Authentication with Shared Secrets SECRET SECRET msg,H(SECRET,msg) Alice Bob Alice wants to ensure that nobody modifies message in transit (both integrity and authentication) Idea:given msg, very hard to compute H(SECRET,msg)without SECRET; easy with SECRET 55 Authentication with Shared Secrets Alice wants to ensure that nobody modifies message in transit (both integrity and authentication) Idea: given msg, very hard to compute H(SECRET, msg) without SECRET; easy with SECRET msg, H( SECRET, msg ) Alice Bob SECRET SECRET