Volume encryption with FileVault in macOS
Mac computers offer FileVault, a built-in encryption capability, to secure all data at rest. FileVault uses the AES-XTS data encryption algorithm to protect full volumes on internal and removable storage devices.
FileVault on a Mac with Apple silicon is implemented using Data Protection Class C with a volume key. On a Mac with the Apple T2 Security Chip as well as a Mac with Apple silicon, encrypted internal storage devices directly connected to the Secure Enclave leverage its hardware security capabilities as well as that of the AES engine. After a user turns on FileVault on a Mac, their credentials are required during the boot process.
Internal storage with FileVault turned on
Without valid login credentials or a cryptographic recovery key, the internal APFS volumes remain encrypted and are protected from unauthorized access even if the physical storage device is removed and connected to another computer. In macOS 10.15, this includes both the system volume and the data volume. Starting in macOS 11, the system volume is protected by the signed system volume (SSV) feature, but the data volume remains protected by encryption. Internal volume encryption on a Mac with Apple silicon as well as those with the T2 chip is implemented by constructing and managing a hierarchy of keys, and builds on the hardware encryption technologies built into the chip. This hierarchy of keys is designed to simultaneously achieve four goals:
Require the user’s password for decryption
Protect the system from a brute-force attack directly against storage media removed from Mac
Provide a swift and secure method for wiping content via deletion of necessary cryptographic material
Enable users to change their password (and in turn the cryptographic keys used to protect their files) without requiring reencryption of the entire volume
On a Mac with Apple silicon and those with the T2 chip, all FileVault key handling occurs in the Secure Enclave; encryption keys are never directly exposed to the Intel CPU. All APFS volumes are created with a volume encryption key by default. Volume and metadata contents are encrypted with this volume encryption key, which is wrapped with the class key. The class key is protected by a combination of the user’s password and the hardware UID when FileVault is turned on.
Internal storage with FileVault turned off
If FileVault isn’t turned on in a Mac with Apple silicon or a Mac with the T2 chip during the initial Setup Assistant process, the volume is still encrypted but the volume encryption key is protected only by the hardware UID in the Secure Enclave.
If FileVault is turned on later—a process that is immediate since the data was already encrypted—an anti-replay mechanism prevents the old key (based on hardware UID only) from being used to decrypt the volume. The volume is then protected by a combination of the user password with the hardware UID as previously described.
Deleting FileVault volumes
When deleting a volume, its volume encryption key is securely deleted by the Secure Enclave. This prevents future access with this key even by the Secure Enclave. In addition, all volume encryption keys are wrapped with a media key. The media key doesn’t provide additional confidentiality of data, but instead is designed to enable swift and secure deletion of data because without it, decryption is impossible.
On a Mac with Apple silicon and those with the T2 chip, the media key is guaranteed to be erased by the Secure Enclave supported technology—for example by remote MDM commands. Erasing the media key in this manner renders the volume cryptographically inaccessible.
Removable storage devices
Encryption of removable storage devices doesn’t utilize the security capabilities of the Secure Enclave, and its encryption is performed in the same manner as an Intel-based Mac without the T2 chip.