public Response createLink(@Context Credentials credentials,
@PathParam("user_id") String userId, @PathParam("name") String name,
@PathParam("reader_id") String readerId) {
final Session session = credentials.buildSession(userDAO, userId);
final User reader = findUser(readerId);
final Document doc = findDocument(session.getUser(), name);
doc.linkUser(reader);
reEncrypt(doc, session.getKeySet());