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+# Measured Boot
+coreboot measured boot is implemented as Google Verified Boot extension. This
+means in order to use it, vboot needs to be available for your platform.
+
+## IBB/CRTM
+The "Initial Boot Block" or "Core Root of Trust for Measurement" is the first
+code block loaded at reset vector and measured by a DRTM solution.
+In case SRTM mode is active, the IBB measures itself before measuring the next
+code block. In coreboot, cbfs files which are part of the IBB are identified
+by a metatdata tag. This makes it possible to have platform specific IBB
+measurements without hardcoding them.
+
+## Known Limitations
+At the moment measuring IBB dynamically and FMAP partitions are not possible but
+will be added later to the implementation.
+
+Also SoCs making use of VBOOT_RETURN_FROM_VERSTAGE are not able to use the
+measured boot extension because of platform constraints.
+
+## SRTM Mode
+The "Static Root of Trust for Measurement" is the easiest way doing measurements
+by measuring code before it is loaded.
+
+![][srtm]
+
+[srtm]: srtm.png
+
+## DRTM Mode
+The "Dynamic Root of Trust for Measurement" is realised by platform features
+like Intel TXT or Boot Guard. The features provide a way of loading a signed
+"Authenticated Code Module" aka signed blob. Most of these features are also
+a "Trusted Execution Environment", e.g. Intel TXT.
+
+DRTM gives you the ability of measuring the IBB from a higher Root of Trust
+instead of doing it yourself without any hardware support.
+
+## Platform Configuration Register
+Normally PCR 0-7 are reserved for firmware usage. In coreboot we use just 4 PCR
+banks in order to store the measurements. coreboot uses the SHA-1 or SHA-256
+hash algorithm depending on the TPM specification for measurements. PCR-4 to
+PCR-7 are left empty.
+
+### PCR-0
+_Hash:_ SHA1
+_Description:_ Google VBoot GBB flags.
+
+### PCR-1
+_Hash:_ SHA1/SHA256
+_Description:_ Google VBoot GBB HWID.
+
+### PCR-2
+_Hash:_ SHA1/SHA256
+_Description:_ Core Root of Trust for Measurement which includes all stages,
+data and blobs.
+
+### PCR-3
+_Hash:_ SHA1/SHA256
+_Description:_ Runtime data like hwinfo.hex or MRC cache.
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