The Android phone won’t be the device on a solar panel hidden in the woods. And hardened android with a secure boot loader, and enhanced verified boot is pretty good anyway. GrapheneOS uses an enhanced version of the modern filesystem-based disk encryption implementation in the Android Open Source Project. The officially supported devices have substantial hardware-based support for enhancing the security of the encryption implementation. GrapheneOS has full support for the hardware-based encryption features just as it does with other hardware-based security features.
The data partition stores all of the persistent state for the operating system. Full disk encryption is implemented via filesystem-based encryption with metadata encryption. All data, file names and other metadata is always stored encrypted. This is often referred to as file-based encryption but it makes more sense to call it filesystem-based encryption. It’s implemented by the Linux kernel as part of the ext4 / f2fs implementation rather than running a block-based encryption layer. The advantage of filesystem-based encryption is the ability to use fine-grained keys rather than a single global key that’s always in memory once the device is booted.
Disk encryption keys are randomly generated with a high quality CSPRNG and stored encrypted with a key encryption key. Key encryption keys are derived at runtime and are never stored anywhere.
File data is encrypted with AES-256-XTS and file names with AES-256-CTS. A unique key is derived using HKDF-SHA512 for each regular file, directory and symbolic link from the per-profile encryption keys, or the global encryption key for non-sensitive data stored outside of profiles. The directory key is used to encrypt the file names. GrapheneOS increases the file name padding from 16 bytes to 32 bytes. AES-256-XTS with the global encryption key is also used to encrypt filesystem metadata as a whole beyond the finer-grained file name encryption.
The OS derives a password token from the profile’s lock method credential using scrypt. This is used as the main input for key derivation.
The OS stores a high entropy random value as the Weaver token on the secure element (Titan M on Pixels) and uses it as another input for key derivation. The Weaver token is stored alongside a Weaver key derived by the OS from the password token. In order to retrieve the Weaver token, the secure element requires the correct Weaver key. A secure internal timer is used to implement hardware-based delays for each attempt at key derivation. It quickly ramps up to 1 day delays before the next attempt. Weaver also provides reliable wiping of data since the secure element can reliably wipe a Weaver slot. Deleting a profile will wipe the corresponding Weaver slot and a factory reset of the device wipes all of the Weaver slots. The secure element also provides insider attack resistance preventing firmware updates before authenticating with the owner profile.
Standard delays for encryption key derivation enforced by the secure element:
- 0 to 4 failed attempts: no delay
- 5 failed attempts: 30 second delay
- 6 to 9 failed attempts: no delay
- 10 to 29 failed attempts: 30 second delay
- 30 to 139 failed attempts: 30 × 2⌊(n - 30) ÷ 10⌋ where n is the number of failed attempts. This means the delay doubles after every 10 attempts. There’s a 30 second delay after 30 failed attempts, 60s after 40, 120s after 50, 240s after 60, 480s after 70, 960s after 80, 1920s after 90, 3840s after 100, 7680s after 110, 15360s after 120 and 30720s after 130
- 140 or more failed attempts: 86400 second delay (1 day)
Invalid input outside the minimum or maximum length limits of the UI won’t trigger an attempt at authentication or key derivation.
The password token, Weaver token and other values like the OS verified boot key are used by the TEE as inputs to a hardware-bound key derivation algorithm provided by the SoC. The general concept is having the SoC perform hardware accelerated key derivation using an algorithm like AES or HMAC keyed with a hard-wired hardware key inaccessible to software or firmware. This is meant to prevent offloading a brute force attack onto more powerful hardware without an expensive process of extracting the hardware key from the SoC.
But, the point is if someone captures the relay node when messages are encrypted with gpg the only thing they will see is:
-----BEGIN PGP MESSAGE-----
-----END PGP MESSAGE-----
The node will be the hardware hidden in the woods, not the android.