These are footnotes from the Saltzer and Schroeder paper.
1A thorough and scholarly discussion of the concept of privacy may be found in [1], and an interesting study of the impact of technology on privacy is given in [2]. In 1973, the U.S. Department of Health Education, and Welfare published a related study [3]. A recent paper by Turn and Ware [4] discusses the relationship of the social objective of privacy to the security mechanisms of modern computer systems.

2 W. Ware [5] has suggested that the term security be used for systems that handle classified defense information, and privacy for systems handling nondefense information. This suggestion has never really taken hold outside the defense security community, but literature originating within that community often uses Ware's definitions.

3 Some authors have widened the scope of the term "protection" to include mechanisms designed to limit the consequences of accidental mistakes in programming or in applying programs. With this wider definition, even computer systems used by a single person might include "protection" mechanisms. The effect of this broader definition of "protection" would be to include in our study mechanisms that may be deliberately bypassed by the user, on the basis that the probability of accidental bypass can be made as small as desired by careful design. Such accident-reducing mechanisms are often essential, but one would be ill-advised to apply one to a situation in which a systematic attack by another user is to be prevented. Therefore, we will insist on the narrower definition. Protection mechanisms are very useful in preventing mistakes, but mistake-preventing mechanisms that can be deliberately bypassed have little value in providing protection. Another common extension of the term "protection" is to techniques that ensure the reliability of information storage and computing service despite accidental failure of individual components or programs. In this paper we arbitrarily label those concerns "reliability" or "integrity," although it should be recognized that historically the study of protection mechanisms is rooted in attempts to provide reliability in multiprogramming systems.

4 The broad view, encompassing all the considerations mentioned here and more, is taken in several current books [6]-[8].

5One can develop a spirited argument as to whether systems originally designed as unprotected, and later modified to implement some higher level of protection goal, should be reclassified or continue to be considered unprotected. The argument arises from skepticism that one can successfully change the fundamental design decisions involved. Most large-scale commercial batch processing systems fall into this questionable area.

6 An easier-to-implement strategy of providing shared catalogs that are accessible among groups of users, who anticipate the need to share was introduced in CTSS in 1962, and is used today in some commercial systems.

7Design principles b), d), f), and h) are revised versions of material originally published in Communications of the ACM [26, p. 398]. © Copyright 1974, Association for Computing Machinery, Inc., reprinted by permission.

8In this paper we have attempted to identify original sources whenever possible. Many of the seminal ideas, however, were widely spread by word of mouth or internal memorandum rather than by journal publication, and historical accuracy is sometimes difficult to obtain. In addition, some ideas related to protection were originally conceived in other contexts. In such cases, we have attempted to credit the person who first noticed their applicability to protection in computer systems, rather than the original inventor.

9We should note that the principle of open design is not universally accepted, especially by those accustomed to dealing with military security. The notion that the mechanism not depend on ignorance is generally accepted, but some would argue that its design should remain secret. The reason is that a secret design may have the additional advantage of significantly raising the price of penetration, especially the risk of detection.

10An interesting suggestion by Hollingsworth [29] is to secretly design what appear to be compromisable implementation errors, along with monitors of attempted exploitation of the apparent errors. The monitors might then provide early warning of attempts to violate system security. This suggestion takes us into the realm Of counterintelligence, which is beyond our intended scope.

11 In most implementations, addresses are also relocated by adding to them the value of the base. This relocation implies that for an address A to be legal, it must lie in the range (0 <= A < bound).

12The concepts of base-and-bound register" and hardware-interpreted descriptors appeared, apparently independently, between 1957 and 1959 on three projects with diverse goals. At M.I.T., J. McCarthy suggested the base-and-bound idea as part of the memory protection system necessary to make time-sharing feasible. IBM independently developed the base-and-bound register as a mechanism to permit reliable multiprogramming of the Stretch (7030) computer system [31]. At Burroughs, R. Barton suggested that hardware-interpreted descriptors would provide direct support for the naming scope rules of higher level languages in the B5000 computer system [32].

13Also called the master/slave bit, or supervisor/user bit.

14For an example, see IBM System VM/370 [11], which provides virtual IBM System/370 computer systems, complete with private storage devices and missing only a few hard-to-simulate features, such as self-modifying channel programs. Popek and Goldberg [33],[34] have discussed the general problem of providing virtual machines.

