This policy setting specifies the constraints for passwords used to unlock BitLocker-protected operating system drives. If non-TPM protectors are allowed on operating system drives you can provision a password enforce complexity requirements on the password and configure a minimum length for the password. For the complexity requirement setting to be effective the Group Policy setting "Password must meet complexity requirements" located in Computer Configuration\Windows Settings\Security Settings\Account Policies\Password Policy\ must be also enabled.Note: These settings are enforced when turning on BitLocker not when unlocking a volume. BitLocker will allow unlocking a drive with any of the protectors available on the drive.If you enable this policy setting users can configure a password that meets the requirements you define. To enforce complexity requirements on the password select "Require complexity".When set to "Require complexity" a connection to a domain controller is necessary when BitLocker is enabled to validate the complexity the password. When set to "Allow complexity" a connection to a domain controller will be attempted to validate the complexity adheres to the rules set by the policy but if no domain controllers are found the password will still be accepted regardless of actual password complexity and the drive will be encrypted using that password as a protector. When set to "Do not allow complexity" no password complexity validation will be done.Passwords must be at least 8 characters. To configure a greater minimum length for the password enter the desired number of characters in the "Minimum password length" box.If you disable or do not configure this policy setting the default length constraint of 8 characters will apply to operating system drive passwords and no complexity checks will occur.Note: Passwords cannot be used if FIPS-compliance is enabled. The "System cryptography: Use FIPS-compliant algorithms for encryption hashing and signing" policy setting in Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options specifies whether FIPS-compliance is enabled.
Windows Components\BitLocker Drive Encryption\Operating System Drives
At least Windows Server 2012 or Windows 8
HKLM\Software\Policies\Microsoft\FVE!OSPassphrase; HKLM\Software\Policies\Microsoft\FVE!OSPassphraseComplexity HKLM\Software\Policies\Microsoft\FVE!OSPassphraseLength HKLM\Software\Policies\Microsoft\FVE!OSPassphraseASCIIOnly