M2 MAC

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Lesson M2: CBC-MAC, retail MAC, CMAC and HMAC: padding, truncation, what is MACed and with which key.

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The opening of the lesson

Why it matters

Encryption hides data, but it does not stop anyone changing it. A message authentication code (MAC) proves that a message comes from someone holding the key and was not altered on the way:

an amount changed in transit;
a response code flipped from 05 to 00;
a replayed approval.

These are the attacks a MAC stops. Terminal–host and host–host links carry one in field 64 or 128.

In the full lesson

The concept
Step by step: retail MAC
Keys
Common mistakes
What to test

What module M2 covers →

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