secure messaging

I have a Chrome Extension that talks to my CI based PHP application. They talk via XMLHttpRequest.  I am concerned with securing the communication between the two. How can I encrypt (or hash) a message in PHP that the javascript script can understand, and how about the other direction? This is not super secure data ie no ss#, birthdays , no pii. But I do send email addresses and sometimes a photo.

proof that an old dog can learn new tricks