When you enable Shield Platform Encryption for files and attachments, all files and attachments
that can be encrypted are encrypted. The body of each file or attachment is encrypted when it’s
uploaded.
These kinds of files are encrypted when you enable file encryption:
• Files attached to an email
• Files attached to feeds
• Files attached to records
• Images included in Rich Text Area fields
• Files on the Content, Libraries, and Files tabs (Salesforce Files, including file previews, and
Salesforce CRM Content files)
• Files managed with Salesforce Files Sync and stored in Salesforce
• Files attached to Chatter posts, comments, and the sidebar
• Notes body text using the new Notes tool
• Files attached to Knowledge articles
• Quote PDFs

These file types and attachments aren’t encrypted:

• Chatter group photos
• Chatter profile photos
• Documents
• Notes previews in the new Notes tool
• Notes and Notes previews in the old Notes tool

Before you enable Shield Platform Encryption in a production environment, inform users about how it affects your business solution. For example, share the information described in Shield Platform Encryption considerations, where it's relevant to your business processes.

The below Doc have complete information about Shield encryption,

https://blog.bessereau.eu/assets/pdfs/salesforce_platform_encryption_implementation_guide.pdf

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