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MUST ATTEMPT ALL QUESTIONS A general piecewise curve definition,…MUST ATTEMPT ALL QUESTIONSA general piecewise curve definition, whether B´ezier, B-spline, or NURBS, can be written as a sum of products of basis functions, Ai(t), and control points, Pi : P(t) = XAi(t)Pi , tmin = t < tmax Give the conditions on the functions Ai that are needed to ensure that: (i) Translation of all of the points by some vector, P0 i = Pi + ?P, causes a translation of the curve by the same vector, P0 (t) = P(t) + ?P.What are the usage and the limitations of the Bootstrap technique in phylogeny?[6 marks](b) We often use Hidden Markov Models (HMM) to predict a pattern (for instancethe exons). How can you compute the number of True Positives, True Negatives,False Positives and False Negatives and use them to evaluate your HMM?[6 marks](c) How can you evaluate the results obtained (number of clusters and their relativeposition) using the K means algorithm for clustering? [5 marks](d) What is the difference between the adjacency list and the accessibility list?[3 marks]2CST.2013.9.3. [3 marks](c) Show that the inter-event times of a Poisson process form a sequence ofindependent random variables each distributed with an exponential distributionwith parameter ?. [3 marks](d) Describe how to use the inverse transform method to simulate exponentialrandom variables with parameter ?. [3 marks](e) Show how your simulated exponential random variables can be used to simulatePoisson random variables with parameter ?. [3 marks](f ) Consider positive numbers ?1, ?2, . . . , ?n and weight factors a1, a2, . . . , an suchthat ai = 0 for i = 1, 2, . . . , n and Pni=1 ai = 1. Show thatf(x) = (Pni=1 ai?ie-?ix x > 00 x = 0is a density for a random variable and describe a procedure to simulate valuesfrom this density. [5 marks]3 (TURN OVER)CST.2013.9.43 Computer Vision(a) Explain why inferring object surface properties from image properties is anill-posed problem in general. In the case of inferring the colours of objects fromimages of the objects, how does knowledge of the properties of the illuminantaffect the status of the problem and its solubility? [4 marks](b) What surface properties can cause a human face to form either a Lambertianimage or a specular image, or an image lying anywhere on a continuum betweenthose two extremes? In terms of geometry and angles, what defines these twoextremes of image formation? What difficulties do these factors create forefforts to extract facial structure from facial images using “shape-from-shading”inference techniques? [4 marks](c) Explain and illustrate the “Paradox of Cognitive Penetrance” as it relates tocomputer vision algorithms that we know how to construct, compared withthe algorithms underlying human visual competence. Discuss how human visualillusions may relate to this paradox. Comment on the significance of this paradoxfor computer vision research. [4 marks](d) Define the “Correspondence Problem”, detailing the different forms that it takesin stereo vision and in motion vision, and discuss its complexity. In both cases,explain why the computation is necessary. What are the roles of space and timein the two cases, and what symmetries exist between the stereo and motionvision versions of the Correspondence Problem? [4 marks](e) When defining and selecting which features to extract in a pattern classificationsystem, what is the goal for the statistical clustering behaviour of the data interms of the variances within and amongst the different classes? [4 marks]4CST.2013.9.54 Denotational Semantics(a) A context-free grammar can be formally defined as a 4-tuple. Give a precise statement of what the components are. [2 marks] (b) Explain the difference between a grammar and the language it generates. [2 marks] (c) Explain what makes a grammar ambiguous, with reference to the grammar which may be commonly expressed as a “rule” E ::= 1 | 2 | X | E + E | E * E | -E where X is an identifier. [2 marks] (d) For the “rule” in part (c), give a formal grammar containing this “rule” and adhering to your definition in part (a). [2 marks] (e) Give non-ambiguous grammars each generating the same language as your grammar in part (d) for the cases: (i) “-” is most tightly binding and “+” and “*” have equal binding power and associate to the left. (ii) “-” is most tightly binding and “+” and “*” have equal binding power and associate to the right. (iii) “-” binds more tightly than “+”, but less tightly than “*”, with “+” left-associative and “*” right-associative so that “-a + -b * c * d + d” is interpreted as “((-a) + (-(b * (c * d)))) + d”. [2 marks each] (f ) Give a simple recursive descent parser for your