Why am I getting package javax.crypto does not exist

Beware! This class is only included in the jdk from oracle. That means your program won;t be portable among different jdk (eg openjdk) It is preferable to use this lib externally, it can be downloaded from here

OK, this was a mistake on my part. The Ant file I was using to compile the code had this attribute on the javac task:

bootclasspath="${java.home}/lib/rt.jar" 

Doh. You can add the JCE jar like this:

bootclasspath="${java.home}/lib/rt.jar:${java.home}/lib/jce.jar"