Encrypt, don't back up passwords & keys

[oakkitten]

Looking at how I seem to have figured out a way to upgrade to AGP 8, and not downgrade sshlib while being able to import tink, I think I want to try to polish and merge my earlier experiment of partial encryption of the preferences. The encrypted preferences would be stored in a file, encrypted with a hardware-based key, which should be as good as we can hope for. These preferences can't be restored from a backup due to the irretrievable nature of the key.

This issue is for discussing what exactly we want to encrypt. My current idea would be:

• Relay password
• SSH password
• SSH key (on supported hardware the key is already stored in hardware, but notably Ed25519 keys are stored by serializing them)
• Client SSL certificate We only store these in AKS, not SP
• Media upload basic auth password

Thumbs if agreed, comment otherwise. Note, however, that hosts, usernames, additional fields and headers in the File sharing section are also potentially sensitive information.

ETA: We are also saving open buffers, sent messages, input history, caching media fetch attempts, uploads, and have some internally used (on-device) statistics for sorting shortcuts, etc.