grammar in part (e)(iii) above which yields a value of type ParseTree. You may assume operations mkplus, mktimes, mkneg acting on type ParseTree. [6 marks] 2 CST.2004.4.3 2 Economics and Law (a) What is “strategy” in game theory? [5 marks] (b) Explain the difference between a dominant strategy equilibrium and a Nash equilibrium. [5 marks] (c) Participants in a peer-to-peer file-sharing system can either cooperate (share their files with others) or cheat (try to download from others without making any contribution themselves). Write down a possible payoff matrix for their behaviour, and identify the Nash equilibrium. [5 marks] (d) Is this equilibrium Pareto-efficient, and, if not, what can be done to make it so? [5 marks] 3 Data Structures and Algorithms (a) Describe the structure of splay trees used to represent a set of key-value pairs. [5 marks] (b) Describe how new key-value pairs are added to the tree, how the value associated with a given key can be looked up, and how to delete a pair with a given key. [5 marks] (c) State without proof the attractive properties of splay trees. [4 marks] (d) Describe the ternary tree structure used to hold a dictionary of key-value pairs where the keys are variable-length strings. Illustrate the mechanism by showing the structure after items with keys MIT, SAD, MAN, APT, MUD, ADD, MAG, MINE, MIKE, MINT, AT, MATE and MINES have been added in that order to an initially empty ternary tree. [6 marks] 3 [TURN OVER CST.2004.4.4 4 Comparative Programming Languages (a) Briefly describe the concept of coroutines as provided in BCPL, and outline the effect of the library functions createco(f, size), deleteco(cptr), callco(cptr, val), and cowait(val). [6 marks] (b) Discuss the relative merits of BCPL coroutines versus those of threads such as provided in Java. [6 marks] (c) Outline the overall design and organisation of a BCPL program to perform discrete event simulation using coroutines to implement the simulated activities. Concentrate on the design of the simulation event loop, the organisation of the priority queue and what functions you would provide to simplify the implementation of the activities. It would probably be sensible to adopt a programming style similar to that used in Simula 67. You should hold simulated time as a global (integer) variable. [8 marks] 5 Operating Systems II (a) Most conventional hardware translates virtual addresses to physical addresses using multi-level page tables (MPTs): (i) Describe with the aid of a diagram how translation is performed when using MPTs. [3 marks] (ii) What problem(s) with MPTs do linear page tables attempt to overcome? How is this achieved? [3 marks] (iii) What problems(s) with MPTs do inverted page tables (IPTs) attempt to overcome? How is this achieved? [3 marks] (iv) What problems(s) with IPTs do hashed page tables attempt to overcome? How is this achieved? [3 marks] (b) Operating systems often cache part of the contents of the disk(s) in main memory to speed up access. Compare and contrast the way in which this is achieved in (i) 4.3 BSD Unix and (ii) Windows 2000. [8 marks] 4 CST.2004.4.5 6 Continuous Mathematics For non-negative integers r and s we have the orthogonality properties Z 2p 0 cos(rx) cos(sx)dx = 2p if r = s = 0 pdrs otherwise Z 2p 0 sin(rx) sin(sx)dx = 0 if r = s = 0 pdrs otherwise Z 2p 0 sin(rx) cos(sx)dx = 0 ? r, s where drs = 1 if r = s 0 otherwise . (a) Derive expressions for the Fourier coefficients a0, an, bn (n = 1, 2, . . .) such that the infinite series a0 2 + X8 n=1 (an cos(nx) + bn sin(nx)) is the Fourier series for the function f(x) in an interval of length 2p. [6 marks] (b) For any fixed integer N = 1 let SN (x) = a0 2 + N X-1 n=1 (an cos(nx) + bn sin(nx)) be the Fourier series for f(x) truncated to the first N terms and let S 0 N (x) = a 0 0 2 + N X-1 n=1 (a 0 n cos(nx) + b 0 n sin(nx)) be any other Fourier series truncated to the first N terms. Show that Z 2p 0 (f(x) – SN (x)) (SN (x) – S 0 N (x)) dx = 0 . [8 marks] (c) Given the function f(x) show that Z 2p 0 (f(x) – S 0 N (x))2 dx is minimised by the unique choice a 0 0 = a0, a 0 n = an, b 0 n = bn (n = 1, 2, . . .), that is, the Fourier series gives the best approximation to f(x) using N terms in the sense of minimising the mean-squared error. [6 marks] 5 [TURN OVER CST.2004.4.6 7 Numerical Analysis I (a) The mid-point rule can be expressed in the form In = Z n+ 1 2 n- 1 2 f(x)dx = f(n) + en where en = f 00(?n)/24 for some ?n in the interval (n- 1 2 , n+ 1 2 ). Assuming that a formula for R f(x)dx is known, and using the notation Sp,q = X q n=p f(n) , describe a method for estimating the sum of a slowly convergent series S1, 8, by summing only the first N terms and estimating the