This security option allows you to pass additional data to the survey using DES encryption. You can pass data through the survey URL in an encrypted format, the data will be decrypted and saved as part of the response.
This authentication requires 8 character key that is used to encrypt or decrypt the custom variables.
For setting the DES Encryption go to:
Login >> Security >> Survey Authentication
For sample code you can refer this following link under DES Encryption Custom Variables section.
This feature is available with the following licenses :