Comment by dhx

BitLocker encrypts data on a disk using what it calls a Full Volume Encryption Key (FVEK).[1][2] This FVEK is encrypted with a separate key which it calls a Volume Management Key (VMK) and the VMK-encrypted FVEK is stored in one to three (for redundancy) metadata blocks on the disk.[1][2] The VMK is then encrypted with one or more times with a key which is derived/stored using one or more methods which are identified with VolumeKeyProtectorID.[2][3] These methods include what I think would now be the default for modern Windows installations of 3 "Numerical password" (128-bit recovery key formatted with checksums) and 4 "TPM And PIN". Previously instead of 4 "TPM And PIN" most Windows installations (without TPMs forced to be used) would probably be using just 8 "Passphrase". Unless things have changed recently, in mode 4 "TPM And PIN", the TPM stores a partial key, and the PIN supplied by the user is the other partial key, and both partial keys are combined together to produce the key used to decrypt the VMK.

Seemingly once you've installed Windows and given the Microsoft your BitLocker keys in escrow, you could then use Remove-BitLockerKeyProtector to delete the VMK which is protected with mode 3 "Numerical password" (recovery key).[4] It appears that the escrow process (possibly the same as used by BackupToAAD-BitLockerKeyProtector) might only send the numerical key, rather than the VMK itself.[5][6] I couldn't find from a quick Internet search someone who has reverse engineered fveskybackup.dll to confirm this is the case though. If Microsoft are sending the VMK _and_ the numerical key, then they have everything needed to decrypt a disk. If Microsoft are only sending the numerical key, and all numerical key protected VMKs are later securely erased from the disk, the numerical key they hold in escrow wouldn't be useful later on.

Someone did however ask the same question I first had. What if I had, for example, a billion BitLocker recovery keys I wanted to ensure were backed up for my protection, safety and peace of mind? This curious person did however already know the limit was 200 recovery keys per device, and found out re-encryption would fail if this limit had been reached, then realised Microsoft had fixed this bug by adding a mechanism to automatically delete stale recovery keys in escrow, then reverse engineered fveskybackup.dll and an undocumented Microsoft Graph API call used to delete (or "delete") escrowed BitLocker recovery keys in batches of 16.[7]

It also appears you might only be able to encrypt 10000 disks per day or change your mind on your disk's BitLocker recovery keys 10000 times per day.[8] That might sound like a lot for particularly an individual, but the API also perhaps applies a limit of 150 disks being encrypted every 15 minutes for an entire organisation/tenancy. It doesn't look like anyone has written up an investigation into the limits that might apply for personal Microsoft accounts, or if limits differ if the MS-Organization-Access certificate is presented, or what happens to a Windows installation if a limit is encountered (does it skip BitLocker and continue the installation with it disabled?).

[1] https://learn.microsoft.com/en-us/purview/office-365-bitlock...

[2] https://itm4n.github.io/tpm-based-bitlocker/

[3] https://learn.microsoft.com/en-us/windows/win32/secprov/getk...

[4] https://learn.microsoft.com/en-us/powershell/module/bitlocke...

[5] https://learn.microsoft.com/en-us/graph/api/bitlockerrecover...

[6] https://learn.microsoft.com/en-us/powershell/module/bitlocke...

[7] https://patchmypc.com/blog/bitlocker-recovery-key-cleanup/

[8] https://learn.microsoft.com/en-us/graph/throttling-limits#in...