This course provides a comprehensive introduction to computer security, covering the foundational principles of secure systems and cryptography. It focuses on the core principles of designing secure systems, including the use of cryptography to ensure data confidentiality and integrity. Students will also learn how to approach systems from an adversarial perspective and study threat modeling to better understand and mitigate security threats. The course serves as an entry point for undergraduates interested in computer security and prepare students for advanced topics such as applied cryptography, systems security, machine learning security, and theoretical cryptography at the graduate level.