If you wish to take advantage of an extra layer of security when collecting data you can use secure links (https). https links use what is called Secure Socket Layer to transport data safely between client and survey by using an encryption algorithm.
You can set this up on a survey-by-survey basis on the Share tab. Click to option to Customize below the Primary Share Link, select Secure, click Done and Save. Your link will now be using the https protocol; ensuring that the data collected in your survey is secure.
Force Secure (https) Links
Account Administrators can choose to set this up at an account level thus forcing all links created to be secure by default. To do so, go to Account > Account Settings and select the option to Force Secure (https) Links. This will force all links for newly created surveys to https. This will not affect any existing survey share links.
Secure Private Domain Links
If you have a private domain set up, you'll notice that you cannot create a link that is both secure and uses your private domain. Secure Private Domains require a custom set up.