Message Authentication Codes

Message Authentication Codes

1. What Is a MAC? (beginner — keyed fingerprint!)

MAC = fingerprint mixed with SECRET key: $tag = MAC(K, m)$. Receiver recomputes with same $K$ — match? (1) unchanged! (2) from key-holder! Eve lacks $K$ → can't forge! Unlike hash (anyone recomputes!).

2. Flow (exam diagram!)

Alice: tag = MAC(K, "Pay ₹1000") → sends (message, tag)
Bob: recomputes MAC(K, received message) — equals tag? accept : reject!
Eve flips ₹1000→₹9000 → Bob's recompute differs → REJECTED!

3. Constructions (exam list!)

Type	Idea	Example
Hash-based	nested hash with key	HMAC (standard!)
Cipher-based	last block of encrypted chain	CBC-MAC, CMAC (AES!)
Universal-hash	polynomial math (fast!)	UMAC, Poly1305 (ChaCha!)

4. Rules (traps!)

• Needs SHARED key first (key-distribution problem — DH/RSA exchange it!).
• NO non-repudiation (both sides hold $K$ — either could forge! need signatures for court!).
• Verify in CONSTANT time (compare digests without early-exit — else timing leak!).
• Never plain $Hash(K + m)$ with Merkle-Damgård (length-extension! use HMAC!).

5. Exam Points

• Define + flow + forge-proof — 5-7 marks, very frequent.
• Constructions + no-non-repudiation — 3-5 marks, frequent.