An Operating Systems Vade Mecum

182 Transput Chapter 5Security: Encryption may be used to provide secrecy and authentication on nonlocalcommunication. Encryption may be used at any level, not just by the devicedriver.The usual point at which the kernel must deal with devices is when a processmakes a transput request. If the kernel keeps a cache of secondary-store blocks in mainstore, it can sometimes bypass dealing with a device. The operating-system designermust choose among many different styles of transput requests. The process can be givenlarger or smaller amounts of control over the way transput is conducted. Recent operatingsystems like Unix have successfully championed the policy of a very simple processinterface with an efficient kernel implementation. However, real-time operating systemstend to require more complex process control over transput.7 FURTHER READINGThe Datapro handbook (Heminway, 1986) has a wealth of information about computersand devices, with both overview sections and details of particular brands. The text byTanenbaum (1981) is a fine reference on communication devices and the protocols thatare used on them. Disk-head scheduling policies have been investigated by Fuller (1974)and reviewed by Teorey and Pinkerton (1972). Public-key cryptosystems were first proposedby Diffie and Hellman (1976). One elegant extension is Gifford’s notion of cryptographicsealing (1982).8 EXERCISES1. Why is it necessary for the escape character itself to be escaped if it appears in amessage?2. When A and B are sending messages to each other using public-key encryption,why does A first apply its own D and then B’s E? Why not perform these operationsin reverse order?3. When A and B are sending authenticated messages to each other, how does therecipient know whose E to apply to the message? After all, the recipient doesn’tknow who sent the message until it can read the message, and the message isinscrutable until it is converted to cleartext.4. The chief of security at Marble Pillar Stocks wants to use some encryption methodfor communication between the main office and the branches. She likes the idea ofthe one-time pad because it is provably secure. She likes the idea of RSA becauseit has not yet been broken. But the one-time pad has a key-distribution problem,and RSA is too time-consuming to calculate. She therefore suggests that RSA be