Having sat through a course and done all the math and derivations for graduate level cryptography, I strongly suggest you learn the basics of cryptographic functions and how to reason about them in a rigorous way. The symmetric stuff isn't that difficult. There are a lot of issues even in the responses here; I don't want to call anybody out in particular but there are way more fundamental issues involved than entropy. Consider something like Bruce Schneier's Applied Cryptography as a good introduction to the field that can be had for cheap.
Some basic problems that are notable immediately:
* We don't have a working definition of what constitutes secure.
* Mersenne Twister isn't a cryptographically secure RNG and is thus highly inappropriate for this type of use.
* The mathematical properties of XOR make the multi-pass thing totally ineffective, as mentioned previously.
* XOR the whole message against some value is guaranteed to return some correct bytes of the message.