特性一：认证authentication

服务端 authentication拦截，主要基于 拦截器！
客户端的拦截，主要基于grpc对 authentication的封装，WithPerRPCCredentials()

实现方式：

func main() {
    flag.Parse()
    fmt.Printf("server starting on port %d...\n", *port)
    cert, err := tls.LoadX509KeyPair(testdata.Path("server1.pem"), testdata.Path("server1.key"))
    if err != nil {
        log.Fatalf("failed to load key pair: %s", err)
    }
    opts := []grpc.ServerOption{
        // The following grpc.ServerOption adds an interceptor for all unary
        // RPCs. To configure an interceptor for streaming RPCs, see:
        // https://godoc.org/google.golang.org/grpc#StreamInterceptor
        grpc.UnaryInterceptor(ensureValidToken),
        // Enable TLS for all incoming connections.
        grpc.Creds(credentials.NewServerTLSFromCert(&cert)),
    }
    s := grpc.NewServer(opts...)
    pb.RegisterEchoServer(s, &ecServer{})
    lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
    if err != nil {
        log.Fatalf("failed to listen: %v", err)
    }
    if err := s.Serve(lis); err != nil {
        log.Fatalf("failed to serve: %v", err)
    }
}

func main() {
    flag.Parse()

    // Set up the credentials for the connection.
    perRPC := oauth.NewOauthAccess(fetchToken())
    creds, err := credentials.NewClientTLSFromFile(testdata.Path("ca.pem"), "x.test.youtube.com")
    if err != nil {
        log.Fatalf("failed to load credentials: %v", err)
    }
    opts := []grpc.DialOption{
        // In addition to the following grpc.DialOption, callers may also use
        // the grpc.CallOption grpc.PerRPCCredentials with the RPC invocation
        // itself.
        // See: https://godoc.org/google.golang.org/grpc#PerRPCCredentials
        grpc.WithPerRPCCredentials(perRPC),
        // oauth.NewOauthAccess requires the configuration of transport
        // credentials.
        grpc.WithTransportCredentials(creds),
    }

    opts = append(opts, grpc.WithBlock())
    conn, err := grpc.Dial(*addr, opts...)
    if err != nil {
        log.Fatalf("did not connect: %v", err)
    }
    defer conn.Close()
    rgc := ecpb.NewEchoClient(conn)

    callUnaryEcho(rgc, "hello world")
}