One of the new features that appeared with Macs that shipped with Lion and Mountain Lion was Apple’s Internet Recovery. If you encounter a situation in which you cannot start from the Mac’s Recovery HD partition, such as where the internal hard drive has failed or when you’ve installed a new disk without an OS on it, Mac models that were released after July 2011 can use Internet Recovery. Internet Recovery lets you start your Mac directly from Apple’s servers using a NetBoot-like process and gives you the same functionality that Recovery HD does.
Because Internet Recovery has the same capabilities as your Mac’s Recovery HD partition, it can be used to unlock or decrypt a FileVault 2-encrypted Mac. This is potentially valuable in case of emergency, as it means that you can do recovery of a FileVault-encrypted drive even in a situation where the Mac’s Recovery HD partition has been damaged or corrupted in some way.
To boot to Internet Recovery, start up your Mac and hold down Command-Option-R on your keyboard.
You should see a gray screen with an animated globe appear. It should say something like “Starting Internet Recovery. This may take a while.” Depending on your connection speed, it may also switch to a countdown clock to show you how long until it’s fully booted.
Once booted to Internet Recovery, you should see the Recovery interface.
From there, you use the methods described in the links below to unlock or decrypt your FileVault 2 encrypted Mac: