Showing papers in "Communications of The ACM in 1978"

Abstract: An encryption method is presented with the novel property that publicly revealing an encryption key does not thereby reveal the corresponding decryption key. This has two important consequences: (1) Couriers or other secure means are not needed to transmit keys, since a message can be enciphered using an encryption key publicly revealed by the intented recipient. Only he can decipher the message, since only he knows the corresponding decryption key. (2) A message can be “signed” using a privately held decryption key. Anyone can verify this signature using the corresponding publicly revealed encryption key. Signatures cannot be forged, and a signer cannot later deny the validity of his signature. This has obvious applications in “electronic mail” and “electronic funds transfer” systems. A message is encrypted by representing it as a number M, raising M to a publicly specified power e, and then taking the remainder when the result is divided by the publicly specified product, n, of two large secret primer numbers p and q. Decryption is similar; only a different, secret, power d is used, where e * d ≡ 1(mod (p - 1) * (q - 1)). The security of the system rests in part on the difficulty of factoring the published divisor, n.

Abstract: This paper suggests that input and output are basic primitives of programming and that parallel composition of communicating sequential processes is a fundamental program structuring method. When combined with a development of Dijkstra's guarded command, these concepts are surprisingly versatile. Their use is illustrated by sample solutions of a variety of a familiar programming exercises.

Abstract: The concept of one event happening before another in a distributed system is examined, and is shown to define a partial ordering of the events. A distributed algorithm is given for synchronizing a system of logical clocks which can be used to totally order the events. The use of the total ordering is illustrated with a method for solving synchronization problems. The algorithm is then specialized for synchronizing physical clocks, and a bound is derived on how far out of synchrony the clocks can become.

Abstract: Use of encryption to achieve authenticated communication in computer networks is discussed. Example protocols are presented for the establishment of authenticated connections, for the management of authenticated mail, and for signature verification and document integrity guarantee. Both conventional and public-key encryption algorithms are considered as the basis for protocols.

Abstract: Conventional programming languages are growing ever more enormous, but not stronger. Inherent defects at the most basic level cause them to be both fat and weak: their primitive word-at-a-time style of programming inherited from their common ancestor—the von Neumann computer, their close coupling of semantics to state transitions, their division of programming into a world of expressions and a world of statements, their inability to effectively use powerful combining forms for building new programs from existing ones, and their lack of useful mathematical properties for reasoning about programs.An alternative functional style of programming is founded on the use of combining forms for creating programs. Functional programs deal with structured data, are often nonrepetitive and nonrecursive, are hierarchically constructed, do not name their arguments, and do not require the complex machinery of procedure declarations to become generally applicable. Combining forms can use high level programs to build still higher level ones in a style not possible in conventional languages.Associated with the functional style of programming is an algebra of programs whose variables range over programs and whose operations are combining forms. This algebra can be used to transform programs and to solve equations whose “unknowns” are programs in much the same way one transforms equations in high school algebra. These transformations are given by algebraic laws and are carried out in the same language in which programs are written. Combining forms are chosen not only for their programming power but also for the power of their associated algebraic laws. General theorems of the algebra give the detailed behavior and termination conditions for large classes of programs.A new class of computing systems uses the functional programming style both in its programming language and in its state transition rules. Unlike von Neumann languages, these systems have semantics loosely coupled to states—only one state transition occurs per major computation.

Abstract: According to traditional conceptions of cryptographic security, it is necessary to transmit a key, by secret means, before encrypted massages can be sent securely. This paper shows that it is possible to select a key over open communications channels in such a fashion that communications security can be maintained. A method is described which forces any enemy to expend an amount of work which increases as the square of the work required of the two communicants to select the key. The method provides a logically new kind of protection against the passive eavesdropper. It suggests that further research on this topic will be highly rewarding, both in a theoretical and a practical sense.

