Message Encryption Source a Destination B M M K E(K, M K (a) Symmetric encryption: confidentiality and authentication A→B:E(K,M Provides confidentiality Only A and B share K Provides a degree of authentication Could come only from A Has not been altered in transit -Requires some formatting/redundancy Does not provide signature Receiver could forge message Sender could deny message (a) Symmetric encryption 復大软件学院12 Message Encryption