Quoting from the site: http://www.ccavenue.com/demo_ccav/integrate2.htm
The Real Time Success Failure Feature works on the basis of Checksum verification. The Checksum refers to a random numeric string generated using a mathematical algorithm (a complex quadratic equation) to ensure that data is not tampered along the way. The way it works is lets say a message has to be sent from A to B. A and B both mutually agree on a Key that only both of them possess. A checksum is calculated with a mathematical function using the parameters passed and the Key as input. This checksum is then sent along with the message to point B. B then recalculates this checksum using the Key and the same algorithm. If the checksum that B calculates is different from the checksum that A has passed then the data has been tampered with along the way.
Hope this helps!