Cryptography has long been an essential tool in safeguarding digital communication and securing sensitive information. As technology has progressed, so has the complexity of the methods used to protect our data. In the wake of quantum computing's rise, traditional cryptographic systems face serious challenges, demanding a new understanding of how quantum algorithms could both undermine and enhance security.

Chapter 1 deals with the Basics of Cryptography lays the groundwork by introducing classical cryptography, tracing its evolution from ancient ciphers to modern cryptosystems. It provides foundational concepts such as stream and block ciphers and distinguishes between symmetric and asymmetric key systems, setting the stage for deeper cryptographic analysis in later chapters.

In Chapter 2, readers are introduced to Quantum Algorithms, the principles of quantum mechanics relevant to computing, including qubits, superposition, and entanglement. The chapter draws distinctions between classical and quantum computation and introduces key quantum algorithms and gates—forming the conceptual basis for understanding quantum cryptanalysis.

The Chapter 3 focuses on Shor’s algorithm, a landmark quantum algorithm that threatens the security of widely used public-key cryptosystems like RSA and ECC. Readers explore its theoretical foundation, circuit implementation, and resource estimation, including practical experiments using IBM Qiskit.

In Chapter 4 Grover’s Algorithm is examined in the context of brute-force attacks on symmetric key cryptography. The chapter includes a detailed cryptanalysis of the simplified Grain cipher using Grover’s technique, and presents experimental results and quantum resource estimates for various attack scenarios.

Chapter 5 focusing on Simon’s Algorithm and its role in breaking cryptographic primitives through structure exploitation. It details the application of Simon’s algorithm to stream ciphers like Grain-128a, with thorough circuit design, implementation strategies, and test case evaluations.

In Chapter 6 a broader discussion about Cryptographic Implications of Quantum Computing is given on how quantum computing affects modern cryptographic systems. It explains the vulnerabilities of RSA, ECC, AES, and other algorithms under quantum threats and underscores the urgent need for transitioning to quantum-resistant alternatives.

Finally, in Chapter 7, the future of cryptography in the quantum era is discussed. It introduces Mosca’s Theorem for risk forecasting and presents a taxonomy of quantum-safe cryptographic primitives. Topics include quantum key distribution (QKD), post-quantum cryptography (PQC), countermeasures, standardization, and infrastructure readiness for quantum networks.