I don't see how they're different then hash functions. Couldn't Alice use Bob's key to produce a MAC and send the message to Jim.

  • Do you mean a HMAC? – BadSkillz Feb 19 '15 at 9:18
  • MAC addresses don't provide any authenticity at all. Where did you read that they do? – AlexH Feb 19 '15 at 9:30
  • Whats the difference between MAC vs hash is closely related, although this question appears to be asking about a specific scenario involving three parties. – apsillers Feb 19 '15 at 13:28
  • @darkf Perhaps from its very name? It's an authentication code after all. – CodesInChaos Feb 19 '15 at 14:12
  • @CodesInChaos Yeah, my bad, I misunderstood the question and thought he was talking about MAC addresses! Thursday mornings eh? – AlexH Feb 19 '15 at 14:22

I assume you mean Message Authentication Code.

The major difference between a MAC and a hash function is that the MAC also takes a secret key as input. Therefore the MAC provides both integrity and authenticity for the message.

Example how it is used:

Both Alice and Bob know the secret key k. Alice wants to send an arbitrary long message m.

  1. She generates a tag. tag <- S(k,m).
  2. She sends message m together with the tag.

Bob verifies this.

  1. V(k,m,tag) = Yes/No

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