A Simple Encryption and Decryption function

I wrote this code to learn a little bit about string manipulation. Although I will deffinatly not claim it's uncrackable, it is deffinatly a little less bulky than some of the others. Not to mention since i submited the code anyone could just use the decryption on it. But anyway, it may be of some help, although I will admit it's a bit slow with large files. I am trying to find ways to improve it's speed. Any ideas are welcome.