You can encrypt the contents of these standard field types.
On the Account object:
Account Name
Description
Fax
Website
Phone
On the Contact object:
Description
Email
Fax
Home Phone
Mailing Address (Encrypts only Mailing Street and Mailing City)
Mobile
Name (Encrypts First Name, Middle Name, and Last Name)
Other Phone
Phone
On the Case object:
Subject
Description
On Case Comments:
Body (including Internal Comments)
Encrypted Custom Fields
You can encrypt the contents of fields that belong to one these custom field types:
Email
Phone
Text
Text Area
Text Area (Long)
URL
Date
Date/Time
Important
Once a custom field is encrypted, you can’t change the field type. For custom phone and email fields, you also can’t change the field format.
You can’t use currently or previously encrypted custom fields in custom formula fields or criteria-based sharing rules.
You can’t use Schema Builder to create an encrypted custom field.
Some custom fields can’t be encrypted:
Fields that have the Unique or External ID attributes or include these attributes on previously encrypted custom fields
Fields that are used in custom formula fields
Fields on external data objects
Fields that are used in an account contact relation
Cheers!!!