Think of a part of a government with no sign on the door. What it does is not announced, and both of its jobs are kinds of protecting: keeping a country's secrets safe, or keeping a country's leader safe. Service here is the old sense of an official body, as in the postal service; secret describes the way it works, out of sight.
Two branches
情報：守住祕密，也去打聽祕密
這一支最貼近字面：一個不公開的政府單位，手上處理的正好就是「祕密」本身。
noun · B2
A part of a government that works out of public view: it guards its own country's military and political secrets, and tries to find out the secrets of other countries. In Britain, this organization is called the Secret Service.
情報機關
保護：美國的特勤局
一樣是那個不張揚的政府單位，只是保護的對象從「祕密」換成了「人」。
noun · B2
In the United States, the government department that does special kinds of police work. The job it is best known for is guarding the president and other important leaders.
特勤局