First make sure you understand how they handle encryption. The features and ease of use come at the cost of some security.
They generate a public/private key pair for the user. The client gets the public key and the server gets the private key. During backups the data is encrypted on the client with a symmetric key (which I believe is generated on the client). The encrypted data is sent to the server. The symmetric key is encrypted using the public key and also sent to the server.
On a restore they use the private key on the server to decrypt the encrypted symmetric key, use that key to decrypt the backup data, and then make the decrypted files available in a zip file that the user can download. The download is over HTTPS so is encrypted in transit.
If you don't like the idea of them having such access to your private key they do offer an option to add additional protection [1]:
> The user’s private key which is stored safely in our data center is protected by a password that is highly guarded. But for some users this is not good enough and we allow the user to secure this file with their own password. When this is done it is impossible to access the data without the user’s password. Unfortunately, this also means we can’t help the user if they ever forget this password so we don’t recommend it for most users.
If you do that then when you restore you have to enter that password on their site when requesting the restore, so their server can decrypt the private key.
They give some more detail in their "Security Question Round-up!" [2]:
> The answer shows a weak point in the Backblaze system. As you prepare a restore, you must type in your private passphrase into the restore server. This is not written to disk, but held in RAM and for the period of time of decrypting all your files, and they are then stored in "clear text" on our very highly secured servers until they are ZIPPED up and offered to you to be downloaded. At that moment you can download them (by HTTPS only), then you can "delete the restore zip" which means you close the window of time that your files are available in plain text.
> So to recap: if you never actually prepare a restore, we cannot possibly know what is in your files, but if you prepare a restore (let's say of a few files) then for the couple minutes they are being prepared and downloaded they are in "plain text" on a HIGHLY SECURE system in the Backblaze datacenter. At that moment, if a Backblaze employee were malicious enough and dedicated enough and was watching (which is actually pretty hard, we get thousands of restores every day so it would fly by quickly) they could see your filenames appear on the Linux servers right before they are ZIPPED up into a new bundle. A few minutes of exposure.
> We actually want to improve this to provide a password encrypted ZIP file for download, and then the FINAL improvement is to actually allow you to download the private encryption key, download the encrypted files, and provide the pass phrase in the privacy of your computer. We hope to add this functionality in the future.
[1] https://www.backblaze.com/blog/how-to-make-strong-encryption...
[2] https://help.backblaze.com/hc/en-us/articles/217664798-Secur...