remainder by integration. [5 marks] (b) Assuming that f 00(x) is a positive decreasing function, derive an estimate of the error |EN | in the method. [5 marks] (c) Given Z dx (1 + x) v x = 2 tan-1 v x illustrate the method by applying it to X8 n=1 1 (1 + n) v n . Verify that f 00(x) is positive decreasing for large x, and estimate the integral remainder to be added to S1,N . [6 marks] (d) How large should N be to achieve an absolute error of approximately 2×10-15? [You may assume N + 1 ‘ N for this purpose.] [4 marks] 6 CST.2004.4.7 8 Concurrent Systems and Applications A multi-threaded application is using a long linked list of integers. The list is accessed through synchronized methods on a ListSet object. The list itself comprises a chain of ListNode objects in ascending numerical order. The chain always starts and ends with special sentinel nodes conceptually containing -8 and +8 respectively. This simplifies the implementation of operations on the list: they do not have to deal with inserting elements at the very start or at the very end. (a) Sketch the definition of ListSet and ListNode as Java classes. You need only give appropriate field definitions and the implementation of an insert method on ListSet. [4 marks] (b) An engineer suggests that, instead of holding a lock on a ListSet object, threads only need to lock a pair of ListNode objects in the region that they are working. (i) Define methods lock and unlock for your ListNode class to allow a thread to acquire a mutual exclusion lock on a given node. [6 marks] (ii) Show how your insert method could be updated to incorporate the engineer’s idea. [8 marks] (iii) Do you think the new implementation will be faster than the original one? Justify your answer. [2 marks] 7 [TURN OVER CST.2004.4.8 9 Computation Theory (a) Explain what is meant by the following statements: (i) f : N ? N is a total recursive (TR) function; [3 marks] (ii) the sequence {fn : N ? N}n?N of TR functions of a single variable is recursively enumerable. [4 marks] (b) Show that no recursive enumeration can include the set of all TR functions of a single variable. [4 marks] (c) Suppose u(n, x) is a recursive enumeration of the sequence of TR functions fn(x) = u(n, x). Show how to define a sequence {gn : N ? N} of TR functions of a single variable such that each gn is distinct from every function fn, and also from each gk for k 6= n. [5 marks] (d) Express the sequence {gn} as an explicit recursive enumeration v(n, x) = gn(x). [4 marks](a) (i) State carefully, without proof, the compositionality, soundness, andadequacy results for PCF. [6 marks](ii) Define the notion of contextual equivalence in PCF. [2 marks](You need not describe the syntax and the operational and denotationalsemantics of PCF.)(b) Show that for all types t and closed terms M and M0 of type t , if [[M]] and[[M0]] are equal elements of the domain [[t ]] then M and M0 are contextuallyequivalent. [4 marks](c) Consider the following closed PCF terms of type nat ? bool ? nat:F0 = fn x : nat. fn y : bool. xF1 = fixfn f : nat ? bool ? nat. fn x : nat. fn y : bool.if zero(x) then 0else succ( f (pred x) y )F2 = fn x : nat. fn y : bool. if y then x else xState whether or not F1 and F2 are contextually equivalent to F0. Justify youranswers. [4 marks each]5 (TURN OVER [2 marks] (ii) The curve lies within the convex hull of the control points. [2 marks] (iii) The curve passes through one of the control points, Pj . [2 marks] (b) The knot vector [0, 0, 0, 1, 1, 1] defines a quadratic B-spline with three control points. Derive the equations of and graph the three basis functions from this knot vector. [6 marks] (c) The basis functions derived in part (b) can be used, in a NURBS curve, to reproduce exactly a quarter-circle. Recall that a NURBS curve can be written as: P(t) = Pn+1 i=1 Ni,k(t)Pihi Pn+1 i=1 Ni,k(t)hi , tmin = t < tmax where hi is the homogeneous co-ordinate associated with point Pi . Place the three control points at P1 = (1, 0), P2 = (1, 1), P3 = (0, 1). (i) Sketch the NURBS curve for the case h1 = h2 = h3 = 1 [1 mark] (ii) Calculate the magnitude of the maximum error between the curve in (c)(i) and a perfect circle of radius 1 centred at (0, 0). [2 marks] (iii) Sketch the NURBS curve for the case h1 = h3 = 1, h2 = 0. [1 mark] (iv) Sketch the NURBS curve for the limit case h1 = h3 = 1, h2 ? 