Best Free Encryption Utility for Cloud Storage

Cloud services, Dropbox for example, take "every precaution" to keep your data secure. They use SSL encryption to make sure that your files are secure in transit. Once your files arrive, they store them encrypted on their servers. They also use internal policies and controls to ensure that employees don't access your files. Of course things can go horribly wrong when you don't have physical control of your documents.

That's where encryption programs like SpiderOak, Tresorit and Viivo come in. They provide client-side encryption to protect your files from access at rest. Or from the more usual threat of a rogue employee gaining access your files. Automatic client-side encryption assures that your files never leave your computer in an unencrypted state.

On-the-fly encryption is the the most convenient way to protect your files the cloud. Your programs have direct access to the unencrypted contents of your files, and the on-the-fly encryption process presents the encrypted files to the cloud. Once on-the-fly encryption is set up, the smooth, client-side conversion requires no direct action by the user. But because encryption adds complexity, it makes local backup even more important.

Notes on "due dilligence"

TrueCrypt is a seasoned product. It fully meets my criteria for selecting encryption software. Unfortunately, it's not a convenient solution in some cases, and it may not work well with the cloud service you choose (see backup trap under True Crypt below).

It's too early to consider Cloudfogger and BoxCryptor seasoned, and they are not open-source software, so while I like all the other indicators I've seen, I'm not ready to declare them fully vetted. On the other hand, your encryption program and your cloud service must both be compromised at the same time to expose your files. Your may feel that the risk of joint compromise may be low enough for you to put your files in the cloud using one of these products.

Viivo and Wuala are not open source either, but they are seasoned products from a source whose business is encryption-centered.

 Cautionary Notes

1. Recent revelations about NSA crippling, and/or hacking encryption software are sobering when you consider storing or transferring data via the Internet. I do not think that it is prudent to trust any of the products listed here to protect information that you would not want agents of any government to have access to. Not unless you have means to independently valididate them. It seems reasonable at this point to trust them for protection from civilian attacks. Example: TrueCrypt might be the most secure alternative for cloud storage security, but "nobody knows" yet (or about any other encryption software/service for that matter).
2. Operating systems are messy: Echos of your personal data -- swap files, temp files, hibernation files, erased files, browser artifacts, etc -- are likely to remain on any computer that you use to access the data. It is a trivial task to extract those echos.
   For example, when you encrypt and compress files, clear-text versions that existed before you compress/encrypt the file or clear-text copies that are created after you decrypt/decompress it remain on your hard drive. Unless you purge -- not just delete -- those clear-text files. :-(
3. The fact that an encryption program "works" does not mean that it is secure. New encryption utilities often appear after someone reads up on applied cryptography, selects or devises an algorithm - maybe even a reliable open source one - implements a user interface, tests the program to make sure it works, and thinks he's done. He's not. Such a program is almost certain to harbor fatal flaws.
   1. "Functionality does not equal quality, and no amount of beta testing will ever reveal a security flaw. Too many products are merely buzzword compliant; they use secure cryptography, but they are not secure." --Bruce Schneier, in Security Pitfalls in Cryptography
4. It is possible to inadvertently upload unencrypted files to cloud services using some of the solutions described here. See the notes under BoxCryptor and Viivo in the discussion below.
5. Further advice about how to use encryption are discussed in Encryption is Not Enough, including what you need to do beyond encryption to be sure your private data is not lost or exposed.

SpiderOak is not just an encryption program. It combines client-side encryption with 2 GB of free cloud storage (more storage is availale for a fee). In other words, you don't need a separate cloud-storage service. SpiderOak also provides sync between PCs and portable devices in addition to backup. In sum, SpiderOak provides encryption backup, sync and storage space. Backup and sync can be automatic.

Your files are unencrypted on all your synced devices, but are always encrypted for transmission and storage in the cloud.You can use SpiderOak for as many folders as you like. Of course you can use up the free 2 GB pretty quickly, but it is inexpensive to get more. It is challenging to discover all the functions of SpiderOak intuitively, but they have excellent "getting started" guides and a users manual.

