    doi: 10.3934/amc.2020070

## Capacity-achieving private information retrieval scheme with a smaller sub-packetization

 1 School of Mathematics, Southwest Jiaotong University, Chengdu, 610031, China 2 State Key Laboratory of Cryptology, P. O. Box 5159, Beijing, 100878, China 3 Department of Computer Science and Software Engineering, University of Melbourne, Melbourne, Australia 4 Institute for Communications Engineering, Technical University of Munich, Germany

Received  April 2019 Revised  February 2020 Published  April 2020

Fund Project: The work of W. Zhang and Z. Zhou was supported by the National Natural Science Foundation of China under Grant 61672028, and partially supported by National Cryptography Development Fund under Grant MMJJ20180103. Vladimir Sidorenko is on leave from Institute for Information Transmission Problems, Russian Academy of Sciences. His work is supported by the Russian Government, Contract No 14.W03.31.0019

Private information retrieval (PIR) allows a user to retrieve one out of $M$ messages from $N$ servers without revealing the identity of the desired message. Every message consists of $L$ symbols (packets) from an additive group and the length $L$ is called the sub-packetization. A PIR scheme with download cost (DC) $D/L$ is implemented by querying $D$ sums of the symbols to servers. We assume that each uncoded server can store up to $tLM/N$ symbols, $t\in\{1,2,\cdots,N\}$. The minimum DC of storage constrained PIR was determined by Attia et al. in 2018 to be $DC_{min} = 1+1/t+1/t^{2}+\cdots+1/t^{M-1}$. The capacity of storage constrained PIR (equivalently, the reciprocal of minimum download cost) is the maximum number of bits of desired symbols that can be privately retrieved per bit of downloaded symbols. Tandon et al. designed a capacity-achieving PIR scheme with sub-packetization $L' = {N\choose t}t^{M}$ of each message. In this paper, we design a PIR scheme with $t$ times smaller sub-packetization $L'/t$ and with the minimum DC for any parameters $N, M, t$. We also prove that $t^{M-1}$ is a factor of sub-packetization $L$ for any capacity-achieving PIR scheme from storage constrained servers.

Citation: Wenqin Zhang, Zhengchun Zhou, Udaya Parampalli, Vladimir Sidorenko. Capacity-achieving private information retrieval scheme with a smaller sub-packetization. Advances in Mathematics of Communications, doi: 10.3934/amc.2020070
##### References:
  M. A. Attia, D. Kumar and R. Tandon, The capacity of uncoded storage constrained PIR, 2018 IEEE International Symposium on Information Theory, (2018), 1959-1963.  doi: 10.1109/ISIT.2018.8437729. Google Scholar  K. Banawan and S. Ulukus, Multi-message private information retrieval: Capacity results and near-optimal schemes, IEEE Trans. Inform. Theory, 64 (2018), 6842-6862.  doi: 10.1109/TIT.2018.2828310.  Google Scholar  K. Banawan and S. Ulukus, The capacity of private information retrieval from byzantine and colluding databases, IEEE Transactions on Information Theory, 65 (2018), 1206–1219, arXiv: 1706.01442. doi: 10.1109/TIT.2018.2869154. Google Scholar  K. Banawan and S. Ulukus, The capacity of private information retrieval from coded databases, IEEE Transactions Information Theory, 64 (2018), 1945-1956.  