File vault full disk encryption (File vault 2) uses XTS-AES-128 encryption with a 256-bit key to help prevent others from accessing the information in your startup disk without authorization.
Open and set up the file vault
File Vault 2 is available in OS X Lion or later. After opening the File Vault, your Mac will always ask you to log in with your account password.
1. Choose Apple menu ()> "System Preferences", and then click "Security and Privacy".
2. Click the "File Vault" tab.
3. Tap Unlock, then enter the administrator name and password.
4. Click "Enable File Vault".
If other users have accounts on your Mac, you may see a message prompting that each user must enter their password to unlock the disk. Please click the "Enable User" button for each user, and then enter the user's password. The user account you add after opening the file vault will automatically enable this feature.
Please select how you want to unlock the disk and reset the password if you forget the password:
If you are using OS X Yosemite or later, you can choose to use your iCloud account to unlock your disk and reset your password.
If you are using OS X Mavericks, you can choose to store the file vault recovery key through Apple by providing three security prompt questions and answers. Please select an answer that you can surely remember.
If you don’t want to use the iCloud File Vault recovery key, you can create a local recovery key. Please save the letters and numbers of this key in a safe place other than the encrypted startup disk.
If both the account password and the FileVault recovery key are lost, you will not be able to log in to your Mac or access the data on your startup disk.
While you are using your Mac, encryption will happen in the background and will only happen when the Mac is awake and connected to AC power. You can check the progress in the "File Vault" section of the "Security & Privacy" preferences. For any new files you create, they will be automatically encrypted when you save them to your startup disk.
After completing File Vault settings and restarting your Mac, you need to use your account password to unlock your disk and let the Mac complete the startup. Whenever your Mac starts, FileVault will ask you to log in, and will not allow any account to automatically log in.
Close the file vault
If you don’t want to encrypt the startup disk anymore, you can close the file vault:
1. Choose Apple menu> "System Preferences", then click "Security and Privacy".
2. Click the "File Vault" tab.
3. Tap Unlock, then enter the administrator name and password.
4. Click "Disable File Vault".
While the Mac is in use, decryption will occur in the background and will only occur when the Mac is awake and connected to AC power. You can check the progress in the "File Vault" section of the "Security & Privacy" preferences.