How to refer kubernetes secrets in properties file?

We have a properties file where all our application configuration is there, in key-value pairs.

All the key-value pairs is in single config.properties file.

So, I kept that data in configmap and linked to this file in kubernetes.

Working fine so far.

But, to increase security, we want to keep some key-value pairs as secrets.

But, how to refer those secrets in the config.properties file without changing any application logic.

Please suggest.