doi: 10.1109/TIT.2018.2791994.  Google Scholar  Z. Chen, Z. Y. Wang and S. Jafar, The capacity of private information retrieval with private side information, (2017), arXiv: 1709.03022. Google Scholar  B. Chor, O. Goldreich, E. Kushilevitz and M. Sudan, Private information retrieval, 36th Annual Symposium on Foundations of Computer Science, (1995), 41-50.  doi: 10.1109/SFCS.1995.492461. Google Scholar  R. Freij-Hollanti, O. W. Gnilke, C. Hollanti and D. A. Karpuk, Private information retrieval from coded databases with colluding servers, SIAM Journal on Applied Algebra and Geometry, 1 (2017), 647-664.  doi: 10.1137/16M1102562.  Google Scholar  S. Kadhe, B. Garcia, A. Heidarzadeh, S. El Rouayheb and A. Sprintson, Private information retrieval with side information, (2017), arXiv: 1709.00112. Google Scholar  H. Sun and S. A. Jafar, The capacity of private information retrieval, IEEE Transactions Information Theory, 63 (2017), 4075-4088.  doi: 10.1109/TIT.2017.2689028.  Google Scholar  H. Sun and S. A. Jafar, The capacity of symmetric private information retrieval, 2016 IEEE Globecom Workshops, Washington, DC, USA, (2016), 1-5.   Google Scholar  H. Sun and S. A. Jafar, The capacity of private information retrieval with colluding databases, IEEE Trans. Inform. Theory, 64 (2018), 2361-2370.  doi: 10.1109/TIT.2017.2777490.  Google Scholar  H. Sun and S. A. Jafar, Private information retrieval from MDS coded data with colluding servers: Settling a conjecture by Freij-Hollanti et al., IEEE Transactions Information Theory, 64 (2018), 1000-1022.  doi: 10.1109/TIT.2017.2779454.  Google Scholar  H. Sun and S. A. Jafar, Optimal download cost of private information retrieval for arbitrary message length, IEEE Transactions Information Forensics and Security, 12 (2017), 2920-2932.   Google Scholar  R. Tajeddine, O. W. Gnilke and S. El Rouayheb, Private information retrieval from MDS coded data in distributed storage systems, IEEE Trans. Inform. Theory, 64 (2018), 7081-7093.  doi: 10.1109/TIT.2018.2815607.  Google Scholar  R. Tandon, M. Abdul-Wahid, F. Almoualem and D. Kumar, PIR from storage constrained databases - coded caching meets PIR, 2018 IEEE International Conference on Communications, (2018), 1-7.   Google Scholar  Q. W. Wang and M. Skoglund, Symmetric private information retrieval for MDS coded distributed storage, IEEE International Conference on Communications, (2017), 1-6.  doi: 10.1109/ICC.2017.7997029. Google Scholar  N. Woolsey, R.-R. Chen and M. Y. Ji, A new design of private information retrieval for storage constrained databases, (2019), arXiv: 1901.07490. doi: 10.1109/ISIT.2019.8849767. Google Scholar  J. K. Xu and Z. F. Zhang, On sub-packetization of capacity-achieving PIR schemes for MDS coded databases, (2017), arXiv: 1712.02466. Google Scholar  Y. W. Zhang and G. N. Ge, A general private information retrieval scheme for MDS coded databases with colluding servers, (2017), arXiv: 1704.06785. doi: 10.1007/s10623-019-00640-x.  Google Scholar  Y. W. Zhang and G. N. Ge, Multi-file private information retrieval from MDS coded databases with colluding servers, (2017), arXiv: 1705.03186. Google Scholar  Z. F. Zhang and J. K. Xu, Capacity-achieving PIR schemes with optimal sub-packetization, (2017), arXiv: 1710.11370. Google Scholar

