Data Protection and Security

I

Secret Key Cryptography

I.III

Message Authentication Codes

Message authentication code (MAC) is a number that is appended to a message. The purpose is not secrecy but only to authenticate the message using secret-key cryptography e. g. to verify that an email is really coming from your advisor. Assume that you and your advisor share a secret key k. The following simulation (steps) demonstates how to achieve message authentication using a MAC algorithm:

1. Your advisor computes a MAC of the email message P with t he shared secret key k yielding an output MAC M1_k.
2. He then sends P as well as M1_k to you.
3. When you get P, you also compute a MAC by using k and obtain the MAC M2_k .
4. If M2_k is as same as M1_k, the email message is really coming from your advisor, otherwise not.

Note that CRC (cyclic redundancy checks) adds redundancy to the message in order to detect unintentional or physical errors in transmission or storage. On the other hand MAC algorithms provide data integrity in spite of intelligent adversaries.

We have already seen how message authentication codes can be generated using hash functions. Another standard way is to use a symmetric cipher in CBC mode of operation and send only the last ciphertext block together with the plaintext message. There are also specially-designed MAC algorithms available.

Animation I.III-I

MAC Workflow simulation . [click to enlarge]