You only need Client-Side Encryption if you are using the Free add-on, or the Pro add-on with Direct Connection feeds. Client-Side Encryption is not used for Recurring Payments or Responsive Shared Page feeds.
If credit card details are entered on your website and not on an eWAY shared page, then you should set the Client Side Encryption key. Sensitive credit card details will then be encrypted in the browser, so that only eWAY can see them. All websites using the Direct Connection method are encouraged to set the Client-Side Encryption key for improved security of credit card details.
If you get the following error, you must add your Client-Side Encryption key:
V6111: Unauthorised API Access, Account Not PCI Certified