Password on Blank Page

Lao Mao has an errand in the Cultural Relics Bureau and is usually diligent and seldom makes mistakes. One day before getting off work, Lao Mao was cleaning up the drawer and found something in the drawer, a sealed envelope. After he opened it, he found a piece of white letter paper inside. The story unfolded from there. What was it? And what kind of secret is hidden behind it?