Abstract: A real-time list processing system is one in which the time required by the elementary list operations (e.g. CONS, CAR, CDR, RPLACA, RPLACD, EQ, and ATOM in LISP) is bounded by a (small) constant. Classical implementations of list processing systems lack this property because allocating a list cell from the heap may cause a garbage collection, which process requires time proportional to the heap size to finish. A real-time list processing system is presented which continuously reclaims garbage, including directed cycles, while linearizing and compacting the accessible cells into contiguous locations to avoid fragmenting the free storage pool. The program is small and requires no time-sharing interrupts, making it suitable for microcode. Finally, the system requires the same average time, and not more than twice the space, of a classical implementation, and those space requirements can be reduced to approximately classical proportions by compact list representation. Arrays of different sizes, a program stack, and hash linking are simple extensions to our system, and reference counting is found to be inferior for many applications.

Abstract: A constraint network representation is presented for a combinatorial search problem: finding values for a set of variables subject to a set of constraints. A theory of consistency levels in such networks is formulated, which is related to problems of backtrack tree search efficiency. An algorithm is developed that can achieve any level of consistency desired, in order to preprocess the problem for subsequent backtrack search, or to function as an alternative to backtrack search by explicitly determining all solutions.

Abstract: This paper describes the CRAY-1, discusses the evolution of its architecture, and gives an account of some of the problems that were overcome during its manufacture.The CRAY-1 is the only computer to have been built to date that satisfies ERDA's Class VI requirement (a computer capable of processing from 20 to 60 million floating point operations per second) [1].The CRAY-1's Fortran compiler (CFT) is designed to give the scientific user immediate access to the benefits of the CRAY-1's vector processing architecture. An optimizing compiler, CFT, “vectorizes” innermost DO loops. Compatible with the ANSI 1966 Fortran Standard and with many commonly supported Fortran extensions, CFT does not require any source program modifications or the use of additional nonstandard Fortran statements to achieve vectorization. Thus the user's investment of hundreds of man months of effort to develop Fortran programs for other contemporary computers is protected.

Abstract: As an example of cooperation between sequential processes with very little mutual interference despite frequent manipulations of a large shared data space, a technique is developed which allows nearly all of the activity needed for garbage detection and collection to be performed by an additional processor operating concurrently with the processor devoted to the computation proper. Exclusion and synchronization constraints have been kept as weak as could be achieved; the severe complexities engendered by doing so are illustrated.

Abstract: By typing requests in English, casual users will be able to obtain explicit answers from a large relational database of aircraft flight and maintenance data using a system called PLANES. The design and implementation of this system is described and illustrated with detailed examples of the operation of system components and examples of overall system operation. The language processing portion of the system uses a number of augmented transition networks, each of which matches phrases with a specific meaning, along with context registers (history keepers) and concept case frames; these are used for judging meaningfulness of questions, generating dialogue for clarifying partially understood questions, and resolving ellipsis and pronoun reference problems. Other system components construct a formal query for the relational database, and optimize the order of searching relations. Methods are discussed for handling vague or complex questions and for providing browsing ability. Also included are discussions of important issues in programming natural language systems for limited domains, and the relationship of this system to others.

Abstract: A broadcast packet is for delivery to all nodes of a network. Algorithms for accomplishing this delivery through a store-and-forward packet switching computer network include (1) transmission of separately addressed packets, (2) multidestination addressing, (3) hot potato forwarding, (4) spanning tree forwarding, and (5) source based forwarding. To this list of algorithms we add (6) reverse path forwarding, a broadcast routing method which exploits routing procedures and data structures already available for packet switching. Reverse path forwarding is a practical algorithm for broadcast routing in store-and-forward packet switching computer networks. The algorithm is described as being practical because it is not optimal according to metrics developed for its analysis in this paper, and also because it can be implemented in existing networks with less complexity than that required for the known alternatives.

Abstract: A language concept for concurrent processes without common variables is introduced. These processes communicate and synchronize by means of procedure calls and guarded regions. This concept is proposed for real-time applications controlled by microcomputer networks with distributed storage. The paper gives several examples of distributed processes and shows that they include procedures, coroutines, classes, monitors, processes, semaphores, buffers, path expressions, and input/output as special cases.

