# Using Data-Oblivious Algorithms for Private Cloud Storage Access. Michael T. Goodrich Dept. of Computer Science

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1 Using Data-Oblivious Algorithms for Private Cloud Storage Access Michael T. Goodrich Dept. of Computer Science

2 Privacy in the Cloud Alice owns a large data set, which she outsources to an honest-but-curious server, Bob. Alice trusts Bob to reliably maintain her data, to update it as requested, and to accurately answer queries on this data. But she does not trust Bob to keep her information confidential. Alice read cell i write x to cell j Bob

3 Encryption is not Sufficient Alice certainly should use a semantically-secure encryption scheme, for each cell of her data. But this is not enough. Alice read E(x) from cell i write E(y) to cell j Bob e.g., Bob can see the hot spots

4 Oblivious Data Storage Alice has a private memory, of size K, which she can use as local scratch space so that she can access her data on a untrusted server in a private fashion. She wants to do this with low overhead She wants to use this to hide her access patterns

5 Data-oblivious Algorithms Alice can encrypt her data and then hide her access patterns by using data-oblivious algorithms. A data-oblivious computation consists of a sequence of data accesses that do not depend on the input values. All functions that combine data values are encapsulated into black box operations, with a constant number of inputs and outputs. The control flow depends only on the input size, and, in the case of randomized algorithms, the values of random variables.

6 Two Approaches Design general methods to efficiently simulate an arbitrary RAM algorithm, A, in a dataoblivious fashion. These methods typically have an overhead per access of O(log n), O(log 2 n), or even O(log 3 n). Design efficient data-oblivious algorithms for specific problems of interest. These methods tend to be more efficient, but are more specialized We are taking a unified view, which allows for both approaches.

7 Our General Simulation Results We give methods for oblivious RAM simulation: O(1) local memory and has O(log 2 n) overhead O(n ε ) local memory and has O(log n) overhead. O(n ε ) local memory and message size, and has O(1) overhead Our methods use the following techniques: MapReduce cuckoo hashing Data-oblivious external-memory sorting cuckoo hashing with a shared stash

8 Our Results for Specific Problems We give data-oblivious algorithms for Planar convex hull construction, Minimum spanning trees, Graph drawing problems, All nearest neighbor finding Image from

9 Cuckoo Hashing Uses two lookup tables T 0 and T 1 and two pseudo-random hash functions, f 0 and f 1. Each item x is stored either in T 0 [h 0 (x)] or T 1 [h 1 (x)]. When an item x is added, we put it in T 0 [h 0 (x)]. If there was already an item y there, we put it in T 1 [h 1 (y)]. If there was already an item z there, we put it in T 0 [h 0 (z)].» Will add a new item in O(log n) time w/ probability 1-1/n.

10 Cuckoo Hashing Technique T 1 T h 1 h

11 Cuckoo Insertion put 7 T 1 T evicts lands in an empty cell

12 Using a Stash [Kirsch et al., 09] introduce the idea of using a stash with a cuckoo table. A small cache where we store items that cannot be added to the cuckoo table without causing an infinite loop. A stash of size c improves the failure probability to be 1/n c. Unfortunately, this is too large a failure bound for us

13 Using a Big Stash We show that a stash of size O(log n) reduces the failure probability to be negligible. But now lookups will no longer be O(1) time. Still, in some cases, like in ORAM simulation, we may have several cuckoo tables that share the same big stash. Ok, but there is still the issue of constructing a cuckoo table obliviously

14 MapReduce A framework for designing computations for large clusters of computers. Decouples location from data and computation Image from taken from Yahoo! Hadoop Presentation: Part 2, OSCON 2007.

