Role instances and kinds

[sam-m-weber]

How many role instances are there, and what kinds are there? From the "Detailed Design of Uptane" it can be inferred that there are at least eight, from the statement that "each" repository has the Root, Timestamp, Snapshot and Target roles. Ie, there is the image repository's Root role, the Director repository's Root role, etc.

Unfortunately, at other places the document talks about "The Root role" and "The Targets role", etc, implying that there is just one Root role, one Targets role, etc. (And therefore that the Director and Image repository have copies of the same roles: ie, the Director's Root is the same as the Image Repository Root). Could this be made clearer by replacing "The Root role" with "A Root role" and similar changes?

Also, in the #targets_role subsection of #roles, it is stated that there are other custom-defined roles. Do these custom-defined roles act like Target roles, and could there existence be listed in places like the "Uptane Role Terminology" where the various roles are enumerated? In particular, in the "Common Metadata Structures" it is not clear if custom roles should be of type "Targets" or if this section is incomplete. Does a Root role sign the public keys for these custom roles as well?