The workshop presents the zero-knowledge-based protocols that are the foundation of current research in cryptography. We will see that it is possible to prove that you have a key without showing it, or showing how it is used, or providing anything that could be used to try to figure out the key. In a practical dynamic we will see how it is possible to play a game of chance over the phone, providing certainty so that the parties are sure that they have not been deceived in the process. This workshop requires a minimum mathematical base (factorization, prime numbers), so it is indicated for high school students, i en concret un perfil de ciències.