Abstract: Maintenance and enhancement of application software consume a major portion of the total life cycle cost of a system. Rough estimates of the total systems and programming resources consumed range as high as 75-80 percent in each category. However, the area has been given little attention in the literature. To analyze the problems in this area a questionnaire was developed and pretested. It was then submitted to 120 organizations. Respondents totaled 69. Responses were analyzed with the SPSS statistical package. The results of the analysis indicate that: (1) maintenance and enhancement do consume much of the total resources of systems and programming groups; (2) maintenance and enhancement tend to be viewed by management as at least somewhat more important than new application software development; (3) in maintenance and enhancement, problems of a management orientation tend to be more significant than those of a technical orientation; and (4) user demands for enhancements and extension constitute the most important management problem area.

Abstract: A data abstraction can be naturally specified using algebraic axioms. The virtue of these axioms is that they permit a representation-independent formal specification of a data type. An example is given which shows how to employ algebraic axioms at successive levels of implementation. The major thrust of the paper is twofold. First, it is shown how the use of algebraic axiomatizations can simplify the process of proving the correctness of an implementation of an abstract data type. Second, semi-automatic tools are described which can be used both to automate such proofs of correctness and to derive an immediate implementation from the axioms. This implementation allows for limited testing of programs at design time, before a conventional implementation is accomplished.

Abstract: A data structure is described which can be used for representing a collection of priority queues. The primitive operations are insertion, deletion, union, update, and search for an item of earliest priority.

Abstract: This paper describes an experiment in program testing, employing 59 highly experienced data processing professionals using seven methods to test a small PL/I program. The results show that the popular code walkthrough/inspection method was as effective as other computer-based methods in finding errors and that the most effective methods (in terms of errors found and cost) employed pairs of subjects who tested the program independently and then pooled their findings. The study also shows that there is a tremendous amount of variability among subjects and that the ability to detect certain types of errors varies from method to method.

Abstract: This paper is a practical study of how to implement the Quicksort sorting algorithm and its best variants on real computers, including how to apply various code optimization techniques. A detailed implementation combining the most effective improvements to Quicksort is given, along with a discussion of how to implement it in assembly language. Analytic results describing the performance of the programs are summarized. A variety of special situations are considered from a practical standpoint to illustrate Quicksort's wide applicability as an internal sorting method which requires negligible extra storage.

Abstract: It is possible to use a small counter to keep approximate counts of large numbers. The resulting expected error can be rather precisely controlled. An example is given in which 8-bit counters (bytes) are used to keep track of as many as 130,000 events with a relative error which is substantially independent of the number n of events. This relative error can be expected to be 24 percent or less 95 percent of the time (i.e. s = n/8). The techniques could be used to advantage in multichannel counting hardware or software used for the monitoring of experiments or processes.

Abstract: A new interprocedural data flow analysis algorithm is presented and analyzed. The algorithm associates with each procedure in a program information about which variables may be modified, which may be used, and which are possibly preserved by a call on the procedure, and all of its subcalls. The algorithm is sufficiently powerful to be used on recursive programs and to deal with the sharing of variables which arises through reference parameters. The algorithm is unique in that it can compute all of this information in a single pass, not requiring a prepass to compute calling relationships or sharing patterns. The algorithm is asymptotically optimal in time complexity. It has been implemented and is practical even on programs which are quite large.

Abstract: A simple algorithm is described for isolating the differences between two files. One application is the comparing of two versions of a source program or other file in order to display all differences. The algorithm isolates differences in a way that corresponds closely to our intuitive notion of difference, is easy to implement, and is computationally efficient, with time linear in the file length. For most applications the algorithm isolates differences similar to those isolated by the longest common subsequence. Another application of this algorithm merges files containing independently generated changes into a single file. The algorithm can also be used to generate efficient encodings of a file in the form of the differences between itself and a given “datum” file, permitting reconstruction of the original file from the diference and datum files.

