Granular file access with PII masking

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Is your feature request related to a problem? Please describe.
In many real-world deployments (especially enterprise and multi-tenant environments), files stored in Nextcloud may contain sensitive or personally identifiable information (PII).

While Nextcloud already provides strong access control and sharing mechanisms, access is currently binary at the file level (read/write/share). This creates a limitation:

• Users who need partial access to a file must be granted full visibility
• There is no built-in way to restrict or mask sensitive sections of a document
• Compliance requirements (e.g., GDPR) often require controlled data exposure rather than full access
  This leads to workarounds such as duplicating files, manually redacting content, or restricting access more than necessary, which reduces usability and increases operational overhead.

Describe the solution you'd like
I would like to propose an access-level aware file interaction layer with optional PII masking capabilities, including:

1. Granular Access Levels

• Extend permissions beyond read/write to include: View-only (no raw download), Redacted view, Editable with restricted fields (future extension)
• Potential integration with roles or groups

2. PII Detection & Masking Layer

• Automatic or configurable detection of sensitive entities: Names, emails, phone numbers, IDs, financial data, etc.
• Masking strategies: Full masking (e.g., *), Partial masking (e.g., e@mail.com), Role-based visibility (e.g., admins see full, others see masked)

3. On-the-fly Rendering

• Masking should be applied dynamically during file preview/rendering
• Original file remains unchanged
• Works with: Text-based files, PDFs, Office documents

4. Policy Engine (maybe)

• Define rules like: “Users in group X can only view masked versions”, “External shares always enforce masking”
• Could integrate with existing sharing and permissions logic

5. Auditability

• Log when masked vs unmasked data is accessed
• Helpful for compliance and monitoring

Describe alternatives you've considered
I have tried manual redaction before upload, however it is error-prone and not scalable. Also, maintaining multiple file versions (masked and unmasked) is cumbersome. I couldn't integrate my local solutions. Plus, instead of masking restricting access entirely complicates workflows, not effective at all.

Additional context
This feature would significantly improve document management, privacy-first deployments, multi-role collaboration scenarios, and GDPR compliance-sensitive environments.

If there are existing ongoing efforts in this area, I’d be happy to collaborate or align with them.

No files, tests, or entry points are named. Start by reviewing Nextcloud’s existing sharing and permissions logic alongside file preview/rendering, then determine whether the proposed access levels, PII detection and masking, policy rules, and audit logging fit the server repository. Done would require an agreed scope and implementation across the requested document types.