15 Map-Shuffle-Reduce Map: (k,v) -> [(k 1,v 1 ),(k 2,v 2 ), ] must depend only on this one pair, (k,v) Shuffle: For each key k used in the first coordinate of a pair, collect all pairs with k as first coordinate [(k,v 1 ),(k,v 2 ), ] Reduce: For each list, [(k,v 1 ),(k,v 2 ), ]: Perform a sequential computation to produce a set of pairs, [(k 1,v 1 ),(k 2,v 2 ), ] Pairs from this reduce step can be output or used in another map-shuffle-reduce cycle. Image from

16 MapReduce Cuckoo Hashing We give a MapReduce Algorithm for constructing a cuckoo table. It performs O(n) parallel steps of item insertions With very high probability, this reduces the number of remaining uninserted items to be n/c, for some constant c. Recursively add these items Total work is O(n). But now we need an oblivious way to simulate a MapReduce algorithm

17 Oblivious Deterministic Sorting For internal-memory: AKS is the only deterministic oblivious method running in O(n log n) time. Randomized Shellsort [Goodrich 10] runs in O(n log n) time and sorts with high probability, but this isn t good enough here. We show how to design an oblivious external-memory sorting method that uses O((N/B)log 2 M/B (N/B)) I/Os.

18 Generalized Odd-Even Sort We divide A into k = (M/B) 1/3 subarrays of size N/k and recursively sort each subarray. Let us therefore focus on merging k sorted arrays of size n = N/k each. If nk < M, then we copy all the lists into internal memory, merge them, and copy them back. Otherwise, let A[i, j] denote the jth element in the ith array. We form a set of m new subproblems, where the pth subproblem involves merging the k sorted subarrays defined by A[i, j] elements such that j mod m = p, for m = (M/B)1/3. Let D[i, j] denote the jth element in the output of the ith subproblem. That is, we can view D as a two-dimensional array, with each row corresponding to the solution to a recursive merge. Lemma: Each row and column of D is in sorted order and all the elements in column j are less than or equal to every element in column j + k. Proof: The lemma follows from Theorem 1 of Lee and Batcher [32]. To complete the k-way merge, then, we imagine that we slide an m x k rectangle across D, from left to right. When it finishes, A will be sorted (obliviously) Runs in O((N/B)log2M/B (N/B)) I/Os. Note that this is O(N)-time sorting if B=1 and M=O(N ε ).

19 Our Simulation Construct O(log n) cuckoo tables in a hierarchy, H 0, H 1, H 2, Each table is twice the size of the previous They all share a single stash of size O(log n) Store all the items (i,v) in these tables Initially, they are all empty except for the largest. H 0 H 1 H 2 H 3

20 For each Access to i First look in H 0 (which is just a list) Then look in H 1, H 2,, doing a cuckoo lookup for i As soon as you find it, say in H 6, store it But to be oblivious, continue doing cuckoo lookups in H 7, H 8,, for a random (previously unused) dummy index When we are done, but the updated value of (i,v) in H 0

21 Cascading Each time a table H i fills up, we dump its contents in H i+1, using the oblivious MapReduce construction ( a few more details please see the paper) We can do ORAM simulation with O(log 2 n) overhead with O(1) local memory or O(log n) overhead with O(n ε ) local memory

22 Convex Hull Representation We want the entire algorithm to be dataoblivious, except for low-level blackbox functions Given a set of points, A, ordered by their x- coordinates, we define the upper hull, UH(A), of A, to be as follows For each point p in A, we label p with the edge, e(p), of the upper convex hull that is intersected by a vertical line through the point p. If p is itself on the upper hull, then we label p with the upper hull edge incident to p on the right. p e(p)

23 Our Approach Do an oblivious sort of A Divide A into left half and right half and recursively find UH of each side

24 Merge Step Find the common upper tangent Relabel points under the tangent

25 Tangent-Finding Cases Case a: p q Case b: p q Case c: p q Case d: p q Case e: p q Case f: Case g: Case h: p q p q p q Case i1: p q Case i2: p q [from Overmars & van Leeuwen]

26 Difficulty The classic binary search algorithm is not dataoblivious We need a new way to do this search We aim to assign each edge e of UH(A 1 ) and UH(A 2 ) one of two labels: L: the tangent line of UH(A 1 U A 2 ) with the same slope as e is tangent to UH(A1). R: the tangent line of UH(A 1 U A 2 ) with the same slope as e is tangent to UH(A2). In some intermediate steps, we may be unable to determine yet whether an edge should be labeled L or R; In such cases, we temporarily label it with an X.

