Inside every Intel Mac with a T2 chip, and every Apple silicon Mac, is a secure enclave, originally referred to as its security enclave. The subject of a flurry of Apple’s patents from 2012 onwards, this was introduced in the A7 chip inside the iPhone 5s and iPad mini 3, 12 years ago in September 2013, where it brought biometric authentication in Touch ID.

iPhone 5s

Protecting the most important secrets in a computer is a great challenge. No matter how secure you try to make the main processor and memory, as they’re exposed to direct attack, isolation can only be relative and temporary. An alternative approach is to move the most secure data and its processing into a secure enclave and its processor, and that’s the architectural solution chosen by Apple in what it patented as a security enclave, filed in September 2012, a year before its release in the iPhone 5s. Engineers credited for that patent are Manu Gulati, Michael J Smith and Shu-Yi Yu.

Successive iPhone chips steadily improved their secure enclaves, and by the time the iPhone 7 was introduced in September 2016, with its A10 Fusion chip, its secure enclave was handling encryption and authentication but not replay prevention. It also had EEPROM secure storage, and an AES engine with DPA protection and lockable seed bits. When the first Intel Mac with a T1 chip was released a couple of months later, that was based not on the A10 but the S2 used in the Apple Watch Series 2. The T1 thus doesn’t really have a secure enclave as such, although it supports Touch ID.

An early and thorough account of these secure enclaves was presented by Tarjei Mandt, Mathew Soling and David Wang at Black Hat USA in 2016. This appears to be the only such account apart from the section in Apple’s Platform Security Guide, most recently updated in December 2024. Apple’s engineers continued to gain new patents, covering trust zone support (filed in 2012), key management (filed in 2014), and most relevant to Macs, Pierre Olivier Martel, Arthur Mesh and Wade Benson’s patent for multi-user storage volume encryption, filed in 2020.

T2 chip

The first Macs with a true secure enclave are those with a T2 chip, starting with the iMac Pro in December 2017. Those are based on the same A10 Fusion chip from the previous year, and were already lagging the iPhone 8 in this respect.

The T2 secure enclave is another co-processor system, run by a Secure Enclave Processor (SEP), a 32-bit ARM CPU running its own operating system, sepOS, based on a specialised L4 microkernel completely different from those used by Macs and Apple’s devices. It has its own secure storage (EEPROM), and a Public Key Accelerator for signing and encryption/decryption using RSA and ECC methods. Outside the enclave is a dedicated AES256 encryption/decryption engine built into the data transfer path between the internal SSD and main system memory.

M-series chips

The big leap forward for Macs was the release of the first models featuring M1 chips, which caught up with the features of late versions (after autumn 2020) of the A12 and A13, with Apple’s second generation Secure Storage Component.

Perhaps the most significant of its improvements are measures to prevent replay attacks. Those are best illustrated with FileVault. Let’s say that you didn’t enable FileVault at first, but left your Apple silicon Mac to handle the encryption of its internal Data volume without the added protection of your password. That would mean that its volume encryption key (VEK) was generated internally by the Secure Enclave, and stored there. If you then turned FileVault on, the VEK would be encrypted using your password and the hardware key. In the T2 chip, it might be possible to use the old VEK to decrypt the volume. In the secure enclave of an M-series chip, that type of replay attack is prevented by the revocation of all previous events and records.

Other improvements include the use of second generation secure storage incorporating counter lockboxes to enforce limits on the number of passcode attempts allowed, instead of an EEPROM, and a better Public Key Accelerator.

Currently, the secure enclave is known to protect the following:

• encryption keys for Touch ID, FileVault, and the Data Protection (iCloud) keychain (but not file-based keychains);
• that Mac’s Unique ID (UID) and Group ID (GID);
• Touch ID control, and (on older devices not Macs) Face ID using a secure neural engine; in recent devices and M-series chips, that’s implemented as a secure mode in the main neural engine (ANE);
• Apple Pay handling;
• Activation Lock, through the Owner and User Identity Keys;
• signing and verification of LocalPolicy for boot environments (Apple silicon).