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##### References:
  M. A. Attia, D. Kumar and R. Tandon, The capacity of uncoded storage constrained PIR, 2018 IEEE International Symposium on Information Theory, (2018), 1959-1963.  doi: 10.1109/ISIT.2018.8437729. Google Scholar  K. Banawan and S. Ulukus, Multi-message private information retrieval: Capacity results and near-optimal schemes, IEEE Trans. Inform. Theory, 64 (2018), 6842-6862.  doi: 10.1109/TIT.2018.2828310.  Google Scholar  K. Banawan and S. Ulukus, The capacity of private information retrieval from byzantine and colluding databases, IEEE Transactions on Information Theory, 65 (2018), 1206–1219, arXiv: 1706.01442. doi: 10.1109/TIT.2018.2869154. Google Scholar  K. Banawan and S. Ulukus, The capacity of private information retrieval from coded databases, IEEE Transactions Information Theory, 64 (2018), 1945-1956.  doi: 10.1109/TIT.2018.2791994.  Google Scholar  Z. Chen, Z. Y. Wang and S. Jafar, The capacity of private information retrieval with private side information, (2017), arXiv: 1709.03022. Google Scholar  B. Chor, O. Goldreich, E. Kushilevitz and M. Sudan, Private information retrieval, 36th Annual Symposium on Foundations of Computer Science, (1995), 41-50.  doi: 10.1109/SFCS.1995.492461. Google Scholar  R. Freij-Hollanti, O. W. Gnilke, C. Hollanti and D. A. Karpuk, Private information retrieval from coded databases with colluding servers, SIAM Journal on Applied Algebra and Geometry, 1 (2017), 647-664.  doi: 10.1137/16M1102562.  Google Scholar  S. Kadhe, B. Garcia, A. Heidarzadeh, S. El Rouayheb and A. Sprintson, Private information retrieval with side information, (2017), arXiv: 1709.00112. Google Scholar  H. Sun and S. A. Jafar, The capacity of private information retrieval, IEEE Transactions Information Theory, 63 (2017), 4075-4088.  doi: 10.1109/TIT.2017.2689028.  Google Scholar  H. Sun and S. A. Jafar, The capacity of symmetric private information retrieval, 2016 IEEE Globecom Workshops, Washington, DC, USA, (2016), 1-5.   Google Scholar  H. Sun and S. A. Jafar, The capacity of private information retrieval with colluding databases, IEEE Trans. Inform. Theory, 64 (2018), 2361-2370.  doi: 10.1109/TIT.2017.2777490.  Google Scholar  H. Sun and S. A. Jafar, Private information retrieval from MDS coded data with colluding servers: Settling a conjecture by Freij-Hollanti et al., IEEE Transactions Information Theory, 64 (2018), 1000-1022.  doi: 10.1109/TIT.2017.2779454.  Google Scholar  H. Sun and S. A. Jafar, Optimal download cost of private information retrieval for arbitrary message length, IEEE Transactions Information Forensics and Security, 12 (2017), 2920-2932.   Google Scholar  R. Tajeddine, O. W. Gnilke and S. El Rouayheb, Private information retrieval from MDS coded data in distributed storage systems, IEEE Trans. Inform. Theory, 64 (2018), 7081-7093.  doi: 10.1109/TIT.2018.2815607.  Google Scholar  R. Tandon, M. Abdul-Wahid, F. Almoualem and D. Kumar, PIR from storage constrained databases - coded caching meets PIR, 2018 IEEE International Conference on Communications, (2018), 1-7.   