27 New Approach Divide UH(A 1 ) and UH(A 2 ) at every n 1/2 edges Do brute-force comparisons See if we can reduce one of A 1 or A 2 to a region of size n 1/2 Repeat until we have found the tangent This sounds non-oblivious, but we can make it oblivious by trying all O(1) possible reductions in turn (one of them will work).

28 New Case Analysis For edge e in H 1, let d be the edge in H 2 with smallest slope greater than e and let f be the edge in H 2 with largest slope less than e

29 Result This gives us an oblivious linear-time method for finding the common upper tangent This, in turn, results in a data-oblivious convex hull algorithm running in O(n log n) time.

30 Data-Oblivious Nearest Neighbors Based primarily on two new oblivious algorithms compressed quadtree construction well-separated pair decomposition The Geography Lesson (Portrait of Monsieur Gaudry and His Daughter), oil on canvas painting by Louis-Léopold Boilly, 1812, Kimbell Art Museum

31 Quadtree Construction Use sorting and bitinterleaving trick (e.g., see Samet) to construct a compressed quadtree in a dataoblivious manner

32 Well-Separated Pairs Given a parameter s construct a set of pairs, (A 1,B 1 ), (A 2,B 2 ),, (A k,b k ), such that every pair of points p and q are represented by a pair (A i,b i ) such that p is in A i and q is in B i, and such that there are balls of radius r containing A i and B i so that these balls are of distance at least sr apart. Images from

33 Conclusion and Open Problems We have shown how to solve several geometric problems efficiently with data-oblivious algorithms These methods lead to efficient SMC protocols for privacy-preserving location-based methods Open: Is there a data-oblivious method for building a representation of the Voronoi diagram (or Delaunay triangulation) of a set of n points in O(n log n) time?

34 Relevant Publications 1. M.T. Goodrich, Randomized Shellsort: A Simple Data-Oblivious Sorting Algorithm," Journal of the ACM, 58(6), Article No. 27, D. Eppstein, M.T. Goodrich, R. Tamassia, Privacy-Preserving Data-Oblivious Geometric Algorithms for Geographic Data," Proc. 18th ACM GIS, 2010, M.T. Goodrich, Spin-the-bottle Sort and Annealing Sort: Oblivious Sorting via Round-robin Random Comparisons," 8 th ANALCO, M.T. Goodrich, Data-Oblivious External-Memory Algorithms for the Compaction, Selection, and Sorting of Outsourced Data," 23rd ACM SPAA, 2011, M.T. Goodrich and M. Mitzenmacher, Privacy-Preserving Access of Outsourced Data via Oblivious RAM Simulation," 38 th ICALP, vol. 6756, 2011, M.T. Goodrich, M. Mitzenmacher, O. Ohrimenko, and R. Tamassia, Oblivious RAM Simulation with Efficient Worst-Case Access Overhead," ACM Cloud Computing Security Workshop (CCSW), , M.T. Goodrich, O. Ohrimenko, M. Mitzenmacher, and R. Tamassia, Privacy-Preserving Group Data Access via Stateless Oblivious RAM Simulation," 23 rd SODA, , M.T. Goodrich, O. Ohrimenko, M. Mitzenmacher, and R. Tamassia, Practical Oblivious Storage," 2nd ACM CODASPY, 13-24, M.T. Goodrich and M. Mitzenmacher, Anonymous Card Shuffling and its Applications to Parallel Mixnets," 39 th ICALP, Springer, LNCS, vol. 6756, , M.T. Goodrich, O. Ohrimenko, and R. Tamassia, Graph Drawing in the Cloud: Privately Visualizing Relational Data using Small Working Storage," 20 th Graph Drawing 2012.

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