Communication between the CPU and SEP is performed using a dedicated mailbox whose function is detailed in Apple’s patents. Further information is also provided in the Platform Security Guide.

FileVault encryption

It has been stated widely (even here) that the secure enclave in T2 and Apple silicon chips contains a hardware encryption/decryption unit and acts as the internal SSD’s storage controller. In fact, as shown in the original patent of Martel and others, and now in the Platform Security Guide, the AES engine responsible is located outside the secure enclave, together with the Flash controller, and has a secure link to the enclave.

During SEP boot, it generates an ephemeral key to wrap keys to be used by the AES engine for encryption and decryption. That key is sent from the secure enclave to the AES engine over the dedicated connection between them, then used to protect keys transferred from the enclave to the AES engine. That ensures an unprotected key is never exposed outside the enclave and AES engine.

The Apple silicon secure enclave is by no means unique. ARM TrustZone, other Trusted Execution Environments, and Trusted Platform Modules offer similar features and facilities. However, the secure enclave is unusual because it has been integrated into all Macs with T2 or Apple silicon chips, and all Apple’s recent devices, and can’t be disabled or bypassed.

References

Manu Gulati, Michael J Smith and Shu-Yi Yu, US Patent 8,832,465 B2, Security enclave processor for a system on a chip, filed 25 September 2012, granted 9 September 2014.
R Stephen Polzin, James B Keller, Gerard R Williams, US Patent 8,775,757 B2, Trust zone support in system on a chip having security enclave processor, filed 25 September 2012, granted 8 July 2014.
R Stephen Polzin, Fabrice L Gautier, Mitchell D Adler, Conrad Sauerwald and Michael LH Brouwer, US Patent 9,419,794 B2, Key management using security enclave processor, filed 23 September 2014, granted 16 August 2016.
Pierre Olivier Martel, Arthur Mesh and Wade Benson, US Patent 11,455,432 B1, Multi-user storage volume encryption via secure processor, filed 8 June 2020, granted 27 September 2022.
Tarjei Mandt, Mathew Soling and David Wang (2016), Demystifying the Secure Enclave Processor, Black Hat USA 16 (PDF)
Apple, Platform Security Guide
Wikipedia’s overview of Apple silicon chips.

With careful observation and a little knowledge of the startup sequence of an Apple silicon Mac, you can learn a lot about what can and can’t happen during that sequence. This article explains how, with examples from the log of a Mac mini M4 Pro.

In broad terms, startup of an Apple silicon Mac consists of the following sequence of events:

• Boot ROM, which ends in DFU mode if there’s a problem, otherwise it hands on to
• the Low-Level Bootloader (LLB) and iBoot (Stage 2), the firmware, that should end in validating and running
• the kernel, which initially runs on a single CPU core before starting others up and launching launchd, and later
• unlocking and accessing the Data volume, and progressing to
• userspace.

The opening entry in the log is the boot announcement of
=== system boot:
followed by the boot UUID. There’s then a gap of 5 seconds or more before the next entry, which marks the start of kernel boot. Those seconds are the silent phase during which the LLB and iBoot are doing their thing. They don’t write to the Unified log, but leave fragments of cryptic information known as breadcrumbs, which you can’t make use of. The kernel then writes its usual welcome of
kprintf initialized
and the following four seconds or so are filled by log entries from the kernel.

Wallclock adjustment

During this phase, the system clock is synchronised, and wallclock time adjusted, usually twice in rapid succession. This is obvious by step changes in timestamp, usually putting the clock back by several seconds in the first sync, then putting it forward slightly in the second. These play havoc with the timestamps, as you can have two or even more instances of the same time being recorded in the log. Beware of the entries
=== system wallclock time adjusted

Early during the kernel phase, it starts up all the other CPU cores in the chip, and records that in the log. Entries become progressively more varied after launchd is loaded, and this first userspace boot (without Data volume access).

