SCIENCE CHINA Information Sciences, Volume 61, Issue 10: 100306(2018) https://doi.org/10.1007/s11432-018-9538-4

## On sub-packetization and access number of capacity-achieving PIR schemes for MDS coded non-colluding servers

• AcceptedJul 23, 2018
• PublishedSep 4, 2018
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### Abstract

Consider the problem of private information retrieval (PIR) over a distributed storage system where $M$ records are stored across $N$ servers by using an $[N,K]$ MDS code. For simplicity, this problem is usually referred as the coded PIR problem. In 2016, Banawan and Ulukus designed the first capacity-achieving coded PIR scheme with sub-packetization $KN^{M}$ and access number $MKN^{M}$, where capacity characterizes the minimal download size for retrieving per unit of data, and sub-packetization and access number are two metrics closely related to implementation complexity.In this paper, we focus on minimizing the sub-packetization and the access number for linear capacity-achieving coded PIR schemes. We first determine the lower bounds on sub-packetization and access number, which are $Kn^{M-1}$ and $MKn^{M-1}$, respectively, in the nontrivial cases (i.e., $N\!>\!K\!\geq\!1$ and $M\!>\!1$), where $n\!=\!N/{\rm~gcd}(N,K)$. We then design a general linear capacity-achieving coded PIR scheme to simultaneously attain these two bounds, implying tightness of both bounds.

### References

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• Figure 1

Suppose $M=2$, $N=3$, $K=2$. (a) is for privately retrieving $W_1$, while (b) is for retrieving $W_2$.

• Figure 2

Query sequence for $\theta=1$ in the $(M=3$, $N=3$, $K=2)$ PIR scheme.

• Figure 3

Query sequence for the $\theta=1$ in the $(M=2$, $N=5$, $K=2)$ PIR scheme.

• Figure 4

Explanations of the functions ${\rm~Dist}_1(\Lambda,\underline{\Lambda})$ and ${\rm~Dist}_2(\Lambda,\Gamma)$ in Example 5.2. (a) Dist$_1~(\{2\},\{1,2\})$; (b) Dist$_2~(\{2\},$ $\{1,2\})$; (c) Dist$_2~(\{2,3\},\{2,3\})$.

• Table 1   Notations in the general scheme
 Notation Meaning $\theta$ rm The index of the desired record $\underline{\Lambda}$ $\Lambda\cup\{\theta\}$rm for some $\Lambda\subseteq[M]-\{\theta\}$ $\Lambda_{j,h}$ rm Suppose $\{\Lambda\subseteq[M]-\{\theta\}\mid|\Lambda|=j\}=\{\Lambda_{j,1},\Lambda_{j,2},\ldots,\Lambda_{j,t}\}$rm where $1\leq~h\leq~t=\binom{M-1}{j}$ $q_{\Lambda,\lambda}$ rm A $\Lambda$-type $|\Lambda|$-sum $q^{(i)}_{\Lambda,h}$ rm The $h$-th $\Lambda$-type $|\Lambda|$-sum provided by Serv$^{(i)}$ $\gamma^{(i)}_j$ rm The number of each type of $j$-sums provided by Serv$^{(i)}$ $\alpha_j$ $\gamma^{(i)}_j$ for $~1\leq~i\leq~N-K$ $\beta_j$ $\gamma^{(i)}_j$ for $N-K+1\leq~i\leq~N$
• Table 2   Notations in the general scheme
 $\Lambda$-type ${\rm~Serv}^{(i)},~1\leq~i\leq~N-K$ ${\rm~Serv}^{(i)},~N-K<~i\leq~N$ ${~q}^{(i)}_{\Lambda,1}$ ${~q}^{(i)}_{\Lambda,1}$ $\forall\Lambda\subseteq[M]$ $\vdots$ $\vdots$ ${q}^{(i)}_{\Lambda,\alpha_{|\Lambda|}}$ ${q}^{(i)}_{\Lambda,\beta_{|\Lambda|}}$

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