Per Salesforce documentation, Shield Platform Encryption gives your data a whole new layer of security while preserving critical platform functionality. It enables you to encrypt sensitive data at rest, and not just when transmitted over a network, so your company can confidently comply with privacy policies, regulatory requirements, and contractual obligations for handling private data. Shield Platform Encryption builds on the data encryption options that Salesforce offers out of the box. Data stored in many standard and custom fields and in files and attachments is encrypted using an advanced HSM-based key derivation system, so it is protected even when other lines of defense have been compromised.

There are certain platform limitations that prevent us from fully supporting this feature.

For example, fields referenced in the WHERE, GROUP BY, or ORDER BY clauses in a SOQL query are not eligible for encryption. Additionally, Salesforce will prevent AppExchange applications from being installed if there are existing encrypted fields in your organization that need to be referenced in the SOQL queries of a packaged component.

App Specific Considerations:

Case Merge Premium

For Probablistic and Deterministic Encryption, the following fields cannot be encrypted:

• Case.Subject

Email to Case Premium

For Probablistic Encryption, the following fields cannot be encrypted:

• Account.Phone
• Contact.Email
• Lead.Email
• Email Message.To Address

For Deterministic Encryption, the following fields cannot be encrypted:

• Account.Phone
• CaseComment.Body
  • Typically assigned a maximum value of 4000 characters, the Comment Body field on Case Comment is reduced to a maximum of 2959 characters when encrypted. If you plan on encrypting the Comment Body field, contact Internet Creations support to change the maximum size of Comments from 4000 to 2959.

Some additional information / limitations on encrypting fields can be found here.