Data volume unlock

With FileVault enabled, by this stage macOS still doesn’t have access to the Data volume. That means all the code run so far, and almost all the data, are immutable, locked in the firmware or the Signed System Volume (SSV). The firmware does access LocalPolicy from another container in the internal SSD, and there’s always the NVRAM, but there’s no access to anything in /Library, including the many property lists there. This also means that processes running before the Data volume is unlocked and mounted can’t write to storage.

Around 10-15 seconds after the start of booting, the login window is displayed, ready for the user to enter their password. Once that has been entered, there’s a watershed moment:
30.845097 com.apple.loginwindow Attempting to unlock the data volume <LFVolume: 0x6000001b8e40: [UUID]: Data>
30.883172 "AppleSEPKeyStore":3814:0: Sending notification for volume [UUID] unlocked (action 1, handle -842987934)
30.885459 com.apple.login volume <LFVolume: 0x6000001b8e40: [UUID]: Data> was unlocked
30.886129 com.apple.loginwindow Unlocked data volume <LFVolume: 0x6000001b8e40: [UUID]: Data>
30.886154 com.apple.loginwindow FileVault volume unlocked, allow authorization
30.887562 com.apple.loginwindowLite -[LWLSystemUnlock unlockSystem]:439: Authorization was successful
30.887587 com.apple.loginwindowLite -[LWLSystemUnlock unlockSystem]:447: logging in user hoakley
The times on those entries were deliberately delayed, as I pressed the Return key for password entry after 30 seconds had elapsed, a good 10 seconds later than I could have done so.

Shortly after that, the kernel manager shuts down, a great many kernel space processes are handed over to continue in userspace, and you’ll then see the kernel report
userspace boot

Before the Data volume is unlocked, log entries are frequent, but hardly a torrent, at around 1,000 per second, and more than 25% of them are written by the kernel. Once the kernel has booted userspace and the Data volume is accessible, log entries are written far more frequently, at an average rate of 5,000 per second, often even higher, with less than 10% of them coming from the kernel.

Phase summary

• Boot ROM, entering DFU mode or handing over to
• Low-Level Bootloader (LLB) and iBoot (Stage 2) firmware, without log entries, handing over to
• the kernel, with wallclock adjustments, until
• Data volume unlocking, then into
• userspace and access to /Library and user files.

There are so many controls in macOS that sometimes you can’t see the wood for the trees. This can leave uncertainty over essentials, such as whether your Apple silicon Mac really is properly secure, or maybe there’s something sinister going on with it? This is a question I’m asked not infrequently, usually when someone has been spreading disinformation or FUD (fear, uncertainty, doubt). So how can you check that your Mac is properly locked down and boots securely?

Quick checks

There are two quick checks that cover the essentials. First, open System Information and select the Controller section in Hardware.

This provides a brief summary of your Mac’s boot security, which should read as shown above. If you still need to use a kernel extension or similar, your Mac might show Reduced Security with Allow All Kernel Extensions enabled, but you should do everything you can to avoid that.

Secure Boot is controlled using Startup Security Utility in Recovery mode, and if you care to start up in that mode, you can confirm or correct its settings there.

Back in normal user mode, open Privacy & Security settings and ensure you have FileVault enabled there.

SilentKnight also checks that XProtect/Gatekeeper checks are enabled, and that security data are up to date, giving you complete confidence.

Details

Although those should be sufficient for most, some want to go further and verify that their Mac’s boot process and security systems are also working correctly. To do that, shut your Mac down, wait ten seconds or so, and start up normally with the startup chime sounding at a known time. Enter your password, wait a few seconds for the Finder to get set up and running, and open LogUI. Set its time to that of the startup chime, and get the first 10 seconds or 10,000 log entries. You may need to adjust the seconds to capture the full boot sequence. When you have, look through the log and identify the following waypoints.

In each of these log entries, I have emboldened a word or two that you can copy from here and paste into LogUI’s Search box, then press Return. That will display the log entry, and sometimes others you might find relevant. Times are given here in seconds, with the startup chime occurring at about 37 seconds. Version numbers shown are those for macOS 15.6.

