The Reading Module. With an appropriately sophisticated and careful supervisor program, we now have an example of a system that completely isolates its users from one another

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With an appropriately sophisticated and careful supervisor program, we now have an example of a system that completely isolates its users from one another. Similarly isolated permanent storage can be added to such a system by attaching some longterm storage device (e.g., magnetic disk) and developing a similar descriptor scheme for its use. Since long-term storage is accessed less frequently than primary memory, it is common to implement its descriptor scheme with the supervisor programs rather than hardware, but the principle is the same. Data streams to input or output devices can be controlled similarly. The combination of a virtual processor, a memory area, some data streams, and an isolated region of long-term storage is known as a virtual machine.¹⁴

Long-term storage does, however, force us to face one further issue. Suppose that the virtual machine communicates with its user through a typewriter terminal. If a new user approaches a previously unused terminal and requests to use a virtual machine, which virtual machine (and, therefore, which set of long-term stored information) should he be allowed to use? We may solve this problem outside the system, by having the supervisor permanently associate a single virtual machine and its long-term storage area with a single terminal. Then, for example, padlocks can control access to the terminal. If, on the other hand, a more flexible system is desired, the supervisor program must be prepared to associate any terminal with any virtual machine and, as a result, must be able to verify the identity of the user at a terminal. Schemes for performing this authentication are the subject of our next example.

4) Authentication Mechanisms: Our second example is of an authentication mechanism: a system that verifies a user's claimed identity. The mechanics of this authentication mechanism differ from those of the protection mechanisms for implementing virtual machines mainly because not all of the components of the system are under uniform physical control. In particular, the user himself and the communication system connecting his terminal to the computer are components to be viewed with suspicion. Conversely, the user needs to verify that he is in communication with the expected computer system and the intended virtual machine. Such systems follow our abstract model of a guard who demands a match between something he knows and something the requester possesses. The objects being protected by the authentication mechanism are the virtual machines. In this case, however, the requester is a computer system user rather than an executing program, and because of the lack of physical control over the user and the communication system, the security of the computer system must depend on either the secrecy or the unforgeability of the user's identification.

In time-sharing systems, the most common scheme depends on secrecy. The user begins by typing the name of the person he claims to be, and then the system demands that the user type a password, presumably known only to that person.

There are, of course, many possible elaborations and embellishments of this basic strategy. In cases where the typing of the password may be observed, passwords may be good for only one use, and the user carries a list of passwords, crossing each one off the list as he uses it. Passwords may have an expiration date, or usage count, to limit the length of usefulness of a compromised one.

The list of acceptable passwords is a piece of information that must be carefully guarded by the system. In some systems, all passwords are passed through a hard-to-invert transformation¹⁵ before being stored, an idea suggested by R. Needham [37, p. 129]. When the user types his password, the system transforms it also and compares the transformed versions. Since the transform is supposed to be hard to invert (even if the transform itself is well known), if the stored version of a password is compromised, it may be very difficult to determine what original password is involved. It should be noted, however, that "hardness of inversion" is difficult to measure. The attacker of such a system does not need to discern the general inversion, only the particular one applying to some transformed password he has available.

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