This lecture covers the fundamental principles of cryptography, focusing on symmetric and asymmetric cryptography for confidentiality, integrity, and responsibility. It explains how digital signatures provide defense against repudiation threats and the compatibility issues between responsibility and confidentiality. The advantages of asymmetric cryptography for ensuring both integrity and responsibility are highlighted, emphasizing the role of digital signatures. The lecture also delves into the practical aspects of RSA signatures and the importance of identifying threats and appropriate defense levels in a cryptographic system.