I like your attitude, so I gave you a ++. Some people get really touchy about their home-grown ciphers. It might be a good idea to include a comment on your thoughts about the cipher's strength, if you're going to be distributing it.

...a key with 20+ digits will be very difficult.

The central problem with your cipher is that there is very little diffusion of key information, which allows me to attack the key bytes one at a time. Adding more key bytes only results in a linear increase in the difficulty of attack. In a properly diffused cipher, it would increase exponentially. Of course, more key bytes means more diffusion required... take a look at CAST5 or AES sometime. There's a lot to be learned by looking at existing ciphers, even though they appear very confusing.