Storage encryption/decryption

[mchack-work]

In Castor we introduce persistent storage for device apps. The apps can allocate a storage area for themselves, which is separated from other apps' areas by the firmware.

Considering that the data is stored unencrypted in the app storage area it would make sense to encrypt data before storing it.

We could investigate the possibility to add transparent encryption/decryption to sys_write()/sys_read() or just add some convenience functions to encrypt data in an easy way for writing/reading instead of manually calling into Monocypher functions.

[mchack-work]

Can we use something based on the CDI (or another secret left by the loader in next_app_data) as a symmetric encryption key? Risks in using it alone? Combined with something on the storage?

[SallSim]

Discussion 2025-06-18:
It should be possible for developers to easily use any crypto lib of their liking, i.e. we should not enforce Monocypher.