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Trusted Platform Module(TPM) supports many crypto functions, and notably the “PCR Extend” and “Seal” operations are used to build this solution.
PCR Extend
TPM can be asked to perform a “PCR Extend” command, where a particular hash value would be added to the existing hash value in a Platform Configuration Register(PCR), and the resultant hash value can be stored back in the same PCR. i.e.
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- The order of measurements - Final value will be the same if and only if the measurements are done in the same order
- Final PCR value captures the whole history of measurements - useful in quick validation of final states against expected state
- Deterministic - If one repeats the same history of measurements, he will end up in the same final PCR value - Useful in validation of a change in one of the input sequences
Seal
TPM can seal a given secret information against the current set of PCR values, through a TPM command called “Seal”. Once sealed, the information can be read back only through an unseal command, which will succeed only if those PCRs hold the same set of values as they were during the sealing operation. In other words, if those PCR values aren’t the same, the secret can not be recovered.
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During these extend operations, the extend operations are recorded by BIOS and Bootloader, in a special firmware table, called the TPM Eventlog table, and this table is handed over to the operating system during OS takeover. By playing the same sequence of extend operations recorded in a given TPM Event Log, one can check if the final PCR values match, and if so, then the Event Log(and hence the software layers) can be trusted.
Building it on PCRs, BootLogs
Based on the above constructs, we present a solution to measure and attest software integrity of EVE node. Just for recap, EVE is the open-source software from LF-Edge for Edge Virtualization, running on IoT Edge gateways. EVC is the controller for managing these EVE instances. Adam under LF-Edge is an open source implementation of one such EVC. In this contextThe APIs between EVE and EVC are specified in EVE API specification. In the context of remote attestation, the EVC is the attesting authority and EVE reports its measurements for attestation.
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