How secure is Time Machine backups?
There is no inherent security in your Time Machine backup. Were it to be stolen, it could be used to recover all your backed-up files. They don’t necessarily need a Mac. There are techniques to encrypt the backups, but they are not for the non-technically inclined. If you are really worried, you might get an external hard drive with built-in encryption.
Time Machine backups are not encrypted, but you can use Apple’s FileVault (built into Mac OS X) to encrypt your entire home directory. Everything in your home directory will stay encrypted in the backup. There may still be ways to get at some of the data (unencrypted caches/temp files, contents of memory, etc…), but this will certainly prevent the “random guy plugs his mac into your HD” situation and most any not involving a highly motivated attacker. You can also create an encrypted disk image if you want to only encrypt some files instead of your whole home directory (for performance).
To clarify, there are certain limitations when using FileVault and Time Machine together. Most significantly, it will only backup your home directory when you are logged out. Logging out will also ensure that no one with physical access to your computer can (easily) access your files.
Time Machine backups are not encrypted, unless the files you are backing up are themselves encrypted. You can combine Time Machine and FileVault in a limited way. The two main drawbacks are that (1) the encrypted home folder will only be backed up after you log out and (2) you can only restore the entire home folder, not individual files or subfolders. Technically there is a way around (2), but it’s fairly technical. It is possible, though, that Apple will enable individual file and folder restore of FileVault protected data in 10.6 (“Snow Leopard”). If you only have a few particular files that you need to keep encrypted, then I would recommend a file-oriented encryption program like MacGPG2.
I don’t really understand your reason for rejecting FileVault:Yes, definitely FileVault is not the best choice, because I need to access to backed up files. So it will be useless if I need to restore all my home folder when I need something. Or at least, the reason I think you’re stating is not correct. If you encrypt with FileVault and then backup with Time Machine, at least to the best of my knowledge, you can still access individual files from the backup volume. It just involves an extra step, versus recovering from an unencrypted volume: you need to mount the “disk image” that contains the encrypted copy of your home folder. See this hint; relevant portion reprinted below:If you need to restore files in your encrypted home directory, the Apple warning is correct in that you can’t use the Time Machine application’s ‘galaxy’ interface to do so. However, you can restore them using the Finder. Double-click on y
