Ongoing File Encryption / Efficiency

I'm looking for advice/ideas in regards to ongoing file encryption.I have an event that fires frequently that contains small amounts of data passed through the eventargs.This data is then written out to disk (currently unencrypted).I need to encrypt this data (which, I can do).My attempt in the first five minutes was unencrypting the file, appending the data and reencrypting the file.This obviously works, but is pretty inefficient and not going to be my solution since the file could grow rather large and the resources to unecrypt/ encrypt grow larger as the file grows larger.My next thought was to encrypt each set of data as it's written out and have some kind of marker in the file to denote where to split so I could unencrypt each chunk to reproduce the original file.I'm leaning towards this approach now (though by encrypting small chunks of data multiple times, the file size will be considerably larger.As an example, if I encrypt each letter of the alphabet the file is approx 8 times larger than if I encrypted a string with all of the letters of the alphabet in this case each files decrypted content would be identitcal).