Google Scholar  Q. W. Wang and M. Skoglund, Symmetric private information retrieval for MDS coded distributed storage, IEEE International Conference on Communications, (2017), 1-6.  doi: 10.1109/ICC.2017.7997029. Google Scholar  N. Woolsey, R.-R. Chen and M. Y. Ji, A new design of private information retrieval for storage constrained databases, (2019), arXiv: 1901.07490. doi: 10.1109/ISIT.2019.8849767. Google Scholar  J. K. Xu and Z. F. Zhang, On sub-packetization of capacity-achieving PIR schemes for MDS coded databases, (2017), arXiv: 1712.02466. Google Scholar  Y. W. Zhang and G. N. Ge, A general private information retrieval scheme for MDS coded databases with colluding servers, (2017), arXiv: 1704.06785. doi: 10.1007/s10623-019-00640-x.  Google Scholar  Y. W. Zhang and G. N. Ge, Multi-file private information retrieval from MDS coded databases with colluding servers, (2017), arXiv: 1705.03186. Google Scholar  Z. F. Zhang and J. K. Xu, Capacity-achieving PIR schemes with optimal sub-packetization, (2017), arXiv: 1710.11370. Google Scholar
Sub-messages stored in the servers
 $Serv^1$ $Serv^2$ $Serv^3$ $A_{12}, A_{13}$ $A_{12}, A_{23}$ $A_{13}, A_{23}$ $B_{12}, B_{13}$ $B_{12}, B_{23}$ $B_{13}, B_{23}$ $C_{12}, C_{13}$ $C_{12}, C_{23}$ $C_{13}, C_{23}$
 $Serv^1$ $Serv^2$ $Serv^3$ $A_{12}, A_{13}$ $A_{12}, A_{23}$ $A_{13}, A_{23}$ $B_{12}, B_{13}$ $B_{12}, B_{23}$ $B_{13}, B_{23}$ $C_{12}, C_{13}$ $C_{12}, C_{23}$ $C_{13}, C_{23}$
Symbols stored in the servers
 $Serv^1$ $Serv^2$ $Serv^3$ $\underline{a^1_{12}},\; \underline{a^2_{12}}$ $\underline{a^1_{12}},\; \underline{a^2_{12}}$ $\underline{a^1_{13}},\; \underline{a^2_{13}}$ $\underline{a^1_{13}},\; \underline{a^2_{13}}$ $\underline{a^1_{23}},\; \underline{a^2_{23}}$ $\underline{a^1_{23}},\; \underline{a^2_{23}}$ $\underline{b^1_{12}},\; \underline{b^2_{12}}$ $\underline{b^1_{12}},\; \underline{b^2_{12}}$ $\underline{b^1_{13}},\; \underline{b^2_{13}}$ $\underline{b^1_{13}},\; \underline{b^2_{13}}$ $\underline{b^1_{23}},\; \underline{b^2_{23}}$ $\underline{b^1_{23}},\; \underline{b^2_{23}}$
 $Serv^1$ $Serv^2$ $Serv^3$ $\underline{a^1_{12}},\; \underline{a^2_{12}}$ $\underline{a^1_{12}},\; \underline{a^2_{12}}$ $\underline{a^1_{13}},\; \underline{a^2_{13}}$ $\underline{a^1_{13}},\; \underline{a^2_{13}}$ $\underline{a^1_{23}},\; \underline{a^2_{23}}$ $\underline{a^1_{23}},\; \underline{a^2_{23}}$ $\underline{b^1_{12}},\; \underline{b^2_{12}}$ $\underline{b^1_{12}},\; \underline{b^2_{12}}$ $\underline{b^1_{13}},\; \underline{b^2_{13}}$ $\underline{b^1_{13}},\; \underline{b^2_{13}}$ $\underline{b^1_{23}},\; \underline{b^2_{23}}$ $\underline{b^1_{23}},\; \underline{b^2_{23}}$
Queries to download message $A$
 $Serv^1$ $Serv^2$ $Serv^3$ $a^1_{12}$ $a^1_{23}$ $a^1_{13}$ $b^1_{12}$ $b^1_{23}$ $b^1_{13}$ $a^2_{13}+ b^1_{13}$ $a^2_{12}+ b^1_{12}$ $a^2_{23}+ b^1_{23}$
 $Serv^1$ $Serv^2$ $Serv^3$ $a^1_{12}$ $a^1_{23}$ $a^1_{13}$ $b^1_{12}$ $b^1_{23}$ $b^1_{13}$ $a^2_{13}+ b^1_{13}$ $a^2_{12}+ b^1_{12}$ $a^2_{23}+ b^1_{23}$
Queries to download message $B$
 $Serv^1$ $Serv^2$ $Serv^3$ $b^1_{12}$ $b^1_{23}$ $b^1_{13}$ $a^1_{12}$ $a^1_{23}$ $a^1_{13}$ $b^2_{13}+a^1_{13}$ $b^2_{12}+a^1_{12}$ $b^2_{23}+a^1_{23}$
