digest algorithm clarifications

Fix #60

Signed-off-by: Thomas Fossati <thomas.fossati@arm.com>

draft-ietf-rats-corim.md

@@ -71,10 +71,12 @@ normative:
     seriesinfo:
       ITU-T: Recommendation X.690
     target: https://www.itu.int/rec/T-REC-X.690
+  IANA.named-information: named-info
 
 informative:
   RFC7942:
   I-D.fdb-rats-psa-endorsements: psa-endorsements
+  I-D.tschofenig-rats-psa-token: psa-token
 
 entity:
   SELF: "RFCthis"
@@ -239,7 +241,14 @@ namespace under which the value must be understood.
 ### Digest {#sec-common-hash-entry}
 
 A digest represents the value of a hashing operation together with the hash
-algorithm used.
+algorithm used.  The type of the digest algorithm identifier can be either
+`int` or `text`.  When carried as an integer value, it is interpreted according
+to the "Named Information Hash Algorithm Registry" {{-named-info}} registry.
+When it is carried as `text`, there are no requirements with regards to its
+format.  In general, the `int` encoding is RECOMMENDED.  The `text` encoding
+should only to be used when the `digest` type conveys reference value
+measurements that must be matched verbatim with evidence that uses the same
+convention - e.g., {{Section 4.4.1.5 of -psa-token}}).
 
 ~~~ cddl
 {::include cddl/digest.cddl}