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What Are Message Authentication Codes?

What Is a MAC?

Preview: Learn more about Message Authentication Codes (MACs) and how they verify the authenticity and integrity of digital messages.

A Message Authentication Code (MAC) is a cryptographic value used to verify that a message has not been altered and that it originated from a trusted sender. A MAC is calculated from both the message itself and a shared secret key known only to the communicating parties. When the message is received, the recipient performs the same calculation using the shared key. If the newly calculated MAC matches the transmitted MAC, the message is considered authentic and unchanged.

The primary purpose of a MAC is to provide message authentication and data integrity. It confirms that the message originated from someone who possesses the shared secret key and that no changes have been made to the message during transmission or storage. If even a single bit of the message is modified, the calculated MAC changes completely, causing the authentication check to fail.

The process is straightforward. Before transmitting a message, the sender combines the message with the secret key using a cryptographic algorithm to generate a short authentication value known as the Message Authentication Code. This MAC is transmitted together with the message. Upon reception, the recipient performs the same calculation using the identical secret key. If both MAC values are identical, the message is accepted; otherwise, it is rejected as either corrupted or unauthorised.

A useful analogy is two people sharing a secret password. Before accepting an important package, the recipient asks the courier to provide the agreed password. If the password is correct, the recipient knows the courier came from the expected sender. A MAC performs a similar function mathematically by using a shared secret key rather than a spoken password.

Unlike a simple checksum or cyclic redundancy check (CRC), which detects accidental transmission errors, a MAC is designed to resist deliberate tampering. An attacker who does not know the secret key cannot generate a valid MAC, even if they understand the algorithm completely. This property makes MACs an essential component of secure communication protocols.

Several MAC algorithms are widely used. One of the most common is Hash-based Message Authentication Code (HMAC), which combines a cryptographic hash function, such as SHA-256, with a secret key. Other algorithms, such as CMAC, are based on symmetric encryption algorithms including the Advanced Encryption Standard (AES). Regardless of the implementation, the underlying objective remains the same: to verify both the authenticity and integrity of the message.

Message Authentication Codes are used extensively in secure communication systems. Internet protocols such as Transport Layer Security (TLS), IPsec, secure wireless networks, banking systems, cloud services, and virtual private networks (VPNs) all employ MACs to ensure that transmitted information has not been modified and originates from an authorised source.

It is important to distinguish a Message Authentication Code from a digital signature. A MAC uses a shared secret key, meaning that both sender and receiver possess the same key. Consequently, either party could have generated the MAC, so it does not provide non-repudiation. A digital signature, by contrast, uses public-key cryptography, allowing anyone to verify the signature while only the owner of the private key can create it. Digital signatures therefore provide authentication, integrity, and non-repudiation, whereas MACs provide authentication and integrity only.

Today, Message Authentication Codes are among the most widely used security mechanisms in digital communications. They provide an efficient and computationally inexpensive means of ensuring that messages remain authentic and unaltered, making them an essential component of modern cryptographic systems and secure network protocols.