HMAC stands for hash-based message authentication code. This authentication is a product of a hash function applied to the body of a message along with a secret key. 
Key: A 36 character key that is used for hashing the time in seconds.
Timestamp: The time window for which the survey URL will be valid. You'll need to pass the following security token fields via the URL: surveyID, ts, hash, mode.
To set up HMAC-SHA1 security, go to:
Login »  Surveys »  Edit »  Security
 - Under Survey Authentication , select HMAC-SHA1 
 
 - Enter the details for the fields: Key, Time Stamp, and check the box for Tamper Proof URL if required.