If you encrypt locally you don't have to trust the backup server with your privacy. This is doubly an issue when the server is running on some random cloud som

If you encrypt locally you don't have to trust the backup server with your privacy. This is doubly an issue when the server is running on some random cloud somewhere instead of the laptop that you have physical possession of. The trick is deduplication across multiple client. In response to your second comment, that's pretty close, the encrypt step is deterministic (unlike the usual with a unique IV and/or nonce) and compressing after encryption is useless. If your encrypted blocks are not effectively random and incompressible you are doing your encryption wrong.