Save encrypted data in server . With no way for server to read actual data

Question

Hello i am developing an app that collects some private data from my clients. My clients wont like it if i am able to read the data. So the data is encrypted in cleint side with a key say "xyz",which is auto generated when client install the app/make a account and only with this key can someone read this data. Now when the client login from a new device i want the client to generate the key, and use it to retrieve the data . How do i do that? I cant save xyz in server since then i will have both key and encrypted data, I cant generate xyz with password or User details since i will have them and i knows how to create the key from that data

Is there a way to achieve this without saying to client Trust me am truest worthy. Or make your own key!?

Answer 1

The usual way is to use a two-level key.

You encrypt the data with key_data, which should probably be a symmetric key.

You keep a separate key-file where key_data is encrypted with the key from the devices (key_dev1, key_dev2, ...)

You do not know the key_data; it must be generated by the first access. You also do not know the key_dev1,... because they are on the device of the client.

The client can decrypt the key_data with an existing device, and pass it to a new device which encrypts it with its new device key before uploading it to your key-file.

Of course, something needs to be done with key management by your client; you may want to facilitate this. Be careful that you never get the decrypted key_data on your devices though.