If a distributed system is built as middleware on an existing network operating system, then its security may depend on the security of the underlying local operating systems. a. true b. false A security policy describes precisely which actions the entities in a system are allowed to take and which ones are prohibited. a. true b. false A secure channel necessarily protects senders and receivers against interception, modification, fabrication, and interruption. a. true b. false The Data Encryption Standard (DES) is used for asymmetric cryptosystems. a. true b. false DES uses 64-bit data blocks. a. true b. false