How to use E-Signature as a session key for the RTC plugin

"com.ibm.workitem.attribute.RationalPassword" is of type E-Signature. PlugIn runs as followup-action when the work item reaches XYZ state. PlugIn uses current session users credentials to login another RTC project area to verify few items.

Currently code:
IWorkItem newState;
private IWorkItemServer fWorkItemService;
IAttribute esig = fWorkItemService.findAttribute(newState.getProjectArea(), "com.ibm.workitem.attribute.RationalPassword", monitor);           
String fSessionKey = getAttributeReadableValue(esig, newState, monitor);
RepositoryLogin repologin = new RepositoryLogin( <<JTS URI>>, fSessionUser, fSessionKey);

String fSessionKey is not printing any value in the ccm logfile.
String fSessionUser is the current session user and is printed correctly.
Login leads to error message in ccm.log: "Unable to login to  JTS Repository. Verify JTS Repo URI and Admin Credentials...CRJAZ0124E The user name or password is invalid....com.ibm.team.repository.transport.client.AuthenticationException: CRJAZ0124E The user name or password is invalid."