BitLocker and the TPM for administrators

A Windows VM made by MacVisor 27.2.1 or later has UEFI Secure Boot, a TPM 2.0 and measured boot, so BitLocker works in it as on a PC. This page is for the person who has to run that across a team: what to turn on, where the recovery keys go, what breaks the TPM's seal, and what your compliance tooling will say. The settings themselves are on Windows 11 guests.

At a glance

What the VM's TPM is

The TPM is software: MacVisor runs it for the VM, in a sandboxed helper process of its own, and Windows drives it with its inbox TPM driver. To Windows it is a TPM 2.0 that is present and ready.

Its state is four files in the VM's bundle (tpm-*.bin), encrypted with AES-256-GCM. The key comes from a key in this Mac's Secure Enclave, which never leaves that Mac. What follows from that:

• A copy of the bundle on another Mac can't open the TPM. That covers a backup, an export, a disk taken out of the Mac, and a bundle copied over the network.
• On this Mac, the macOS account that owns the VM can start it, and the TPM unlocks for that start. The Secure Enclave key is usable once someone has signed in to the Mac after it started up, so VMs can start with the screen locked.
• So a TPM-only BitLocker protector protects the Windows disk away from this Mac, not from someone signed in to this Mac as the VM's owner. That is the same position as a PC with TPM-only BitLocker and automatic sign-in. Protect the Mac itself with FileVault and a password, as Security model says.

MacVisor keeps the Windows account's password in the bundle too, unless you delete it (mvz password <vm> --delete). For a VM that holds anything sensitive, delete it once Windows is set up, and turn automatic sign-in off (--no-autologin when you create the VM).

Turn BitLocker on

Windows doesn't encrypt a MacVisor VM by itself: MacVisor sets PreventDeviceEncryption so that a clone or a move doesn't lock anyone out by surprise. A value you set yourself, by Group Policy or MDM, is left alone.

1. Check the VM in an administrator PowerShell:

   Get-Tpm | Select-Object TpmPresent, TpmReady     # both True
   Confirm-SecureBootUEFI                           # True

2. Turn BitLocker on with a recovery password. Windows adds the TPM protector itself:

   manage-bde -on C: -RecoveryPassword -UsedSpaceOnly -SkipHardwareTest

   It prints the 48-digit recovery password once. Escrow it before you go on: see Escrow the recovery key.

3. Check what it sealed to, once encryption has finished:

   manage-bde -status C:
   manage-bde -protectors -get C:

Keep Secure Boot on. With it, the seal depends only on the Secure Boot keys and on who signed the boot files, so replacing a boot file with a newer signed one doesn't ask for the recovery key.

BitLocker policies from Group Policy or an MDM (encryption method, required protectors, escrow before enabling) apply as on a PC.

From the Mac, a VM made with --ssh takes the same commands through mvz exec:

mvz exec win11 -- "manage-bde -status C:"

Escrow the recovery key

The recovery key is the only way into the disk when the TPM's seal breaks, and several ordinary VM operations break it. Escrow the key before anyone clones, moves or resets the VM.

Entra ID or Active Directory, in an administrator PowerShell in the VM:

$id = ((Get-BitLockerVolume -MountPoint C:).KeyProtector |
    Where-Object KeyProtectorType -eq RecoveryPassword).KeyProtectorId
BackupToAAD-BitLockerKeyProtector -MountPoint C: -KeyProtectorId $id   # Entra ID
Backup-BitLockerKeyProtector -MountPoint C: -KeyProtectorId $id        # Active Directory

Your own vault, from the Mac:

mvz exec win11 -- "manage-bde -protectors -get C: -Type RecoveryPassword"

The output has the protector's ID and the 48-digit password. Store both with the VM's name: the recovery screen shows the first eight characters of the ID, which is how you find the right key.

What makes BitLocker ask for the recovery key

BitLocker asks when the TPM it sealed to is gone, or when what the TPM measured at boot has changed.

Two rules follow:

• Make templates and golden images before you turn BitLocker on. Every VM made from an encrypted image would start at the recovery screen. Turn BitLocker on in each VM, by policy or with the commands above, so each seals to its own TPM and escrows its own key.

