MAC

Message authentication code

Not to be confused with MAC addresses.

• Add a secret to the end of each message that is also hashed. It is extremely unlikely that anyone who doesn’t know the secret to come up with an appropriate hash
• Shared secret $s$ (this is a symmetric key)
• Hash is computed not on message $m$, but on $m+s$
  • Bob sends message $h=H(m+s)$
  • Alice receives $(m,h)$ and computes $H(m+s)$
  • If $h=H(m+s)$, message is considered signed
• Fast because asymmetric key crypto is not necessary