The start of boot is recorded as
37.562774 === system boot: [UUID]
and a little while after that, the kernel declares its version details
42.759300 Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6041
for macOS 15.6.

Further down you’ll come across more information about key security components, including the Trusted Execution Monitor
43.060422 [Log]: Code Signing Monitor Image4 Module Version 7.0.0: Fri Jul 11 16:51:29 PDT 2025; root:AppleImage4_txm-320.100.22~1090
43.060447 [Log]: build variant: txm.macosx.release.TrustedExecutionMonitor_Guarded-135.100.37

Then the iBoot firmware version
43.061758 iBoot version: iBoot-11881.140.96
43.061760 iBoot Stage 2 version: iBoot-11881.140.96

CoreCrypto support is vital, and another Image4 extension
43.137635 FIPSPOST_KEXT [133796636] fipspost_post:154: [FIPSPOST][Module-ID] Apple corecrypto Module v18.3 [Apple silicon, Kernel, Software, SL1]
43.242334 Darwin Image4 Extension Version 7.0.0: Mon Jul 14 11:23:46 PDT 2025; root:AppleImage4-320.100.22~2585/AppleImage4/RELEASE_ARM64E

You should see entries reporting the loading of security policy components
43.242343 Security policy loaded: AppleImage4 hooks (AppleImage4)
43.242961 Security policy loaded: Apple Mobile File Integrity (AMFI)
43.243092 Security policy loaded: Seatbelt sandbox policy (Sandbox)

The Secure Enclave Processor or SEP is another key component that has to be started up
43.264594 "AppleSEPKeyStore":326:0: starting (BUILT: Jul 14 2025 23:34:10) ("normal" variant 🌽 , 1827.120.2)
43.264639 "AppleSEPKeyStore":471:0: _sep_enabled = 1

Apple System Policy should follow a bit later
43.760156 Security policy loaded: Apple System Policy (ASP)
43.760188 AppleSystemPolicy has been successfully started

The root of the file system is then identified in two entries whose origins go right back to the start of Mac OS X
43.940643 BSD root: disk3s1
43.940644 , major 1, minor 13

And APFS mounts the root file system, using the SSV snapshot
43.941048 apfs_vfsop_mountroot:2984: apfs: mountroot called!
44.034685 apfs_vfsop_mount:2763: disk3s1 Rooting from snapshot with xid 1724240.

One of the most important entries comes shortly after that, where successful validation of the SSV’s root hash is reported
44.038830 authenticate_root_hash:642: disk3s1 successfully validated on-disk root hash

It’s now time to start user space processes, and for that launchd must be loaded so it can launch everything else
44.103761 load_init_program: attempting to load /sbin/launchd

How Secure Boot works

Apple silicon Macs have a small ROM to support DFU mode in case a full Restore is required, and to check and load the first stage of the ‘firmware’, the Low-Level Bootloader or LLB. Only if that matches its signature will the ROM firmware hand over to it and proceed with the boot process. The LLB in turn performs the same checks on the second stage ‘firmware’, iBoot proper. That goes on to check the kernel, before loading that and handing over for kernel boot to take over.

iBoot ‘firmware’ doesn’t write anything in the log, but once the kernel takes over its log entries provide a detailed account of its progress. The great majority of its log entries are unintelligible to anyone outside Apple, but the waypoints I have given above identify some of the most important steps it takes. When it’s ready, the kernel validates the root hash for the SSV snapshot, as noted above, enabling the boot process to proceed to load and run other parts of macOS. The remaining hash checking of the SSV, to confirm that it’s exactly as Apple intends, proceeds in a ‘lazy’ fashion, as access is needed to its contents.

This chain of validation before loading the next stage ensures that nothing in the boot process can be tampered with or changed, and the boot is secure throughout. Apple provides further details in its Platform Security Guide.

We rely on FileVault and APFS to protect our secrets by encrypting the volumes containing our documents and data. How they do that is a mystery to many, and raises important questions such as the role our passwords play, and how recovery keys work. This article attempts to demystify them.

