Encrypting files uploaded by users

Hello everyone,
I am working on an app for an accounting firm that would allow their end clients to upload accounting data for the accounting firm to treat it. In return, the accounting firm would upload accounting documents in the tool for the clients to retrieve.
These files are quite sensitive and would need to be encrypted both when the end client submit their data and when the accountant submits the end result accounting data.
Any suggestion on how I should go about it ?
Thank you for your help.

What do you want to be protected against? What the medium to send files?

Hi @redvivi , the users will upload accounting data into the app and these files will be saved to Amazon S3 which I understand is not encrypted.
However, this is sensitive information that needs to be encrypted. I hope that clarifies.

It is encrypted, see Amazon S3 Security Features - Amazon Web Services, Encryption section.

Should you require additional encryption, it means you would protect yourself against an insider at AWS who might have access to your buckets, or somebody external who may have an unauthorised access to said buckets (e.g. AWS hacking or credentials misconfiguration/compromised).

Thank you redvivi.