being able to use the stock OS. Send an email to the
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developers and leave a review of the app with a link to this information. Share it
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with other users and create pressure to support GrapheneOS rather than locking users
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into the stock OS without a valid security reason. GrapheneOS not only upholds the
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app security model but substantially reinforces it, so it cannot be justified with
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reasoning based on security, anti-fraud, etc.</p>
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