Lecture Notes: Message Authentication Codes (MACs) What are MACs? Message Authentication Codes (MACs) are cryptographic functions used to verify the integrity and authenticity of a message. They provide a way to ensure that a message has not been modified or tampered with during transmission. How MACs Work 1. Key Generation: A shared secret key is generated and distributed to both the sender and receiver. 2. MAC Calculation: The sender calculates a MAC value for the message using the shared key and a cryptographic algorithm. 3. Transmission: The message and the MAC value are transmitted to the receiver. 4. MAC Verification: The receiver calculates a MAC value for the received message using the same key and algorithm. 5. Comparison: If the calculated MAC value matches the received MAC value, the message is considered authentic and has not been modified. Types of MACs ● HMAC (Hash-based Message Authentication Code): A popular MAC algorithm that combines a cryptographic hash function (such as SHA-256) with a secret key. ● CBC-MAC (Cipher Block Chaining Message Authentication Code): A MAC algorithm that uses a block cipher in CBC mode to calculate the MAC value. ● CMAC (Cipher-based Message Authentication Code): A MAC algorithm that is similar to CBC-MAC but uses a different keying mechanism. Advantages of MACs ● Efficiency: MACs are generally more efficient than digital signatures, especially for short messages. ● Simplicity: MACs are relatively simple to implement and use. ● Integrity and Authenticity: MACs can verify the integrity and authenticity of a message, ensuring that it has not been modified or tampered with. Disadvantages of MACs ● Shared Secret Key: The shared secret key must be securely distributed to both the sender and receiver. ● Limited Non-Repudiation: MACs do not provide non-repudiation, meaning that the sender can deny sending the message. Applications of MACs ● Message Integrity: MACs are used to verify the integrity of messages, ensuring that they have not been modified during transmission. ● Authentication: MACs can be used to authenticate the sender of a message. ● Secure Protocols: MACs are used in various secure protocols, such as IPsec and TLS. Conclusion Message Authentication Codes are essential tools for ensuring the integrity and authenticity of digital messages. By understanding the principles of MACs and selecting appropriate algorithms, organizations can protect their sensitive information and prevent unauthorized access or modifications.