8. [1 mark] (v) Derive the value for h2 that makes the NURBS curve perfectly match a quarter circle of radius 1 centred at (0, 0). [3 marks] 2 CST.2013.7.3 2 Artificial Intelligence II Princess Precious is a very light sleeper, and insists that every night she must sleep on brand new silk sheets. Her younger brother, however, is in the habit of secretly scattering toast crumbs in her bed, to make sure she sleeps badly. In order to get the week started well, he does this every Sunday with probability 0.9. For the rest of the week, he tends to relent on any given night if he placed crumbs in her bed the previous night, and hence leaves them with a probability of 0.1. On the other hand, if he did not leave crumbs on a given night, his mischievous nature compels him to leave crumbs the next night with probability 0.6. Precious, being a true princess, tends to be grumpy in the morning if she has not slept well. Consequently, if she has slept with crumbs in her bed she is grumpy with probability 0.95. Being a light sleeper, even if there are no crumbs she is grumpy with probability 0.55. (a) Give a detailed definition of a Hidden Markov Model (HMM) and show how the scenario described can be modelled as an HMM. [4 marks] (b) Give a detailed description of the Viterbi algorithm for computing the most probable sequence of states, given that an HMM produces a given sequence of observations. [8 marks] (c) It is observed that Princess Precious is grumpy on Monday and Tuesday. However on Wednesday she is radiantly happy. Use the Viterbi algorithm to compute the most likely sequence of activities performed by her brother. [8 marks] 3 (TURN OVER) CST.2013.7.4 3 Bioinformatics Given the two DNA sequences: GCACTT and CCCAAT (a) Compute the alignment (using the edit graph) and the final score with the following rules: match score = +1, mismatch = -1, gap penalty = -1. [4 marks] (b) Discuss how the alignment score and the quality of the result depend on the match score, mismatch, and gap penalty. [6 marks] (c) Generate four, short DNA sequences (a,b,c,d) such that their relations as a tree are approximately the following: ((a,b),(c,d)). [5 marks] (d) How is the score matrix used in phylogenetic tree building techniques? [5 marks] 4 CST.2013.7.5 4 Business Studies This question is about Intellectual Property and Copyright. (a) List 5 types of Intellectual Property. Comment on their use to protect a software program. [5 marks] (b) Explain what Intellectual Property Rights (IPR) an employee has on their inventions. Does this differ if the invention is made in their own time? [5 marks] (c) Professor Elbowpatch, a professor of ancient languages and keen amateur gardener, invents a new sort of lawnmower. What are his IPR options? [5 marks] (d) Discuss what actions Professor Elbowpatch should take to exploit his invention. [5 marks] 5 (TURN OVER) CST.2013.7.6 5 Comparative Architectures (a) Why might a heterogeneous or asymmetric chip-multiprocessor be preferable to a homogeneous or symmetric one? [5 marks] (b) You are a computer architect working on the design of a new processor for the mobile phone market. An initial analysis of applications suggests that there would be worthwhile gains in producing a processor that could offer two different power-performance tradeoffs. The first configuration would maximise the exploitation of ILP and consume the most power. The second would on average perform less well, but would consume less power. (i) Describe how the microarchitecture of a single processor could be modified in order to offer the ability to switch between the two configurations described at run-time. [9 marks] (ii) How might one determine when to switch from one configuration to the other in order to reduce overall power consumption while minimising the impact on the user experience? [6 marks] 6 CST.2013.7.7 6 Denotational Semantics (a) Let D be a poset and let f : D ? D be a monotone function. (i) Give the definition of the least pre-fixed point, fix (f), of f. Show that fix (f) is a fixed point of f. [5 marks] (ii) Show that whenever D is a domain and f is a continuous function, fix (f) exists. [5 marks] (b) A poset (P, v) has binary meets if for every pair of elements x, y ? P there is a necessarily unique element (x u y) ? P such that • (x u y) v x and (x u y) v y, and • for all z ? P, z v x and z v y imply z v (x u y). (i) Let (P, v) be a poset with binary meets. Show that the function meet : P × P ? P given by meet(x, y) = x u y is monotone. [5 marks] (ii) Exhibit a domain with binary meets for which the function meet is not continuous. Justify your answer. [5 marks] 7 (TURN OVER) CST.2013.7.8 7 Hoare Logic (a) Briefly explain the concepts: mechanised program verification and verification conditions (VCs). [4 marks] (b) Consider three consecutive assignments: { P } V1 := E1; V2 := E2; V3 := E3 { Q } Write down the VCs that are generated for such a program. Give a detailed proof which shows that, if the VCs are true, then the specification above is provable in Hoare Logic. [6 marks] (c) Write down the VCs for the following annotated