The first method that came to mind (since I will be the only one encrypting or decrypting) was to XOR the files bit-by-bit (or byte-by-byte depending on how the i/o stream works) with some key. Basically a Symmetric Key Cipher.
I'm not sure how to generate the key though. Could I just use a password-length key and do the following?
for (i=0; i<length(plaindata); i++) encrypted = key[i%length(key)]^plaindata;
It seems simple enough, secure enough, reversible, and at first glance I don't see anything horribly inefficient about it.
However, I've never done anything like this before so I wouldn't be surprised if I was doing something silly or pointless.