How to convert protocol buffers message to byte[]

here is my code i'm still new with this protocol buffer how can i convert the protocol buffer to byte[] so that i can encrypte

for (ClientRequest req : check.getClientRequestGroup().getClientRequests()) {
            ClientRequestProtos.GetUnixDormantUsers.Builder guduBuilder =     ClientRequestProtos.GetUnixDormantUsers.newBuilder();
            guduBuilder.setCheckFiles(true);
            guduBuilder.setCheckProcesses(true);
            guduBuilder.setThreshold(30);
            guduBuilder.addExcludeUser("root");
            guduBuilder.addExcludeUser("bin");
            ClientRequestProtos.GetUnixDormantUsers guduMessage = guduBuilder.build();

            ClientRequestProtos.ClientRequest.Builder clientBuilder = ClientRequestProtos.ClientRequest.newBuilder();
            clientBuilder.setId(req.getId());
            clientBuilder.setSentAt(req.getSentAt().getTime());
            clientBuilder.setType(ClientRequestType.GET_UNIX_DORMANT_USERS);
            clientBuilder.setGetUnixDormantUsers(guduMessage);
            ClientRequestProtos.ClientRequest clientMessage = clientBuilder.build();



String ipAddress = mapHostIp.get(req.getHost());
                GenericUrl url = new GenericUrl("http://"+ ipAddress+":5736/request");
byte[] content; // convert protocol buffers to byte[]


            byte[] encrypted = Encrypt.encrypt(content);
            HttpRequest request = requestFactory.buildPostRequest(url, new ByteArrayContent(null, encrypted));
            request.execute();
        }