Abstract: Optimal decision table conversion has been tackled in the literature using two approaches, dynamic programming and branch-and-bound. The former technique is quite effective, but its time and space requirements are independent of how “easy” the given table is. Furthermore, it cannot be used to produce good, quasioptimal solutions. The branch-and-bound technique uses a good heuristic to direct the search, but is cluttered up by an enormous search space, since the number of solutions increases with the number of test variables according to a double exponential. In this paper we suggest a heuristically guided top-down search algorithm which, like dynamic programming, recognizes identical subproblems but which can be used to find both optimal and quasioptimal solutions. The heuristic search method introduced in this paper combines the positive aspects of the above two techniques. Compressed tables with a large number of variables can be handled without deriving expanded tables first.

Abstract: Based on an empirical study of more than 10,000 lines of program text written in a GOTO-less language, a machine architecture specifically designed for structured programs is proposed. Since assignment, CALL, RETURN, and IF statements together account for 93 percent of all executable statements, special care is given to ensure that these statements can be implemented efficiently. A highly compact instruction encoding scheme is presented, which can reduce program size by a factor of 3. Unlike a Huffman code, which utilizes variable length fields, this method uses only fixed length (1-byte) opcode and address fields. The most frequent instructions consist of a single 1-byte field. As a consequence, instruction decoding time is minimized, and the machine is efficient with respect to both space and time.

Abstract: The problem of recovering an image (a function of two variables) from experimentally available integrals of its grayness over thin strips is of great importance in a large number of scientific areas. An important version of the problem in medicine is that of obtaining the exact density distribution within the human body from X-ray projections. One approach that has been taken to solve this problem consists of translating the available information into a system of linear inequalities. The size and the sparsity of the resulting system (typically, 25,000 inequalities with fewer than 1 percent of the coefficients nonzero) makes methods using successive relaxations computationally attractive, as compared to other ways of solving systems of inequalities. In this paper, it is shown that, for a consistent system of linear inequalities, any sequence of relaxation parameters lying strictly between 0 and 2 generates a sequence of vectors which converges to a solution. Under the same assumptions, for a system of linear equations, the relaxation method converges to the minimum norm solution. Previously proposed techniques are shown to be special cases of our procedure with different choices of relaxation parameters. The practical consequences for image reconstruction of the choice of the relaxation parameters are discussed.

Abstract: A mathematical model of a transaction-oriented system under intermittent failures is proposed. The system is assumed to operate with a checkpointing and rollback/recovery method to ensure reliable information processing. The model is used to derive the principal performance measures, including availability, response time, and the system saturation point.

Abstract: A method for inducing knowledge by abstraction from a sequence of training examples is described. The proposed method, interference matching, induces abstractions by finding relational properties common to two or more exemplars. Three tasks solved by a program that uses an interference-matching algorithm are presented. Several problems concerning the description of the training examples and the adequacy of interference matching are discussed, and directions for future research are considered.

Abstract: A simple technique for reasoning about equalities that is fast and complete for ground formulas with function symbols and equality is presented. A proof of correctness is given as well.

Abstract: This paper presents several numerical methods which may be used to obtain the stationary probability vectors of Markovian models. An example of a nearly decomposable system is considered, and the results obtained by the different methods examined. A post mortem reveals why standard techniques often fail to yield the correct results. Finally, a means of estimating the error inherent in the decomposition of certain models is presented.

Abstract: Kellerman has presented a method for determining keyword conflicts and described a heuristic algorithm which solves a certain combinatorial optimization problem in connection with this method. This optimization problem is here shown to be equivalent to the problem of covering the edges of a graph by complete subgraphs with the objective of minimizing the number of complete subgraphs. A relationship between this edge-clique-cover problem and the graph coloring problem is established which allows algorithms for either one of these problems to be constructed from algorithms for the other. As consequences of this relationship, the keyword conflict problem and the edge-clique-cover problem are shown to be NP-complete, and if P ≠ NP then they do not admit polynomial-time approximation algorithms which always produce solutions within a factor less than 2 from the optimum.

Abstract: A parallel bucket-sort algorithm is presented that requires time O(log n) and the use of n processors. The algorithm makes use of a technique that requires more space than the product of processors and time. A realistic model is used in which no memory contention is permitted. A procedure is also presented to sort n numbers in time O(k log n) using n1+1/k processors, for k an arbitrary integer. The model of computation for this procedure permits simultaneous fetches from the same memory location.