15For example, Purdy [35] suggests using the password as the parameter in a high-order polynomial calculated in modulo arithmetic, and Evans, Kantrowitz, and Weiss [36] suggest a more complex scheme based on multiple functions.

16 Actually, there is still one uncovered possibility: a masquerader could exactly record the enciphered bits in one communication, and then intercept a later communication and play them back verbatim. (This technique is sometimes called spoofing.) Although the spoofer may learn nothing by this technique, he might succeed in thoroughly confusing the user or the computer system. The general countermeasure for spoofing is to include in each enciphered message something that is unique, yet predictable, such as the current date and time. By examining this part of the message, called the authenticator the recipient can be certain that the deciphered message is not a replayed copy of an old one. Variations on this technique are analyzed in detail by Smith et al. [38].

17As shown later, in a computer system, descriptors can be used on the tickets.

18 Called an agency by Branstad [40]. The attendance of delegates at the various sessions of a convention is frequently controlled by an agency‹upon presentation of proof of identity, the agency issues a badge that will be honored by guards at each session. The agency issuing the badges is list-oriented, while the individual session guards (who ignore the names printed on the badges) are ticket-oriented.

19 The terms "process," "execution point," and "task" are sometimes used for this abstraction or very similar ones. We will use the term "virtual processor" for its self-evident operational definition, following a suggestion by Wilkes.

20The word "principal," suggested by Dennis and Van Horn [41], is used for this abstraction because of its association with the legal concepts of authority, accountability, liability, and responsibility. The detailed relationships among these four concepts are an interesting study, but inside the computer system, accountability is the only one usually mechanized. In defining a principal as the agent of accountability, we are restricting our attention to the individual guiding the course of the computation. We are avoiding the complication that responsibility for any specific action of a processor may actually be shared among.the user, the programmer, and the maintainer of the program being executed, among others.

21In some systems, more bits are used, separately controlling, for example, permission to call as a subroutine, to we indirect addressing, or to store certain specialized processor registers. Such an extension of the idea of separately controllable permissions is not important to the present discussion.

22Actually, this constraint has been introduced by our assumption that descriptors must be statically associated with a virtual processor. With the addition of protected subsystems, described later, this constraint is relaxed.

23Of course, program A cannot allocate any arbitrary set of addresses for this purpose. The specifications of the math routine would have to include details about what addresses it is programmed to use relative to the first descriptor; program A must expect those addresses to be the ones used when it calls the math routine. Similarly, program B, if it wishes to use the shared math routine, will have to reserve the same addresses in its own area. Most systems that permit shared procedures use additional hardware to allow more relaxed communication conventions. For example, a third descriptor register can be reserved to point to an area used exclusively as a stack for communication and temporary storage by shared procedures; each virtual processor would have a distinct stack. Similar consideration must be given to static (own) variables. See, for example, Daley and Dennis [43].

24Extension of the discussion of information protection beyond multiple descriptors requires an understanding of descriptor-based addressing techniques. Although subsection II-A contains a brief review, the reader not previously familiar with descriptor-based architecture may find the treatment too sketchy. References [37] and [44] provide tutorial treatments of descriptor-based addressing, while the papers by Dennis [42] and Fabry [45] provide in-depth technical discussion. A broad discussion and case studies are given in [46] and [47]. The reader who finds this section moving too rapidly is invited to skip to Section III, which requires fewer prerequisites.

25Since the unique identifier will be relied upon by the protection System, it may be a good idea to guard against the possibility that an accidental hardware error in manipulating a unique identifier results coincidentally in access to the wrong segment. One form of guard is to encode the clock reading in some larger number of bits, using a multiple-error detecting code, to use the encoded value as, the unique identifier, and to have the memory system check the coding of each unique identifier presented to it.

26A detailed analysis of the resulting architectural implications was made by Fabry and Yngve [49]. The capability system is a close relative of the codeword organization of the Rice Research Computer [50], but Dennis and Van Horn seem to be the first to have noticed the application of that organization to interuser protection.

27Tagged architectures were invented for a variety of applications other than protection. The Burroughs B5700 and its ancestors, and the Rice Research Computer [50], are examples of architectures that use multibit tags to separately identify instructions, descriptors, and several different types of data. All examples of tagged architecture seem to trace back to suggestions made by J. Iliffe. A thorough discussion of the concept is given by Feustel [51].