program. For this part, do not attempt to define Inv. [4 marks] { T } I := 0; X := 0; Y := 1; WHILE (I 6= N) DO { Inv } I := I + 1; X := X + Y; Y := X + Y OD { X = fib(2 × N) } Here fib(0) = 0, fib(1) = 1 and fib(n + 2) = fib(n) + fib(n + 1) for n ? N. (d) Provide a definition of Inv such that the VCs are provable. Sketch a proof of the VCs. [6 marks] 8 CST.2013.7.9 8 Human-Computer Interaction Imagine you have been commissioned to design the user interface for a head-up display (e.g. based on Google Project Glass) that can be used while riding a bicycle, as a reminder of appointments around Cambridge. (a) In order to be safe while riding, the visual design of appointment reminders and instructions should be as simple as possible. Describe three specific ways this can be achieved, using formal elements of visual design. [6 marks] (b) Consider the possibility that users might wish to modify their appointment schedules while riding. Choose three different Cognitive Dimensions of Notations, and discuss their implications. [6 marks] (c) Describe ways that features of the bicycle itself might form the basis for (i) a tangible user interface; and (ii) an augmented reality interface to this system. For each of these, explain what sensor processing would be involved. [4 marks] (d) How might these two alternative interfaces be compared experimentally? Describe the structure of the experimental design and procedure for analysis of the results. [4 marks] 9 (TURN OVER) CST.2013.7.10 9 Information Theory and Coding (a) Consider an alphabet of 5 symbols whose probabilities are as follows: A B C D E 1 16 1 4 1 8 1 16 1 2 One of these symbols has been selected at random and you need to discover which symbol it is by asking 'yes/no' questions that will be truthfully answered. (i) What would be the most efficient sequence of such questions that you could ask in order to discover the selected symbol? [2 marks] (ii) By what principle can you claim that each of your proposed questions in the sequence is maximally informative? [2 marks] (iii) On average, how many such questions will need to be asked before the symbol is discovered? What is the entropy of the symbol set? [2 marks] (iv) Construct a uniquely decodable prefix code for the symbols. Explain why it is uniquely decodable and why it has the prefix property. [2 marks] (v) Relate the bits in the code words forming your prefix code to the 'yes/no' questions that you proposed in (i). [2 marks] (b) Explain how the bits in an IrisCode are set by phase sequencing. Discuss how quantisation of the complex plane into phase quadrants sets each pair of bits; why it is beneficial for quadrant codes to form a Gray Code; how much entropy is thereby typically extracted from iris images; and why such bit sequences enable extremely efficient identity searches and matching. [5 marks] (c) Consider a noisy analog communication channel of bandwidth ? = 1 MHz, which is perturbed by additive white Gaussian noise whose total spectral power is N0? = 1. Continuous signals are transmitted across such a channel, with average transmitted power P = 1,000. Give a numerical estimate for the channel capacity, in bits per second, of this noisy channel. Then, for a channel having the same bandwidth ? but whose signal-to-noise ratio P N0? is four times better, repeat your numerical estimate of capacity in bits per second. [5 marks] 10 CST.2013.7.11 10 Natural Language Processing Consider the following context-free grammar: S -> NP VP N -> dog V -> sees NP -> Det N N ->cat V -> hates VP -> V N -> mouse V -> sneezes VP -> V NP Det -> the (a) Which of the following sentences are recognised by this grammar, and why? [4 marks] (i) the dog sneezes the cat (ii) the mouse hates (iii) the cat the mouse hates (iv) the mouse hates the mouse (b) Modify the grammar so that the following sentence is now accepted in addition: the dog the cat the mouse sees hates sneezes Your modification should express the linguistic phenomenon as efficiently and elegantly as possible. Justify your choice. [6 marks] (c) The semantics of natural language expressions can be expressed in first order predicate logic (FOPL). For instance, “the dog sneezes” can be approximately expressed as ?x dog(x) n sneeze(x) Following this pattern, express the semantics of the sentence in part (b) in FOPL. [4 marks] (d) Consider the following sentence: the mouse that sees the cat that hates the dog that sneezes Contrast this construction to the one in part (b) in terms of semantics and syntax. How would you modify the original grammar in part (a) to account for this construction? [6 marks] 11 (TURN OVER) CST.2013.7.12 11 Optimising Compilers (a) Give a semantic notion of a variable being live at a program point, explaining why this is problematic to calculate. Now give a simpler-to-calculate notion of liveness and explain how it relates to the semantic notion. Formulate dataflow equations whose solution(s) give the liveness at each program point. You need only consider liveness of simple non-address-taken variables. [4 marks] (b) Suppose we have a basic block of p simple statements. Give a formula relating the liveness on entry to the block to those of its q neighbouring blocks in the control flow graph. This formula naturally uses O(p) +O(q) operations – justify this statement. It is claimed that this formula can be re-arranged to require only O(q) time to calculate by only using one ‘?’ and one ” operator. Determine whether this is true. [Hint: you may wish to consider examples, and to start by solving the case p = 2. Partial credit will be given for a good set of concrete examples arguing for or against.] [5 marks] (c) To solve the dataflow equations, an initial approximation to liveness at the start of each basic block is required. What is it, and indicate why this leads to a preferable solution. [2 marks] (d) Solving dataflow equations is usually expressed iteratively, where each iteration is of the form “for every basic block re-calculate the set of live variables from the current sets of live variables of its neighbours”. We want to determine whether some basic-block orderings in “for every basic block” result in fewer overall iterations than others. Suppose the program has k basic blocks, but no cycle in the control flow graph; give an optimal ordering which only requires one dataflow iteration to calculate liveness (a second would only calculate the same value of the first). Also give such a program and an ordering which maximises the number of iterations required, giving the number of iterations in terms of k. [5 marks] (e) Consider the program with four labelled blocks (with B1 as entry node): B1: x = read(); y = read(); z = read(); goto B2; B2: z = z+1; x = x-1; if (x>0) goto B3; else goto B4; B3: z = z+1; y = y-1; if (y>0) goto B2; else goto B4; B4: print(z); Show (i) there is no basic block ordering for which a single iteration gives the correct liveness at each label, but (ii) there is an ordering for which two iterations suffice (in the sense that a third would agree with the second). Give your ordering both explicitly as a permutation of {B1, B2, B3, B4} and also as a general principle along the lines of your answer to part (d). [4 marks] 12 CST.2013.7.13 12 Principles of Communications Suppose that you read about the design of an end-to-end transport protocol for an early version of the Internet, which uses window-based flow control with a fixed size window, with Go-Back-N retransmission of all un-acknowledged packets when there is a time-out awaiting an acknowledgement for any given data packet. How would you convince the designer that they are going to have real problems with such a simplistic scheme? [20 marks] 13 (TURN OVER) CST.2013.7.14 13 Security II (a) Formally state the two rules of the Bell-LaPadula (BLP) security policy model and then re-state them informally in terms of a single rule about the direction of information flow. [2 marks] (b) Consider a distributed system in which A is a TOP SECRET process running on machine Alice and B is a CONFIDENTIAL object residing on machine Bob. (i) Explain and justify whether A is allowed to read and/or write from B according to the BLP policy. [2 marks] (ii) Discuss the claim made by some researchers that this scenario highlights a fundamental problem with the BLP policy. [4 marks] (c) Consider the following description of Brewer and Nash’s Chinese Wall security policy model. • Simple rule: Read or write access to object o2 by subject s is granted if and only if, for all objects o1 to which s has had access, we have: (class(company(o1)) 6= class(company(o2)) or (company(o1) = company(o2)). • *-rule: Write access to object o2 by subject s is granted if and only if access is granted by the simple rule and there does not exist any unsanitized object o1, readable by s, for which company(o1) 6= company(o2). (i) Explain the context and goal of the Chinese Wall security policy model. Then explain what each of the two rules is intended to enforce or prevent. [4 marks] (ii) Some researchers have claimed that the formal rules of Chinese Wall do not match the policy that Brewer and Nash intended to enforce, to the extent that the resulting policy is unusable in practice. Explain precisely why the policy would be unusable and give a clear proof of this claim. [8 marks] Computer ScienceEngineering & TechnologyNetworking CS 001

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