LocalPolicy signing-key creation and management
When macOS is first installed in the factory, or when a tethered erase-install is performed, the Mac runs code from temporary restore RAM disk to initialise the default state. During this process, the restore environment creates a new pair of public and private keys, which are held in the Secure Enclave. The private key is referred to as the Owner Identity Key (OIK). If any OIK already exists, it’s destroyed as part of this process. The restore environment also initialises the key used for Activation Lock; the User Identity Key (UIK). Part of that process that is unique to a Mac with Apple silicon is when UIK certification is requested for Activation Lock, a set of requested constraints to be enforced at validation-time on the LocalPolicy are included. If the device can’t get a UIK certified for Activation Lock (for example, because the device is currently associated with a Find My Mac account and reported as lost), it’s unable to proceed further to create a LocalPolicy. If a device is issued a User identity Certificate (ucrt), that ucrt contains server-imposed policy constraints and user-requested policy constraints in an X.509 v3 extension.
When an Activation Lock/
ucrt is successfully retrieved, it’s stored in a database on the server side and also returned to the device. After the device has a ucrt, a certification request for the public key that corresponds to the OIK is sent to the Basic Attestation Authority (BAA) server. BAA verifies the OIK certification request using the public key from the ucrt stored in the BAA accessible database. If the BAA can verify the certification, it certifies the public key, returning the Owner Identity Certificate (OIC), which is signed by the BAA and contains the constraints stored in ucrt. The OIC is sent back to the Secure Enclave. From then on, whenever the Secure Enclave signs a new LocalPolicy, it attaches the OIC to the Image4. LLB has built-in trust in the BAA root certificate, which causes it to trust the OIC, which causes it to trust the overall LocalPolicy signature.
All Image4 files, not just Local Policies, contain constraints on Image4 manifest evaluation. These constraints are encoded using special object identifiers (OIDs) in the leaf certificate. The Image4 verification library looks up the special certificate constraint OID from a certificate during signature evaluation and then mechanically evaluates the constraints specified in it. The constraints are of the form:
X must exist
X must not exist
X must have a specific value
So, for instance, for “personalised” signatures, the certificate constraints will contain “ECID must exist”, and for “global” signatures, it will contain “ECID must not exist”. These constraints are designed to ensure that all Image4 files signed by a given key must conform to certain requirements to avoid erroneous signed Image4 manifest generation.
In the context of each LocalPolicy, these Image4 certificate constraints are referred to as the RemotePolicy. A different RemotePolicy can exist for different boot environments’ LocalPolicies. The RemotePolicy is used to restrict the recoveryOS LocalPolicy so that when recoveryOS is booted it can only ever behave as if it’s booting with Full Security. This increases trust in the integrity of the recoveryOS boot environment as a place where policy can be changed. The RemotePolicy restricts the LocalPolicy to contain the ECID of the Mac on which the LocalPolicy was generated, and the specific Remote Policy Nonce Hash (
rpnh) stored in the Secure Storage Component on that Mac. The
rpnh, and therefore the RemotePolicy, change only when actions are taken for Find My Mac and Activation Lock, such as enrolment, unenrolment, remote lock and remote wipe. RemotePolicy constraints are determined and specified at User Identity Key (UIK) certification time and are signed in to the issued User identity Certificate (ucrt). Some RemotePolicy constraints, such as ECID, ChipID and BoardID, are determined by the server. This is designed to prevent one device from signing LocalPolicy files for another device. Other RemotePolicy constraints may be specified by the device to help prevent Security downgrade of the LocalPolicy without providing both the local authentication required to access the current OIK and remote authentication of the account to which the device is Activation Locked.