28The construction of a capability for a newly created object requires loading a protection descriptor register with a capability for the new segment. This loading can be accomplished either by giving the supervisor program the privilege of loading protection descriptor registers from untagged locations, or else by making segment creation a hardware supported function that includes loading the protection descriptor register.

29Our model assumes that we are using a "one-level" storage system that serves both as a repository for permanent storage and as the target for address references of the processor. The primitive filing system based on capabilities is the only one needed to remember objects permanently.

30Imagery inspired by Lampson [30].

31A fourth problem, not directly related to protection, is the "garbage collection" or "lost object" problem. If all copies of some capability are overwritten, the object that capability described would become inaccessible to everyone, but the fact of its inaccessibillty is hard to discover, and it may be hard to recover the space it occupies. The simplest solution is to insist that the creator of an object be systematic in his use of capabillties and remember to destroy the object before discarding the last capability copy. Since most computer operating systems provide for systematic resource accounting, this simple strategy is usually adequate. See, for example, Robinson et al. [52].

32In early plans for the HYDRA system [21], revocation was to be provided by allowing capabilities to be used as indirect addresses and by separately controlling permission to use them that way. This strategy, in contrast to Redell's, makes the fact of indirection known to the user and is also not as susceptible to speedup tricks.

33For example, in the Multics system [55], capabilities are recognized by the hardware only if they are placed in special capability-holding segments, and the supervisor domain never gives out copies of capabilities for those segments to other domains. The supervisor also associates with each access control list a thread leading to every copy it makes of a capability, so that revocation is possible.

34We should note that nothing prevents a program running under an authorized principal from copying the data of segment X into some other segment where other principals might be authorized to read it. In general, a program running under an authorized principal may "give away" any form of access permission, for example, by writing into the segment whenever it receives a message from an unauthorized accomplice. Partly because of this possibility, the importance of direct accountability of each principal has been emphasized.

35If there is more than one match, and the multiple access control list entries specify different access permissions, some resolution strategy is needed. For example, the INCLUSIVE-OR of the individually specified access permissions might be granted.

36In some systems (notably CAL TSS [17]), principal identifiers are treated as a special case of a capability, known as an access key, that can be copied about, stored anywhere, and passed on to friends. Although this approach appears to produce the same effect as protection groups, accountability for the use of a principal identifier no longer resides in an individual, since any holder of a key can make further copies for his friends.

37We have thus merged, for speed, the protection descriptor and the addressing descriptor.

38Variations of this strategy are implemented in software in TENEX [15] and UNIX [18]. This idea seems to have originated in the University of California SDS-940 TSS [56].

39The mechanics of adjustment of the access control list require using a special "store" instruction (or calling a supervisor entry in a software implementation) that interprets its address as direct, rather than indirect, but still performs the access control list checks before performing the store. This special instruction must also restrict the range of addresses it allows so as to prevent modifying the addressing descriptor stored in the access controller.

40The simplest way to handle the first access controller is to have it refer to itself. This approach provides self control at one point in the system; the difficulty of providing for unanticipated changes in authority is real and must be countered by careful planning by the system administrator.

41A variation is the use of the segments controlled by access controllers higher in the hierarchical authority structure as catalogs for the segments below. This variation, if carried to the extreme, maps together the authority control hierarchy and the cataloging hierarchy. Some mechanical simplification can be made, but trying to make dual use of a single hierarchy may lead to cataloging strategies inappropriate for the data bases, or else to pressure to distort the desired authority structure. The Multics system [58], for example, uses this variation.

42A term suggested by R. Schell [60].

43The dual strategy of maintaining a "low water mark" has been suggested as a way of monitoring the trustworthiness, as contrasted to the contamination level, of a computation. The Multics temporary ring register maintains such a low water mark on indirect address evaluation [63].

44This notion of a dynamically defined type is an enforced version of the class concept of Simula 67 [65].

45Encapsulation of a borrowed program in a protected subsystem is done with a different goal than confinement of a borrowed program within a compartment. Encapsulation may be used to limit the access a borrowed program has to the borrower's data. Confinement is intended to allow a borrowed program to have access to data, but ensure that the program cannot release the information. The two threats from borrowed programs that are countered by encapsulation and confinement are frequently combined under the name "Trojan Horse," suggested by D. Edwards [66].