If by encrypt you mean using MD5 algorithm, and by data you mean parameter values or body message, you could use Jmeter-Plugins' md5 function ( http://jmeter-plugins.org/wiki/Functions/).
The articles below, from JMeter official site, talks about writing your own plugin. https://jmeter.apache.org/extending/jmeter_tutorial.pdf http://wiki.apache.org/jmeter/BuildingPlugInWithEclipse Checkout JMeter and JMeter-Plugins source codes and have a look at them.
