parsec-evidence-tpm.md

TPM Parsec Evidence

This document describes the format of the credential produced by the Parsec service as a key attestation credential for TPM-backed keys.

The Parsec service produces a blob of binary data as a response to a key attestation operation issued by a client. The service takes in a nonce and uses it as freshness for the two attestation tokens it produces.

The attestation token produced by Parsec is a CBOR encoded structure, namely a Combined Attestation Bundle (CAB). The full high-level format is shown below.

parsecTpmAttestation = {
    "kat" => parsecTpmKeyStmtFormat,
    "pat" => parsecTpmPlatStmtFormat
}

The definition of parsecTpmKeyStmtFormat can be found in this document, while the definition of parsecTpmPlatStmtFormat can be found in this document.

The CAB produced by Parsec is then encapsulated in a Conceptual Message Wrapper, which is simply a CBOR-encoded array with the first element identifying the type of the second element. In this specific case, the type is represented by the string application/vnd.parallaxsecond.key-attestation.tpm, a media type string:

[
    "application/vnd.parallaxsecond.key-attestation.tpm"
    bytes .cbor parsecTpmAttestation 
]