Service encryption is the baseline. Strong access controls and carefully tested file protection complete the picture.
Know what is already encrypted
Microsoft provides encryption in transit and at rest for SharePoint Online. You do not need to encrypt every library manually to establish that service baseline.
This guidance focuses on SharePoint Online. A self-managed SharePoint Server deployment requires a separate review of your infrastructure and configuration.
Match controls to the risk
Use permissions to define who can reach a site or library. Consider supported sensitivity-label encryption when protection must remain attached to a file beyond its original location.
Do not select advanced key-management options solely because they sound stronger. Document the business requirement, operational responsibilities, licensing, and recovery expectations first.
Test real collaboration
Create a small set of representative documents and user accounts. Verify that authorized staff can open and edit the documents, and that a user outside the permitted group is denied.
Repeat the exercise with your actual guest-sharing workflow and supported applications. Record expected behavior for downloads and offline use rather than assuming every client works identically.
Keep a practical checklist
Maintain an owner for each sensitive workspace, review access regularly, test policy changes before broad release, and keep a documented process for legitimate access problems.
Revisit the design when business processes change. A merger, a new partner, or a migration can change the assumptions behind an otherwise sound protection policy.
Further reading
Validate configuration choices against Microsoft’s current documentation and your tenant’s available licenses.
Microsoft: Data encryption in OneDrive and SharePoint ↗Microsoft: Sensitivity labels for SharePoint and OneDrive files ↗