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The Attestation Process

Given the discussion in the above section about the state and maturity of attestation techniques, let's look at the details of the static attestation protocol and the overall integrity measurement flow.

The integrity measurement flow describes the steps required to measure the platform integrity measurements. It includes:

• A means of generating and collecting the measurements through an RTM.

• A means of storing the measurements that is either tamper resistant or tamper evident, with a TPM for RTS and RTR.

• A means of conveying the measurements to a challenger via the attestation agents, as described in the attestation protocol below.

• A means of analyzing the measured result, and a means of asserting the trustability of the machine based on the results of that determination through a trust assessment authority or trust attestation authority (TAA).

Remote Attestation Protocol

Figure 4-1 illustrates the attestation protocol providing the means for conveying measurements to the challenger. The endpoint attesting device must have a means of measuring the BIOS firmware, low-level device drivers, operating system, virtual machine monitor components, and be able to forward those measurements to the attestation authority. The attesting device must do this while protecting the integrity, authenticity, nonrepudiation, and some cases, the confidentiality of those measurements.

Figure 4-1. Remote attestation protocol

Let's walk through the steps of the remote attestation protocol:

1. The challenger, at the request of a requester, creates a nonpredictable nonce (NC) and sends it to the attestation agent on the attesting node, along with the selected list of platform configuration registers (PCRs).

2. The attestation agent sends that request to the TPM as a TPMQuote request with the nonce, and the PCR list.

3. In reponse to the TPMQuote request, the TPM loads the attestation identity key from protected storage in the TPM by using the storage root key (SRK), performs a TPM Quote command, which is used to sign the selected PCRs and the provided nonce (NC) with the private key, AIKpriv. Additionally, the attesting agent retrieves the stored measurement log (SML).

4. Called the integrity response, the attesting agent sends the response consisting of the signed quote, signed nonce (NC), and the SML to the challenger. The attesting agent also delivers the AIK credential, which consists of the AIKpub that was signed by a privacy CA.

5. The challenger validates if the AIK credential was signed by a trusted privacy CA thus belonging to a genuine TPM. The challenger also verifies whether AIKpub is still valid by checking the certificate revocation list of the trusted issuing party.

6. The challenger verifies the signature of the quote and checks the freshness of the quote.

7. Based on the received stored measurement log and the PCR values, the challenger processes the SML, compares the individual module hashes that are extended to the PCRs against the known-good or golden values, and recomputes the received PCR values. If the individual values match the golden values, and if the computed values match the signed aggregate, the remote node is asserted to be in a trusted state.

This protocol is highly resistant to replay attacks, tampering, and masquerading.

How does this remote attestation protocol get implemented and manifested in an IT environment? Figure 4-2 illustrates a sample IT architecture supporting the generation, forwarding, and analysis of platform boot integrity measurements, as well as assertion of the trustability of the attestation at each decision point via a trust assertion authority, or TAA. These solutions come from a set of compatible components available from a variety of suppliers.

Figure 4-2. Trust attestation authority

 
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