I see your point lowflyinghawk, but I still think that having encryption of the data stored to S3 is a rather good idea. It is true, that using encryption requires a bit more thought with regards to key and algorithm handling. My guess is that there are quite a lot paranoid people out there that would enjoy such a feature. (I for one would like the added security). But you might have a very relevant point in whether this is something an rsync-like program should do, it would certainly make s3sync.rb more a dedicated backup-program.

With regards to the storage of the unencrypted md5 sum, ferrix, I have noticed that s3sync.rb creates nodes for each directory. This might be an appropriate place for storing the unencrypted md5sum, rather than having to issue separate HEAD requests for each file. But then again, I might have missed the point entirely :-)