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Merge pull request #3284 from VLG17/patch-13

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Justin Hall
2019-04-26 15:09:03 -07:00
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windows/security/information-protection/tpm/switch-pcr-banks-on-tpm-2-0-devices.md
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@ -50,6 +50,24 @@ As a result, if the currently used PCR bank is switched all keys that have been
Before switching PCR banks you should suspend or disable BitLocker – or have your recovery key ready. For steps on how to switch PCR banks on your PC, you should contact your OEM or UEFI vendor.
## How can I identify which PCR bank is being used?
A TPM can be configured to have multiple PCR banks active. When BIOS is performing measurements it will do so into all active PCR banks, depending on its capability to make these measurements. BIOS may chose to deactivate PCR banks that it does not support or "cap" PCR banks that it does not support by extending a separator. The following registry value identifies which PCR banks are active.
- Registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\IntegrityServices<br>
- DWORD: TPMActivePCRBanks<br>
- Defines which PCR banks are currently active. This is a bitmap defined in the TCG Algorithm Registry.<br>
Windows checks which PCR banks are active and supported by the BIOS. Windows also checks if the measured boot log supports measurements for all active PCR banks. Windows will prefer the use of the SHA-256 bank for measurements and will fall back to SHA1 PCR bank if one of the pre-conditions is not met.
You can identify which PCR bank is currently used by Windows by looking at the registry.
- Registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\IntegrityServices<br>
- DWORD: TPMDigestAlgID<br>
- Algorithm ID of the PCR bank that Windows is currently using. (For the full list of supported algorithms, see the TCG Algorithm Registry.)<br>
Windows only uses one PCR bank to continue boot measurements. All other active PCR banks will be extended with a separator to indicate that they are not used by Windows and measurements that appear to be from Windows should not be trusted.
## Related topics
- [Trusted Platform Module](trusted-platform-module-top-node.md) (list of topics)