How to secure the key in IVR?

In our IVR project, one of the feature is user can enter their 4 digit pin, and IVR generate the PIN block, encrypt the PIN block and send to the web service to save database on their end. Now I am facing an issue that how to secure the key for encryption, since the client provide the key for encrypt/decrypt the PIN. I did my research over the weekend, and I found I can use jasypt to encrypt the properties file. But this API is free and cannot guarantee is fully secured. How we can secure the key in IVR?

When I was searching for the same issue, I got help with given URL:

Feature Configuration Guide for Cisco Unified Communications Manager, Release 11.0(1) - Conference Now [Cisco Unified C…