This chapter presents a collection of problems focused on authentication methods, systems, and associated attacks. Given the widespread and persistent reliance on passwords, several exercises explore strategies for breaking them. These include calculating the size of the password space and evaluating the computational effort required for password-cracking attempts. Additional problems address threats to password secrecy and examine the controls systems employ to secure password databases against unauthorized access. A second set of exercises broadens the scope to consider alternative authentication methods, such as one-time passwords, lookup secrets, out-of-band authentication devices, and federated identity systems. These problems highlight the principles and challenges of implementing these methods in real-world scenarios. Finally, the chapter includes exercises introducing foundational cryptographic protocols for authentication, utilizing both symmetric and public-key cryptography. Emphasis is placed on identifying weak constructions, analyzing potential vulnerabilities, and understanding common attack vectors. Together, these exercises aim to provide a comprehensive exploration of the complexities and considerations inherent in modern authentication systems.