This thesis explores key aspects of API security, emphasizing their role in protecting sensitive data and preventing unauthorized access. It analyzes the fundamental security mechanisms essential for controlling access to resources and safeguarding user information. Understanding these mechanisms enables better management of security risks in a dynamic environment. Cryptographic techniques are examined, including symmetric and asymmetric cryptosystems, which ensure the confidentiality, integrity, and non-repudiation of data in transit. These cryptographic foundations facilitate the development of more advanced security mechanisms necessary for effective API protection. Particular emphasis is placed on proof of possession techniques for cryptographic keys, such as mTLS and DPoP, which ensure that only authorized users can access protected resources. Through a comparison of these techniques, the thesis illustrates their advantages and limitations in various security scenarios. These methods significantly reduce the risk of attacks on communication channels and unauthorized access, making them essential in the design of secure and reliable APIs.