The SpiderOak statement on privacy and passwords is a good example of what you should look for to evaluate the security of any encryption service for cloud storage. In particular, be very leary of any service that offers password recovery. If there is a mechanism for password recovery, it is likely your data on the server is also accessible to a determined hacker or agency.

I have have been using SpiderOak for the small set of sensitive files that I want to backup and sync. One thing to understand is that SpiderOak breaks files into blocks so that only the changed or added sections of files need to be stored. That way many versions of the file by just storing the incremental blocks. It offers fine-grain control of the backup/sync process, which helps you stay within the 2 GB of free storage. It's a bit tricky to use SpiderOak until you get used to how it processes backups and syncing.

Wuala is similar to SpiderOak, and also provides selective sharing by file or folder. They offer a more-generous 5 GB free storage. I had intended to use Wuala, but it requires Java, which exposes your PC to a seemingly perpetual string of serious vulnerabilities. I truncated my evaluation after learning that it worked well.

I liked the general capabilities of Wuala, and judging from some papers they have written it is likely that their encryption is sound. If you're willing to live with the hazards of Java, it is nice to work with.

Wuala uses AES- 256 for encryption, RSA 2048 for signatures and for key exchange when sharing folders, and SHA-256 for integrity checks, which is good in principle, but keep in mind that AES is an NSA "approved" algorithm. Their servers are in Switzerland, Germany, and France, which may offer you more privacy.

Tresorit is the latest significant entry in this "client plus cloud" active encryption arena, with a free 5 GB plan. The Tresorit interface is gloriously simple, and they describe their approach to encryption quite well. It could be the most secure one on this list. They have an impressive analysis of why they doubt that Tresorit has been hacked. Being based in Switzerland doesn't hurt either. ;)

Tresorit may be little puzzling when you first set it up. You might think nothing is happening. Their support is comprehensive and well written though, so you should be able to figure out how to get Tresorit going. I've been using Tresorit for my most important data since September, 2013, and it has performed flawlessly. There is a sizable development team at Tresorit, and they are actively introducing new apps and features. They plan to implement file versioning soon, for example. [whitepaper]

SafeMonk is similar in operation to Cloudfogger, but quite a bit simpler to use. It is a bit unsual in that it uses public/private encryption instead of shared key. That enables a very flexible sharing capability that works on a folder-by-folder basis. SafeMonk presents the same hazard that many cloud encryption products do. If you copy a file to the SafeMonk folder without SafeMonk running it will be unencrypted on your computer and in the cloud.

BoxCryptor and Viivo provide most of the same functions that Cloudfogger does. They are integrated with the file-system in a different way though. Both use an encrypted virtual-drive interface that is linked to an ordinary folder. They encrypt a single folder, and augment it with the virtual-folder overlay to give cleartext access. With this approach, you work directly with an unencrypted local files, which is faster, but not as secure against local attack.

Their two folder approach also leaves users open to fatal mistakes. All files to be encrypted must be placed in the unencrypted local folder. or they will not be encrypted in the cloud-facing folder. Any files placed directly in the encrypted folder will not be encrypted. That could be hard to remember, and there is no warning or other indication of mistakes.

TrueCrypt is a top-rated product for most uses, but there is a potential backup trap when it is used for files that will be synced or stored in the cloud.

Encryption programs that create encrypted "volumes" (files that contain encrypted files) do not change the size of the volume (container file), and often - intentionally - do not change the modified date of the volume, even though files in the volume have been changed or added. The result can be that your cloud service does not recognize that the volume file has changed, and will fail to update the online copy.

TrueCrypt is an example of an encryption program that does not change the modified date of volume files (encrypted file container). However, some cloud backup services - Dropbox for example - check the hash value of volume files, not the date, and if that changes Dropbox stores the latest copy of the volume file. If you're using Dropbox, that makes TrueCrypt an excellent way to implement client-side encryption for your most sensitive files. SkyDrive, monitors the modified date - not a hash value - so TrueCrypt volumes are not updated in the cloud by SkyDrive after their content changes client-side.