http://lomteqnxehqjdemqgzse7xchjlusgnpoxq2fpmz3yuukyfzc22t5gwyd.onion/jira_projects/browse/LOC-10.html
That can be addressed, to some extent, by Alice asking questions to verify the authenticity of Bob (i.e. only questions he would know the answer to) - at least, so long as bob isn't actively co-operating with those authorities. However, if we accept that messages within the system may not be authentic, once Bob passed the test, Chuck could subsequently start spoofing his headers to appear as though Bob is sending messages (particularly if Bob has gone quiet).