Add Group Key term.

pep-keysync/draft-hoeneisen-pep-keysync.mkd

@@ -116,6 +116,10 @@ not leaked or exposed to theft -- end-device security being out-of-scope.
   cryptographic keys, trust information, calendar, configuration and other
   data. The data is synchronized through a common data channel for a given
    identity, e.g., a mailbox in email.
+* Group Key: A key pair primarily used by a Device Group for a certain 
+  identity. Each uncorrelated network address, e.g. an email address, has its 
+  own Group Key. That is, a Device Group can have several Group Keys. One for
+  each identity.
 * Sole Device: A device which is not part of a Device Group.
 * Grouped Device: A device which is already part of a Device Group.
 * Beacon (message): A technical text message that is broadcasted by Sole
@@ -242,7 +246,7 @@ her peers are be reflected on any device she uses.
 ## Interaction Diagrams
 
 The following interaction diagrams depict the implementations for some
-of the above Use Case in a simplified manner (i.e. some aspects are
+of the above Use Case in a simplified manner (i.e., some aspects are
 skipped for the sake of better readability). A decription is added
 after each diagram.