• Before a planned change to Secure Boot or its keys, suspend BitLocker for one restart, as you would before a firmware update on a PC. Windows seals to the new state at the next start:

  Suspend-BitLocker -MountPoint C: -RebootCount 1

After entering the recovery key, sign in and check manage-bde -protectors -get C:. If the VM has a new TPM (a clone, a reset, a move without its TPM), remove the old TPM protector and add a new one, or BitLocker asks again at every start:

manage-bde -protectors -delete C: -Type TPM
manage-bde -protectors -add C: -TPM

Move a VM to another Mac

A VM moved without its TPM doesn't start on the other Mac until you choose: start it without its TPM (mvz set <vm> --tpm off), or give it a new one (mvz tpm reset <vm>). Either way BitLocker asks for the recovery key. To keep the same TPM instead:

1. On the first Mac, shut the VM down and run mvz tpm export <vm> <file>. It asks for a passphrase of at least 8 characters and writes the TPM sealed with that passphrase (PBKDF2-HMAC-SHA256 with 600,000 rounds, AES-256-GCM) instead of the Secure Enclave.
2. Move the VM (an export, a backup or a copy of its bundle) and the file.
3. On the other Mac, with the VM shut down, run mvz tpm import <vm> <file>.

Windows sees the same TPM, so BitLocker unlocks and Windows Hello carries on. The TPM is then sealed to the second Mac.

For a script, the passphrase comes on standard input or from --passphrase-file, never on the command line, and import needs --yes:

mvz tpm export win11 /Volumes/Transfer/win11.tpm --passphrase-file /path/to/passphrase
mvz tpm import win11 /Volumes/Transfer/win11.tpm --passphrase-file /path/to/passphrase --yes

What to plan for:

• Treat the export file as you treat the recovery key. With its passphrase it is the VM's TPM. Delete it after the import, or keep it in the vault where the recovery key is.
• Export before a Mac is erased, replaced or sent for repair. Don't count on a TPM opening on a Mac that was erased or whose logic board was changed. Migration Assistant and Time Machine copy the bundle, and the copy has the same limit as any other.
• If the first Mac is gone, use the recovery key: reset the TPM on the new Mac, start the VM, enter the key, and add a new TPM protector as shown above.
• Snapshots taken on the first Mac hold the TPM sealed to that Mac. A revert to one on the second Mac keeps the TPM you imported, and says so.

Reset a TPM

Reset TPM… under Firmware in VM Settings → System, or mvz tpm reset <vm>, erases the VM's TPM. The VM must be shut down, with no suspended session, and gets a new TPM at its next start. Use it for a VM that arrived without its TPM, or one whose TPM can't be read.

It is the same as clearing a PC's TPM. Afterwards:

• BitLocker asks for the recovery key, then needs a new TPM protector.
• Windows Hello asks for a new PIN.
• Keys and certificates that Windows kept in the TPM are gone. That includes the device's registration with Entra ID or a domain's certificates, where it had them: join or enroll again as you would after clearing a PC's TPM.

A reset can't be undone, except by reverting to a snapshot taken before it, which brings the old TPM back with the disk.

Backups and disaster recovery

One TPM export serves until that TPM is reset: it holds the TPM's identity, which later use doesn't change. For most teams the escrowed recovery key is enough, and the TPM export is for VMs where re-enrolling Windows Hello and certificates would be costly.

What compliance tools report

If a compliance policy requires attestation or Memory Integrity, a MacVisor VM can't meet it. Give those VMs a policy of their own that checks Secure Boot, the TPM and BitLocker by their state instead.

What isn't tested

We haven't yet run a Windows VM through these, so treat them as unknown rather than supported:

• Joining Entra ID, enrolling in Intune, user-driven Autopilot and Windows Hello for Business.
• BitLocker turned on silently by an Intune policy.
• A BitLocker startup PIN (TPM and PIN), typed at the pre-boot screen.
• Windows feature updates. An in-place repair install of the same version works.

If you run one of these on MacVisor, tell us how it went.

Checklist

• Secure Boot and the TPM on (the default for new Windows VMs).
• Templates made before BitLocker is turned on.
• BitLocker on in each VM, with a recovery password.
• The recovery key escrowed outside the VM, findable by the protector's ID.
• The Windows account's password deleted from the bundle, and automatic sign-in off, where the data calls for it.
• FileVault on the Mac.
• A TPM export, or the recovery key at hand, before a Mac is erased or replaced.
• A compliance policy for these VMs that doesn't need attestation or Memory Integrity.