3 Message authentication codes

Mix a hash function with a secret key, and you obtain something called a message authentication code (MAC). The addition of a secret key is the foundation behind any type of security: without keys there can be no confidentiality, and there can be no authentication. While hash functions can provide authentication or integrity for arbitrary data, they did this thanks to an additional trusted channel. In this chapter you will see how a MAC can be used to create such a trusted channel, and what else it can do.

For this chapter you’ll need to have read:

3.1 Stateless cookies, a motivating example for message authentication codes

Let’s picture the following scenario: you are a webpage. You’re bright, full of colors, and above all you’re proud of serving a community of loyal users. To interact with you, visitors must first log-in by sending you their credentials, which you must then validate. If the credentials match those that were used when the user first signed up, then you have successfully authenticated the user.

3.2 An example in code

3.3 Security properties of a message authentication code

3.3.1 Forgery of authentication tag

3.3.2 Lengths of authentication tag

3.3.3 Replay attacks

3.3.4 Verifying authentication tags in constant-time

3.4 MAC in the real world

3.5 Message authentication codes in practice

3.5.1 HMAC, a hash-based message authentication code

3.5.2 KMAC, a hash based on cSHAKE

3.6 SHA-2 and length-extension attacks

3.7 Summary