You can encrypt a custom text field provided by Encryption for Jira. There are two types of text fields: Single line text and Multiline text. These custom fields are text type fields and you can find it in the Administration menu under Issues.
Creating a new custom field for encryption purposes is performed in the exact same way as any other custom field in Jira.
After giving a name to your custom field, choose the appropriate screen(s) you want to see this newly created Encrypted Text Field to appear using the Screen checkboxes.
Once created, the value of the text field will be encrypted within the Jira Database.
By default, Encrypted Custom Fields have the Encrypted Custom Field Searcher template.
Users can search for texts in the same way as they do with JIRA's native text custom fields.
To be able to index the fields, Lucene stores the decrypted values. Therefore, a user with access to the file system could access the Lucene indexes and see the values of the fields.
If this is a problem, you can unlink the field from the searcher by editing the field and selecting the Search Template "None".
In this way, Lucene will not store any decrypted value, but neither any user will be able to search by this custom field in a filter.