The sci.crypt faq has more information. http://www.contrib.andrew.cmu.edu/~shadow/crypt.html

http://www.faqs.org/faqs/cryptography-faq/

It might be an idea to read about different attacks - known plaintext etc - and seein if your system is still secure. (It won't be, so if you can't find the flaw you know you need to look more).