how to do this base64encode in xml element value using string manipulation function?

how to do this base64encode in xml element value using string manipulation
function?

I have this xml file, and I need to fill out the hash element with the
following formula
<?xml version="1.0" encoding="UTF-8"?>
<data>
<time>xxxx</time>
<id>12345678</id>
<code>abcd</code>
<hash>base64encoded(a XOR b)</hash>
<data>
with
a = {time} + {last 4 digit of id} => xxxx5678
b = {reversed last 4 digit of id} + {code} => 8765abcd
Anyone can point me How I can do this ? No idea at all about this problem.
Really appreciate your help.
Thanks very much.
Kind Regards, LL