Sealed Key Protection (SKP)
On Apple devices that support Data Protection, the key encryption key (KEK) is protected (or sealed) with measurements of the software on the system, as well as being tied to the UID available only from the Secure Enclave. On a Mac with Apple silicon, the protection of the KEK is further strengthened by incorporating information about security policy on the system, because macOS supports critical security policy changes (for example, disabling secure boot or SIP) that are unsupported on other platforms. On a Mac with Apple silicon, this protection encompass FileVault keys, because FileVault is implemented using Data Protection (Class C).
The key that results from entangling the user password, long-term SKP key, and Hardware key 1 (the UID from Secure Enclave) is called the password-derived key. This key is used to protect the user keybag (on all supported platforms) and KEK (in macOS only), and then enable biometric unlock or auto unlock with other devices such as Apple Watch.
The Secure Enclave Boot Monitor captures the measurement of the Secure Enclave OS that's loaded. When the Application Processor Boot ROM measures the Image4 manifest attached to LLB, that manifest contains a measurement of all other system-paired firmware that's loaded as well. The LocalPolicy contains the core security configurations for the macOS which are loaded. The LocalPolicy also contains the nsih field which is a hash of the macOS Image4 manifest. The macOS Image4 manifest contains measurements of all of the macOS-paired firmware and core macOS boot objects such as the Boot Kernel Collection or signed system volume (SSV) root hash.
If an attacker is able to unexpectedly change any of the above measured firmware, software, or security configuration components, it modifies the measurements stored in the hardware registers. The modification of the measurements causes the crypto-hardware-derived system measurement root key (SMRK) to derive to a different value, effectively breaking the seal on the key hierarchy. That causes the system measurement device key (SMDK) to be inaccessible, which in turn causes the KEK, and thus the data, to be inaccessible.
However, when the system isn’t under attack, it must accommodate legitimate software updates that change the firmware measurements and the nsih field in the LocalPolicy to point at new macOS measurements. In other systems that attempt to incorporate firmware measurements but that don’t have a known good source of truth, the user is required to disable the security, update firmware, and then reenable so that a new measurement baseline can be captured. This significantly increases the risk that the attacker could tamper with firmware during a software update. The system is helped by the fact that the Image4 manifest contains all the measurements needed. The hardware that decrypts the SMDK with the SMRK when the measurements match during a normal boot can also encrypt the SMDK to a proposed future SMRK. By specifying the measurements that are expected after a software update, the hardware can encrypt an SMDK, which is accessible in a current operating system, so that it remains accessible in a future operating system. Similarly, when a customer legitimately changes their security settings in the LocalPolicy, the SMDK must be encrypted to the future SMRK based on the measurement for the LocalPolicy, which LLB computes on the next restart.