What is Zero-Knowledge Protocol?

Zero-Knowledge Protocol

A Zero-Knowledge Protocol (ZKP) in cryptography allows sharing and verifying information without providing data that is not necessary.

We are faced with a procedure with two agents: the “tester” who says something is true and the “verifier” who verifies it. Thus, the second can demonstrate the integrity of the information without needing the first to provide sensitive information.

 Therefore, we face a data protection system that allows encryption, leading to a technological revolution. We will see an example written in 1992 by Louis Guillou, Jean-Jacques Quisquater, and Thomas Berson.

Origin of the Zero-Knowledge protocol

 The zero-knowledge protocol has more than 50 years of research behind it. Let’s look at some of the most relevant events in its history. 

  • Cryptography has always been a way to protect sensitive or important information that we don’t want to be known. The old systems were very simple and based on secret codes.
  • With the advent of computers, it became a much more complex problem. However, these allowed the generation of codes based on mathematics that offered greater security in encryption.
  • Asymmetric encryption was revolutionized by its creators, Whitfield Diffie and Martin Hellman. These researchers, in 1976, baptized (using their surnames) the algorithm used in Internet security.
  • For his part, David Chaum devised the blind signature protocol in 1982, which allowed digital signatures without exchanging sensitive information. He created a group system that offered the possibility of digitally signing several people.
  • Shafi Goldwasser, Silvio Micali, and Charles Rackoff were the fathers of the zero-knowledge protocol, a technological revolution compared to previous systems.

 Characteristics of the Zero-Knowledge Protocol

Let’s see some of the characteristics of this protocol that, in addition, are essential requirements for its use. 

  • Solvency: The probability that the “tester” cheats the verifier is very small, as we see in the example.
  • Whole: If the statement is true, it can be tested and verified with an almost infinite probability.
  • The protocol allows this exchange of information without providing additional data in operation. For this reason, the system protects the user’s privacy.

 Zero-Knowledge Protocol Types

We can classify the zero-knowledge protocol, based on the knowledge tests performed, into two groups:

  • In the interactive ones, IZKP, both agents must be present while exchanging information. Normally, the tester sends a message to the verifier, who checks that he is telling the truth. This situation is repeated several times.
  • In the non-interactive NZKP, only the tester must be present. This generates a protocol that can be reviewed later by the verifier.

 The Fiat-Shamir heuristic is used to go from an interactive test to a non-interactive one, allowing one to be transformed into the other. Finally, an example of IZKP is the Chaum-Pedersen Protocol, which verifies mathematical calculations with pairs of numbers.

 For example, the cave of Ali Baba.

 Let’s see the example used by Louis Guillou, Jean-Jacques Quisquater, and Thomas Berson. Let us remember the story of Ali Baba and her cave and imagine two people (Juan and Ana) in front of her. Of course, only Ana (tester) knows the magic word that opens the door.

The cave is ring-shaped and has two entrance options; in one, B, the door is closed. Ana (the tester) uses a path to enter, chosen at random. Meanwhile, Juan (the verifier) ​​waits outside.

Now Juan enters and tells Ana to return via path B. He does not know the path Ana took to enter, but now he stands at the crossroads and can know the path she will take to return. In this way, he will check whether or not Ana knows her magic word without her telling him what it is.

She can know how to get in and, therefore, get out. But the other option is that she doesn’t know her and can only go back through A (she can’t open the door). Thus, the probability that Juan is correct is 50%, which would be due to chance and would not be useful.

Now let’s repeat the experiment 30 times, and the error probability approaches zero. Therefore, Ana is honest and does not lie when she says she knows the way or will have a problem returning, thanks to the zero-knowledge protocol.


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