The joint concepts of Secure and Private are relative and subjective. Relative in that there are very few absolutes, but there are an infinite number of variations that may be better or worse. Qualifying “better or worse” is where the subjective comes into play. It is subjective in terms of who / what you are trying protect your files from. Is it your family, co-workers, your neighbors, the Internet, some large corporation trying to characterize you (in order to better sell to you), or the government? Depending on how good of a solution and who you are trying to protect your privacy from, we can look at a few easy (and practical) solutions.
Off the Shelf
There are off the shelf solutions that provide file sharing options. Dropbox, Box and Google Drive are three popular examples or cloud storage solutions – meaning your files are on their servers. Each one of these provides some degree of privacy / security. Each of these services use a username / password to restrict access, and additionally Google and Dropbox support two factor authentication using Google Authenticator. Each of these services uses SSL/TLS to provide a secure channel from the client to their servers. What they do not provide is any explicit privacy or security from the respective services or anybody with a NSL.
Fundamentally these services are not particularly private, secure, but they do provide some degree of security / privacy. If you use them and Two Factor Authentication is an option – use it.
A Better Option(s)
If the convenience of these services is appealing, but you have some real need for something more secure, we have a better solution. TrueCrypt is an disk encryption tool that can create secure containers for files. Specifically, Truecrypt can be used to create a secure file container in your GDrive/Dropbox/Box sync directory on your client system. This container can be opened by Truecrypt, files placed inside, and then be closed – at which point the service will sync the file up to their servers. They services will have access to the file, but its contents will be completely hidden from all except the keyholder. Note – a large container will hold lots of files, but the entire file will need to be synced even if there is a minor change – so consider wisely how large / small this container should be.
Another tool is Keepass, a secure password locker that is similar – but only for password / account information. Both of these tools are also cross platform and open source.
An Even Better Option
One of the core flaws with each of these cloud storage solutions identified above (as examples) is that ultimately all of your data resides on their servers within the providers data centers. BitTorrent Sync is a solution that breaks that paradigm by distributing files using the bittorrent protocols in a peer to peer (P2P) fashion. The result is that files can be distributed and shared between multiple users / platforms, but they do not exist on any cloud server – greatly reducing the risk of compromise-ever. BitTorrent Sync is easy to setup and use. Specifically, the app is installed and then you can create a share – and then generate a key – initiating a share. If you are connecting to an existing share, you create a share and provide the key for that share, and it will automagically be synced from the other clients on that share.
The most significant upside (other than P2P architecture) is that there are no storage or transfer limits – the only limitation being your local capacity.
The only significant downside to BitTorrent Sync is that synchronizations must be synchronous – since there is no cloud storage server, it requires that at least two members be online to synchronize.
For the truly insecure, TrueCrypt can be used on top of BitTorrent Sync.
These are a few examples of how to secure / privatize file sharing on the Internet using relatively non-private services coupled with a few open source applications. However, it is very important to understand key management – since this security / privacy is only as secure as the keys you use to contain it. The applications themselves are fairly mature, well reviewed and generally accepted as secure.