# Polylogarithmic Private Approximations and Efficient Matching

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Polylogarithmic Private Approximations and Efficient Matching
David Woodruff MIT, Tsinghua Piotr Indyk MIT TCC 2006

Secure communication a  {0,1}n b  {0,1}n
Alice Bob a  {0,1}n b  {0,1}n Want to compute some function F(a,b) Security: protocol does not reveal anything except for the value F(a,b) Semi-honest: both parties follow protocol Malicious: parties are adversarial Efficiency: want to exchange few bits

Secure Function Evaluation (SFE)
[Yao, GMW]: If F computed by circuit C, then F can be computed securely with O~(|C|) bits of communication [GMW] + … + [NN]: can assume parties semi-honest Semi-honest protocol can be compiled to give security against malicious parties Problem: circuit size at least linear in n * O~() hides factors poly(k, log n)

Secure and Efficient Function Evaluation
Can we achieve sublinear communication? With sublinear communication, many interesting problems can be solved only approximately. What does it mean to have a private approximation? Efficiency: want SFE with communication comparable to insecure case

Private Approximation
[FIMNSW’01]: A protocol computing an approximation G(a,b) of F(a,b) is private, if each party can simulate its view of the protocol given the exact value F(a,b) Not sufficient to simulate non-private G(a,b) using SFE Example: Define G(a,b): bin(G(a,b))i =bin((a,b))i if i>0 bin(G(a,b))0=a0 G(a,b) is a 1 -approximation of (a,b), but not private Popular protocols for approximating (a,b), e.g., [KOR98], are not private

Approximating Hamming Distance
[FIMNSW01]: A private protocol with complexity O~(n1/2/ ) (a,b) small: compute (a,b) exactly in O~((a,b)) bits (a,b) high: sample O~(n/(a,b)) (a-b)i, estimate (a,b) Our main result: Complexity: O~(1/2) bits Works even for L2 norm, i.e., estimates ||a-b||2 for a,b  {1…M}n * O~() hides factors poly(k, log n, log M, log 1/)

Crypto Tools Efficient OT1n:
P1 has A[1] … A[n] 2 {0,1}m , P2 has i 2 [n] Goal: P2 privately learns A[i], P1 learns nothing Can be done using O~(m) communication [CMS99, NP99] Circuits with ROM [NN01] (augments [Yao86]) Standard AND/OR/NOT gates Lookup gates: In: i Out: Mgate[i] Can just focus on privacy of the output Communication at most O~(m|C|)

High-dimensional tools
Random projection: Take a random orthonormal nn matrix D, that is ||Dx|| = ||x|| for all x. There exists c>0 s.t. for any xRn, i=1…n Pr[ (Dx)i2 > ||Dx||2/n * k] < e-ck

Approximating ||a-b||
Recall: Alice has a 2 [M]d, Bob has b 2 [M]d Goal: privately estimate ||a-b||, x=a-b Suffices to estimate ||a-b||2

Protocol Intuition Alice and Bob agree upon a random orthonormal matrix D Efficient by exchanging a seed of a PRG Alice and Bob rotate vectors a,b, obtaining Da, Db ||Da-Db|| = ||a-b|| D “spreads the mass” of the difference vector uniformly across the n coordinates. Can now try obliviously sampling coordinates as in [FIMNSW01]

Protocol Intuition Con’d
Alice and Bob agree upon random orthonormal D Alice and Bob rotate a,b, obtaining Da, Db Use secure circuit with ROMs Da and Db to: Circuit obtains (Da)i and (Db)i for many random indices i Problem: Now what? Samples leak a lot of info! Fix: - Suppose you know upper bound T with T ¸ ||a-b||2 - Flip a coin z with heads probability n((Da)i – (Db)i)2/(kT) - Then E[z] = n||Da-Db||2/(nkT) = ||a-b||2/(kT) - E[z] only depends on ||a-b||, and z only depends on E[z]!

Protocol Intuition Con’d
Alice and Bob agree upon random orthonormal D Alice and Bob rotate a,b, obtaining Da, Db Use secure circuit with ROMs Da, Db, to: Obtain (Da)i and (Db)i for L random i Generate Bernoulli z1, … , zL with E[zi] = ||a-b||2/(kT) Output kT  zi/L Privacy: View only depends on ||a-b|| Problem: Correctness! A priori bound T=M2 n, but ||a-b||2 may be (1), so (n) samples required. Fix: Private binary search on T

Protocol Intuition Con’d
Use secure circuit with ROMs Da, Db to: Obtain (Da)i and (Db)i for L random i Generate Bernoulli z1, … , zL with E[zi] = ||a-b||2/(kT) Output kT  zi/L Fix: - Private binary search on T - If many zi = 0, then intuitively can replace T with T/2 - Eventually T = ~(||a-b||2) - We will show: final choice of T is simulatable!

One last detail Want to show final choice of T is simulatable
Estimate is kT zi/L and we stop when “many” zi = 1 Recall E[zi] = ||a-b||2/(kT) Key Observation: Since orthonormal D is uniformly random, can guarantee that if many zi = 0, then ||a-b||2 << T. Note: - Suppose didn’t use D, and a = (M, 0, …, 0), b = (0, …, 0) - Then ||a-b||2 = M2 is large, but almost always zi = 0, so you’ll choose T < ||a-b||2. - Not simulatable since T depends on the structure of a, b

Algorithm vs. Simulation
Repeat Generate L independent bits zi such that Pr[zi=1]= ||a-b|| 2/Tk T=T/2 Until Σi zi ≥ (L/k) Output E= Σi zi /L * 2Tk as an estimate of ||a-b||2 ALGORITHM Repeat Generate L independent bits zi such that Pr[zi=1]= ||D(a-b)|| 2/Tk T=T/2 Until Σi zi ≥ (L/k) Output E= Σi zi /L * 2Tk as an estimate of ||a-b||2 Recall:||D(a-b)||=||a-b|| Communication = O~(L) = O~(1/2)

Other Results Use homomorphic encryption tricks to get better upper bounds for private nearest neighbor and private all-pairs nearest neighbors. Define private approximate nearest neighbor problem: Requires a new definition of private approximations for functionalities that can return sets of values. Achieve small communication in this setting.