The private tag uses the Xtea encryption algorithm.  This a relatively secure encryption technique though there are some weaknesses.  See http://en.wikipedia.org/wiki/XTEA for more information.  Support for this algorithm is a built-in feature of iKog.

 

The code for the algorithm base on the public domain code by Paul Chakravarti; see http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/496737 .