Naïve encryption

A simple scheme to encrypt a disk or volume might be to take the user password, somehow turn it into a key suitable for the encryption method to be used, and employ that to encrypt and decrypt the data as it’s transferred between disk storage and memory.

There are lots of weaknesses and difficulties with that. Even using a ‘robust’ user password, it’s not going to be memorable, sufficiently long or hard to crack, and there’s no scope for recovery if that password is lost or forgotten.

FileVault base encryption

In Macs with T2 or Apple silicon chips when FileVault is disabled, everything in the Data volume stored on their internal SSD is still encrypted, but without any user password. This is performed in the Secure Enclave, which both handles the keys and performs the encryption/decryption. That ensures the keys used never leave the Secure Enclave, so are as well-protected as possible.

Generating the key used to encrypt the volume, the Volume Encryption Key or VEK, requires two huge numbers, a hardware key unique to that Mac, and the xART key generated by the Secure Enclave as a random number. The former ties the encryption to that Mac, and the latter ensures that an intruder can’t repeat generation of the same VEK even if it does know the hardware key. When you use Erase All Content and Settings (EACAS), the VEK is securely erased, rendering the encrypted data inaccessible, and there’s no means to either recover or recreate it.

This scheme lets the Mac automatically unlock decryption, but doesn’t put that in the control of the user, who therefore needs to enable FileVault to get full protection.

FileVault full encryption

Rather than trying to incorporate a user password or other key into the VEK, like many other encryption systems FileVault does this by encrypting the VEK using a Key Encryption Key or KEK, a process known as wrapping.

When you enter your FileVault password, that’s passed to the Secure Enclave, where it’s combined with the hardware key to generate the KEK, and that’s then used together with hardware and xART keys to decrypt or unwrap the VEK used for decryption/encryption.

This has several important benefits. As the KEK can be changed without producing a new VEK, the user password can be changed without the contents of the protected volume having to be fully decrypted and encrypted again. It’s also possible to generate multiple KEKs to support the use of recovery keys that can be used to unlock the VEK when the user’s password is lost or forgotten. Institutional keys can be created to unlock multiple KEKs and VEKs where an organisation might need access to protected storage in multiple Macs.

APFS encryption

True FileVault requires all keys to be stored in the Secure Enclave, and never released outside it. Intel Macs without T2 chips, and other protected volumes such as those on external storage can’t use that, and in the case of removable storage need an alternative that stays on the disk. For that, APFS uses the AES Key Wrap Specification in RFC 3394, using a secret such as a password to maintain confidentiality of every key.

APFS also uses separate VEKs and KEKs, so enabling the use of multiple KEKs for a single VEK, and the potential to change a KEK without having to decrypt and re-encrypt the whole volume, as in FileVault. In APFS, VEKs and KEKs are stored in and accessed from Keybags associated with both containers and volumes. The Container Keybag contains wrapped VEKs for each encrypted volume within that container, together with the location of each encrypted volume’s keybag. The Volume Keybag contains one or more wrapped KEKs for that volume, and an optional passphrase hint. These are shown in the diagram below.

Apple’s documentation refers to several secrets that can be used to wrap a KEK, including a user password, an individual recovery key, an institutional recovery key, and an unspecified mechanism implemented through iCloud. Currently, for normal software encryption in APFS, only two of those appear accessible: a user password is supported in both Disk Utility and diskutil‘s apfs verb, while diskutil also supports use of an institutional recovery key through its -recoverykeychain options. Individual and iCloud recovery keys only appear available when using FileVault, in this case implemented in software, either on Intel Macs without a T2 chip, or on all Macs when encrypting an external volume.

Because keybags are stored on the disk containing the encrypted volume, if the disk is connected to another Mac, when macOS tries to mount that volume, the user will be prompted to enter its password, and can then gain access to its contents. When FileVault is used to protect a Data volume on the internal SSD of a T2 or Apple silicon Mac, that volume can only be unlocked through the Secure Enclave of that Mac, and it isn’t possible to unlock it from another Mac (that’s also true when FileVault hasn’t been enabled on that volume).