International Association for Cryptologic Research

International Association
for Cryptologic Research


Duhyeong Kim


Efficient Homomorphic Comparison Methods with Optimal Complexity 📺
Comparison of two numbers is one of the most frequently used operations, but it has been a challenging task to efficiently compute the comparison function in homomorphic encryption~(HE) which basically support addition and multiplication. Recently, Cheon et al.~(Asiacrypt~2019) introduced a new approximate representation of the comparison function with a rational function, and showed that this rational function can be evaluated by an iterative algorithm. Due to this iterative feature, their method achieves a logarithmic computational complexity compared to previous polynomial approximation methods; however, the computational complexity is still not optimal, and the algorithm is quite slow for large-bit inputs in HE implementation. In this work, we propose new comparison methods with \emph{optimal} asymptotic complexity based on \emph{composite polynomial} approximation. The main idea is to systematically design a constant-degree polynomial $f$ by identifying the \emph{core properties} to make a composite polynomial $f\circ f \circ \cdots \circ f$ get close to the sign function (equivalent to the comparison function) as the number of compositions increases. We additionally introduce an acceleration method applying a mixed polynomial composition $f\circ \cdots \circ f\circ g \circ \cdots \circ g$ for some other polynomial $g$ with different properties instead of $f\circ f \circ \cdots \circ f$. Utilizing the devised polynomials $f$ and $g$, our new comparison algorithms only require $\Theta(\log(1/\epsilon)) + \Theta(\log\alpha)$ computational complexity to obtain an approximate comparison result of $a,b\in[0,1]$ satisfying $|a-b|\ge \epsilon$ within $2^{-\alpha}$ error. The asymptotic optimality results in substantial performance enhancement: our comparison algorithm on $16$-bit encrypted integers for $\alpha = 16$ takes $1.22$ milliseconds in amortized running time based on an approximate HE scheme HEAAN, which is $18$ times faster than the previous work.
Numerical Method for Comparison on Homomorphically Encrypted Numbers
We propose a new method to compare numbers which are encrypted by Homomorphic Encryption (HE). Previously, comparison and min/max functions were evaluated using Boolean functions where input numbers are encrypted bit-wise. However, the bit-wise encryption methods require relatively expensive computations for basic arithmetic operations such as addition and multiplication.In this paper, we introduce iterative algorithms that approximately compute the min/max and comparison operations of several numbers which are encrypted word-wise. From the concrete error analyses, we show that our min/max and comparison algorithms have $$\varTheta (\alpha )$$ and $$\varTheta (\alpha \log \alpha )$$ computational complexity to obtain approximate values within an error rate $$2^{-\alpha }$$, while the previous minimax polynomial approximation method requires the exponential complexity $$\varTheta (2^{\alpha /2})$$ and $$\varTheta (\sqrt{\alpha }\cdot 2^{\alpha /2})$$, respectively. Our algorithms achieve (quasi-)optimality in terms of asymptotic computational complexity among polynomial approximations for min/max and comparison operations. The comparison algorithm is extended to several applications such as computing the top-k elements and counting numbers over the threshold in encrypted state.Our method enables word-wise HEs to enjoy comparable performance in practice with bit-wise HEs for comparison operations while showing much better performance on polynomial operations. Computing an approximate maximum value of any two $$\ell $$-bit integers encrypted by HEAAN, up to error $$2^{\ell -10}$$, takes only 1.14 ms in amortized running time, which is comparable to the result based on bit-wise HEs.


Jung Hee Cheon (2)
Dongwoo Kim (2)
Hun Hee Lee (1)
Keewoo Lee (1)