Standard measurement format

Signed-off-by: Steven Bellock <[email protected]>

Author: steven-bellock

doc/standard_measurement_report.md

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+# Standard Measurement Report
+
+The SPDM specification grants the Requester flexibility when retrieving measurements from a
+Responder via `GET_MEASUREMENTS`. This flexibility includes the presence or absence of a signature,
+multiple `GET_MEASUREMENTS` requests to individual measurement indices, and the ability to indicate
+whether the Requester desires the measurements to be encoded as a raw bitstream or a cryptographic
+hash. However, it is possible for a Requester to construct measurement artifacts that are not easily
+consumed by a Verifier, if at all. For example a Verifier may reject the evaluation of multiple
+`GET_MEASUREMENTS` requests to the same Responder.
+
+This document describes a standard measurement report for an SPDM Responder that is constructed by a
+SPDM Requester and consumed by a Verifier while all agents are operating in production mode. In
+particular, a production Verifier may only support a measurement report of this type and can point
+to this document to advertise that restriction to other agents.
+
+## Standard Measurement Report Definition
+
+The standard measurement report is a byte buffer that consists of the L1/L2 transcript along with
+the signature over the transcript if the Responder supports signing. It is comprised of a single
+`GET_MEASUREMENTS` request and a single `MEASUREMENTS` response.
+
+For SPDM 1.0 and 1.1, the byte buffer is {`GET_MEASUREMENTS`, `MEASUREMENTS`}. For SPDM 1.2 and
+later, the byte buffer is {`VCA`, `GET_MEASUREMENTS`, `MEASUREMENTS`}. The `GET_MEASUREMENTS`
+request has the following properties:
+* `Param2 = 0xFF`
+    * All measurement indices are requested.
+* If the Responder supports signature generation (`MEAS_CAP = 10b`) then `SignatureRequested` is
+  set, else it is not set.
+* For SPDM 1.2 and later, `RawBitStreamRequested` is not set.
+    * This is a hint to the Responder to hash measurements instead of producing their raw values.
+* For SPDM 1.3 and later, `NewMeasurementRequested` is not set.
+    * This requests the current state of the Responder and not its future state.
+
+## Rationale
+
+### Single Request and Response
+
+Capturing all measurements in a single response provides an atomic snapshot of the state of the
+Responder at a specific point in time. As such, a Verifier need not have to reason about the state
+of the Responder through multiple measurement requests and responses with a possibly unknown amount
+of time between each message.
+
+### Byte Buffer
+
+A raw byte buffer allows the Verifier to verify the signature, if supported by the Responder, over
+the rest of the measurement report without any transformation of data.
+
+### `RawBitStreamRequested`
+
+In the interest of message size, `RawBitStreamRequested` is not set, indicating preference, when
+given the opportunity, for hashed measurements instead of raw measurements. In particular, certain
+raw measurements may only be examined for the purpose of debugging, whereas the hashed measurements
+are evaluated by the Verifier while the Responder is in production.
+
+### `NewMeasurementRequested`
+
+Presumably the Verifier evaluates the current state of the Responder and not its future state. As
+such `NewMeasurementRequested` is not set.