GrapheneOS is an open source privacy and security focused mobile OS with Android app compatibility.
The official GrapheneOS releases are supported by the Auditor app and attestation service for hardware-based attestation. For more details, see the about page and tutorial. You can also extend these with support for your own builds.
GrapheneOS is a privacy / security research and engineering project that has been under way for over 5 years. It recently became rebranded as GrapheneOS and is taking a different direction based on obtaining funding for the research and development work as a non-profit open source project rather than being a company. The reborn project is still in a very early stage and lots of the past work on privacy and security has not yet been restored for the new incarnation of the OS.
The grapheneos.org site is very new and is currently being put together. It will have lots of additional documentation and tutorials in the future along with coverage of various software, firmware and hardware privacy/security topics.
GrapheneOS is being supported with funding and developers from various companies and other organizations interested in contributing to this shared base for a feature rich private and secure mobile operating system able to run many existing applications. It will take more time to organize and deploy these resources in order for the project to have a strong development team with proper infrastructure behind it.
Details on the roadmap of the project will be posted on the site in the near future. In the long term, it aims to move beyond a hardened fork of the Android Open Source Project. Achieving the goals requires moving away from relying the Linux kernel as the core of the OS and foundation of the security model. It needs to move towards a microkernel-based model with a Linux compatibility layer, with many stepping stones leading towards that goal including adopting virtualization-based isolation.
The initial phase for the long-term roadmap of moving away from the current foundation will be to deploy and integrate a hypervisor like Xen to leverage it for reinforcing existing security boundaries. Linux would be running inside the virtual machines at this point, inside and outside of the sandboxes being reinforced. In the longer term, Linux inside the sandboxes can be replaced with a compatibility layer like gVisor, which would need to be ported to arm64 and given a new backend alongside the existing KVM backend. Over the longer term, i.e. many years from now, Linux can fade away completely and so can the usage of virtualization. The anticipation is that many other projects are going to be interested in this kind of migration, so it's not going to be solely a GrapheneOS project, as demonstrated by the current existence of the gVisor project and various other projects working on virtualization deployments for mobile. Having a hypervisor with verified boot still intact will also provide a way to achieve some of the goals based on extensions to Trusted Execution Environment (TEE) functionality even without having GrapheneOS hardware.
Hardware and firmware security are core parts of the project, but it's currently limited to research and submitting suggestions and bug reports upstream. In the long term, the project will need to move into the hardware space.
In the current early stage of the project, GrapheneOS provides production releases for the Pixel, Pixel XL, Pixel 2, Pixel 2 XL, Pixel 3, Pixel 3 XL, Pixel 3a and Pixel 3a XL. The recommended devices with the best hardware, firmware and software security along with the longest future support time are the Pixel 3a, Pixel 3a XL, Pixel 3 and Pixel 3 XL. It will support other devices in the future, but devices are carefully chosen based on their merits rather than the project aiming to have broad device support. Broad device support is counter to the aims of the project, and the project will eventually be engaging in hardware and firmware level improvements rather than only offering suggestions and bug reports upstream for those areas. Much of the work on the project involves changes that are specific to different devices, and officially supported devices are the ones targeted by most of this ongoing work. GrapheneOS also has source level support without device-specific hardening for the Android emulator, HiKey, HiKey 960 and also generic targets providing basic support for many other devices.
Devices need to be meet the standards of the project in order to be considered as potential targets. In addition to support for installing other operating systems, standard hardware-based security features like the hardware-backed keystores, verified boot, attestation and various hardware-based exploit mitigations need to be available. Devices also need to have decent integration of IOMMUs for isolating components such as the GPU, radios (NFC, Wi-Fi, Bluetooth, Cellular), media decode / encode, image processor, etc. as if the hardware / firmware support is missing or broken, there's not much that the OS can do to provide an alternative. Devices with support for alternative operating systems as an afterthought will not be considered. Devices need to have proper ongoing support for their firmware and software specific to the hardware like drivers in order to provide proper full security updates too. Devices that are end-of-life and no longer receiving these updates will not be supported.
In order to support a device, the appropriate resources also need to be available and dedicated towards it. Releases for each supported device need to be robust and stable, with all standard functionality working properly and testing for each of the releases.
Hardware, firmware and software specific to devices like drivers play a huge role in the overall security of a device. The goal of the project is not to slightly improve some aspects of insecure devices and supporting a broad set of devices would be directly counter to the values of the project. A lot of the low-level work also ends up being fairly tied to the hardware.
Some of the GrapheneOS sub-projects support other operating systems on a broader range of devices. Device support for Auditor and AttestationServer is documented in the overview of those projects. The hardened_malloc project supports nearly any Linux-based environment due to official support for musl, glibc and Bionic along with easily added support for other environments. It can easily run on non-Linux-based operating systems too, and supporting some like HardenedBSD is planned but depends on contributors from those communities.