CalyxOS based on LineageOS
CalyxOS is an operating system for smartphones based on Android with mostly free and open-source software.  It is produced by the Calyx Institute as part of its mission to "defend online privacy, security and accessibility."CalyxOS preserves the Android security model, using Android's Verified Boot system of cryptographic signing of the operating system, and running with a locked bootloader, partly thanks to an installer that guides the user through the process of unlocking and then re-locking the bootloader.
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