In the early 1980s, Goldwasser and others put forward the concept of zero knowledge proof. In essence, zero-knowledge proof is a protocol. The so-called agreement is a series of steps taken by two or more participants to complete a specific task, including the following three characteristics: 1. The agreement is an orderly process from beginning to end, and each step should be executed in turn. Before the previous step is completed, the following steps cannot be executed. 2. An agreement needs at least two participants. One person can complete a task by performing a series of steps, but it does not constitute an agreement. 3. A zero-knowledge proof must include two aspects, one is the prover and the other is the verifier. The prover tries to prove to the verifier that an assertion is correct or that the prover has some knowledge. But it will not reveal any useful information to the verifier. Zero-knowledge proof is widely used in cryptography, especially in authentication protocols and digital signatures. Many excellent algorithms have been designed by using digital signature. Explain zero knowledge with a story about a cave. There is a secret in the cave. Those who know the spell can open the secret door between C and D. For others, both passages are dead ends. Peggy knows the secret of this cave. She wanted to prove it to Victor, but she didn't want to reveal the spell. Here's how she persuaded Victor: (1) Victor stood at point A. (2) Peggy went into the cave and reached point C or D. (3) After Peggy disappeared into the cave, Victor went to point B. (4) Victor shouted to Peggy to come out from the left or right channel. (5) Peggy agrees If necessary, she will use a spell to open the secret door. Peggy and Victor repeat steps (1) to (5) n times. Suppose Victor has a camera that can record everything he sees. He records the scene that Peggy disappears in the cave, records the time when he calls Peggy out of the place he chooses, and records the time when Peggy comes out. He recorded all the n experiments. What if he recorded this without knowing the spell? What if Peggy and Victor agreed in advance what Victor would shout? Peggy will definitely walk into the road that Victor tells her to come out, and then she can come out on the road that Victor tells her to come out every time without knowing the spell. Maybe they won't do that. Peggy walked into one of the passages, and Victor made a random request. If Victor is right, great. If he guesses wrong, they will delete the experiment from the video. In short, Victor can get a record that shows exactly the same sequence of events as Peggy's actual proof that she knew the spell. This shows two things. First, Victor can't convince the third party of the validity of this proof; Secondly, it is proved that the protocol is zero-knowledge. When Peggy didn't know the spell, Victor obviously couldn't get any information from the record. However, because it is impossible to distinguish between real records and forged records, Victor can't get any information from the actual proof-it must be zero knowledge. In other words, Peggy didn't reveal any secret knowledge in the process of proving to Victor, which is called zero knowledge.