Clarify mkey as local to a single attester for one appraisal context.

To allow for mkey to have effective use, add an mkeyvalues codepoint to
measurement-map to allow for property/claim names to be (locally to the
environment) named by an mkey to make the measurement-values-map more
reusable.

Signed-off-by: Dionna Glaze <dionnaglaze@google.com>

cddl/corim-frags.mk

@@ -29,6 +29,7 @@ COMID_FRAGS += mac-addr-type-choice.cddl
 COMID_FRAGS += measured-element-type-choice.cddl
 COMID_FRAGS += measurement-map.cddl
 COMID_FRAGS += measurement-values-map.cddl
+COMID_FRAGS += mkeyvalue-pair.cddl
 COMID_FRAGS += non-empty.cddl
 COMID_FRAGS += oid.cddl
 COMID_FRAGS += raw-value.cddl

cddl/measured-element-type-choice.cddl

@@ -1,3 +1,4 @@
 $measured-element-type-choice /= tagged-oid-type
 $measured-element-type-choice /= tagged-uuid-type
 $measured-element-type-choice /= uint
+$measured-element-type-choice /= tstr

cddl/measurement-map.cddl

@@ -1,5 +1,6 @@
 measurement-map = {
   ? &(mkey: 0) => $measured-element-type-choice
-  &(mval: 1) => measurement-values-map
+  ? &(mval: 1) => measurement-values-map
   ? &(authorized-by: 2) => [ + $crypto-key-type-choice ]
+  ? &(mkeyvalues: 3) => [ + mkeyvalue-pair ]
 }

cddl/mkeyvalue-pair.cddl

@@ -0,0 +1,4 @@
+mkeyvalue-pair = [
+  mkey: $measured-element-type-choice
+  mval: measurement-values-map
+]

draft-ietf-rats-corim.md

@@ -795,25 +795,43 @@ the `authorized-by` statement can be supplied in the `measurement-map`.
 {::include cddl/measurement-map.cddl}
 ~~~
 
+where `mkeyvalue-pair` is
+
+~~~ cddl
+{::include cddl/mkeyvalue-pair.cddl}
+~~~
+
 The following describes each member of the `measurement-map`:
 
-* `mkey` (index 0): An optional unique identifier of the measured
-  (sub-)environment.  See {{sec-comid-mkey}}.
+* `mkey` (index 0): DEPRECATED (remove for final RFC) An optional identifier of the measured element.
+  Mutually exclusive with `mkeyvalues`.
+  See {{sec-comid-mkey}}.
 
-* `mval` (index 1): The measurements associated with the (sub-)environment.
+* `mval` (index 1): The measurements associated with the environment's measured element.
+  If not present, `mkeyvalues` MUST be present. Mutually exclusive with `mkeyvalues`.
   Described in {{sec-comid-mval}}.
 
 * `authorized-by` (index 2): The cryptographic identity of the individual or organization that is
  the designated authority for this measurement. For example, producer of the measurement or a delegated supplier.
 
+* `mkeyvalues` (index 3): Measurements associated with the environment with local names to distinguish measured elements of the same measurement value type.
+  Each `mkey` in the list must be unique within the list. If not present, `mval` MUST be present. Mutually exclusive with use of `mkey` and `mval`.
+
 ###### Measurement Keys {#sec-comid-mkey}
 
-The types defined for a measurement identifier are OID, UUID or uint.
+The types defined for a measurement key are ~OID, UUID~ (DEPRECATED: remove for final RFC), uint, or a textual string.
 
 ~~~ cddl
 {::include cddl/measured-element-type-choice.cddl}
 ~~~
 
+A measurement key serves is a local identifier within the scope of the Environment.
+A measurement key SHOULD be associated with a `measurement-values-map` to disambiguate values of the same type that are associated with different measured elements.
+For example, Evidence may include both the firmware version at boot and the firmware version at evidence collection time when a firmware hotloading feature does not require a reboot.
+Both versions use codepoint 0 in the `measurement-values-map`, but they are semantically distinct measured elements.
+
+A measurement key MUST NOT be used as a refinement on the `environment-map`, since it names an element from a specific Environment's collection of measurements in one appraisal context.
+
 ###### Measurement Values {#sec-comid-mval}
 
 A `measurement-values-map` contains measurements associated with a certain
@@ -2027,12 +2045,13 @@ indicates that the authority named by `measurement-map`/`authorized-by`
 asserts that the actual state of one or more Claims within the
 Target Environment, as identified by `environment-map`, have the
 measurement values in `measurement-map`/`mval`.
+If the Claim name is associated with an `mkey` the measurement value may be in `measurement-map`/`mkeyvalues`[`mkey`]/`mval`.
 
 ECT authority is represented by cryptographic keys. Authority
 is asserted by digitally signing a Claim using the key. Hence, Claims are
 added to the ACS under the authority of a cryptographic key.
 
-Each Claim is encoded as an ECT. The `environment-map` and a
+Each Claim is encoded as an ECT. The `environment-map`, the `mkey`, and the
 key within `measurement-values-map` encode the name of the Claim.
 The value matching that key within `measurement-values-map` is the actual
 state of the Claim.