Understanding How MACs Provide Authenticity

Understanding How MACs Provide Authenticity

In computer security, Message Authentication Codes (MACs) play a crucial role in providing authenticity for messages. A MAC is different from a hash function in that it also takes a secret key as input, ensuring both integrity and authenticity for the message.

Let’s take a closer look at how MACs work:

Imagine Alice wants to send a message to Bob, and both Alice and Bob know the secret key k. Here’s how the process unfolds:

1. Alice generates a tag for the message using the secret key: tag <- S(k,m).
2. Alice sends the message m along with the tag to Bob.

Bob then verifies the authenticity of the message:

1. Bob uses the same secret key, message, and tag to check if the verification is successful: V(k,m,tag) = Yes/No.

By using MACs, it is virtually impossible for an attacker, like Alice in your example, to produce a MAC using Bob’s key and send a forged message to Jim. The secret key is essential for the authentication process, ensuring the integrity and authenticity of the message.