For example, this means I cannot even _try_ GrapheneOS anymore, without losing my ability to keep using LineageOS if I change my mind.
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As stated, all operating systems on Pixel devices must be on Android 13 to get full security updates. There isn't any leeway there.
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As a user, you have the freedom to stay on an old version, at the cost of having missing security content, which I don't really suggest.
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But it's entirely possible to install (a modified) Android 13 and NOT lose the ability to downgrade.
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You do lose the ability to downgrade the Titan M2 firmware regardless of which OS you use and based on our experience with the migration from Android 11 to Android 12, they do not preserve backwards compatibility. We're the only ppl who noticed since others don't use what broke.
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I probably already lost that, then, since I tried the A13 beta...? But Android 12 continues to work right now.
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The features which tend to break are only used by a small portion of apps such as our Auditor app.
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But most apps are versioned separately from the OS anyway? Auditor is the exception since it's bundled with Graphene, but in theory, if you supported an older version at all, you could fix the compatibility of the app, right?
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No, stuff actually breaks at the OS level because the OS HAL doesn't know how to cope with backwards incompatible changes in the secure element firmware and it can't be downgraded.
Users who upgraded to Android 12 Beta couldn't use Auditor on Android 11 GrapheneOS anymore.
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They did backwards incompatible changes in Android 12 Beta for the secure element firmware and Android 11 couldn't downgrade the firmware because the Android 12 Beta version counter was newer. It was a significant issue because it happened with the 12 Beta, long before 12.
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We shipped Android 12 days after the stock OS shipped it so there wasn't much of a window where anyone would have cared if the Android 12 Beta hadn't caused it. For this Android 13 boot chain counter update, they only shipped it in the stable release with 2022-08-05 sec patch.