 $Serv^1$ $Serv^2$ $Serv^3$ $b^1_{12}$ $b^1_{23}$ $b^1_{13}$ $a^1_{12}$ $a^1_{23}$ $a^1_{13}$ $b^2_{13}+a^1_{13}$ $b^2_{12}+a^1_{12}$ $b^2_{23}+a^1_{23}$
Sub-messages stored in the servers
 $Serv^1$ $Serv^2$ $Serv^3$ $Serv^4$ $Serv^5$ $A_{12}, A_{13}$ $A_{12}, A_{23}$ $A_{13}, A_{23}$ $A_{14}, A_{24}$ $A_{15}, A_{25}$ $A_{14}, A_{15}$ $A_{24}, A_{25}$ $A_{34}, A_{35}$ $A_{34}, A_{45}$ $A_{35}, A_{45}$ $B_{12}, B_{13}$ $B_{12}, B_{23}$ $B_{13}, B_{23}$ $B_{14}, B_{24}$ $B_{15}, B_{25}$ $B_{14}, B_{15}$ $B_{24}, B_{25}$ $B_{34}, B_{35}$ $B_{34}, B_{45}$ $B_{35}, B_{45}$ $C_{12}, C_{13}$ $C_{12}, C_{23}$ $C_{13}, C_{23}$ $C_{14}, C_{24}$ $C_{15}, C_{25}$ $C_{14}, C_{15}$ $C_{24}, C_{25}$ $C_{34}, C_{35}$ $C_{34}, C_{45}$ $C_{35}, C_{45}$
 $Serv^1$ $Serv^2$ $Serv^3$ $Serv^4$ $Serv^5$ $A_{12}, A_{13}$ $A_{12}, A_{23}$ $A_{13}, A_{23}$ $A_{14}, A_{24}$ $A_{15}, A_{25}$ $A_{14}, A_{15}$ $A_{24}, A_{25}$ $A_{34}, A_{35}$ $A_{34}, A_{45}$ $A_{35}, A_{45}$ $B_{12}, B_{13}$ $B_{12}, B_{23}$ $B_{13}, B_{23}$ $B_{14}, B_{24}$ $B_{15}, B_{25}$ $B_{14}, B_{15}$ $B_{24}, B_{25}$ $B_{34}, B_{35}$ $B_{34}, B_{45}$ $B_{35}, B_{45}$ $C_{12}, C_{13}$ $C_{12}, C_{23}$ $C_{13}, C_{23}$ $C_{14}, C_{24}$ $C_{15}, C_{25}$ $C_{14}, C_{15}$ $C_{24}, C_{25}$ $C_{34}, C_{35}$ $C_{34}, C_{45}$ $C_{35}, C_{45}$
Queries to download message $A$
 $Serv^1$ $Serv^2$ $Serv^3$ $Serv^4$ $Serv^5$ Step $1$ $a^1_{12},\; a^1_{13}$ $a^1_{23},\; a^1_{24}$ $a^1_{34},\; a^1_{35}$ $a^1_{14},\; a^1_{45}$ $a^1_{15},\; a^1_{25}$ $b^1_{12},\; b^1_{13}$ $b^1_{23},\; b^1_{24}$ $b^1_{34},\; b^1_{35}$ $b^1_{14},\; b^1_{45}$ $b^1_{15},\; b^1_{25}$ $c^1_{12},\; c^1_{13}$ $c^1_{23},\; c^1_{24}$ $c^1_{34},\; c^1_{35}$ $c^1_{14},\; c^1_{45}$ $c^1_{15},\; c^1_{25}$ Step $2$ $a^2_{14}+b^1_{14}$ $a^2_{12}+b^1_{12}$ $a^2_{13}+b^1_{13}$ $a^2_{24}+b^1_{24}$ $a^2_{35}+b^1_{35}$ $a^3_{14}+c^1_{14}$ $a^3_{12}+c^1_{12}$ $a^3_{13}+c^1_{13}$ $a^3_{24}+c^1_{24}$ $a^3_{35}+c^1_{35}$ $a^2_{15}+b^1_{15}$ $a^2_{25}+b^1_{25}$ $a^2_{23}+b^1_{23}$ $a^2_{34}+b^1_{34}$ $a^2_{45}+b^1_{45}$ $a^3_{15}+c^1_{15}$ $a^3_{25}+c^1_{25}$ $a^3_{23}+c^1_{23}$ $a^3_{34}+c^1_{34}$ $a^3_{45}+c^1_{45}$ $b^2_{14}+c^2_{14}$ $b^2_{12}+c^2_{12}$ $b^2_{13}+c^2_{13}$ $b^2_{24}+c^2_{24}$ $b^2_{35}+c^2_{35}$ $b^2_{15}+c^2_{15}$ $b^2_{25}+c^2_{25}$ $b^2_{23}+c^2_{23}$ $b^2_{34}+c^2_{34}$ $b^2_{45}+c^2_{45}$ Step $3$ $a^4_{12}+b^2_{12}+c^2_{12}$ $a^4_{23}+b^2_{23}+c^2_{23}$ $a^4_{34}+b^2_{34}+c^2_{34}$ $a^4_{14}+b^2_{14}+c^2_{14}$ $a^4_{15}+b^2_{15}+c^2_{15}$ $a^4_{13}+b^2_{13}+c^2_{13}$ $a^4_{24}+b^2_{24}+c^2_{24}$ $a^4_{35}+b^2_{35}+c^2_{35}$ $a^4_{45}+b^2_{45}+c^2_{45}$ $a^4_{25}+b^2_{25}+c^2_{25}$
 $Serv^1$ $Serv^2$ $Serv^3$ $Serv^4$ $Serv^5$ Step $1$ $a^1_{12},\; a^1_{13}$ $a^1_{23},\; a^1_{24}$ $a^1_{34},\; a^1_{35}$ $a^1_{14},\; a^1_{45}$ $a^1_{15},\; a^1_{25}$ $b^1_{12},\; b^1_{13}$ $b^1_{23},\; b^1_{24}$ $b^1_{34},\; b^1_{35}$ $b^1_{14},\; b^1_{45}$ $b^1_{15},\; b^1_{25}$ $c^1_{12},\; c^1_{13}$ $c^1_{23},\; c^1_{24}$ $c^1_{34},\; c^1_{35}$ $c^1_{14},\; c^1_{45}$ $c^1_{15},\; c^1_{25}$ Step $2$ $a^2_{14}+b^1_{14}$ $a^2_{12}+b^1_{12}$ $a^2_{13}+b^1_{13}$ $a^2_{24}+b^1_{24}$ $a^2_{35}+b^1_{35}$ $a^3_{14}+c^1_{14}$ $a^3_{12}+c^1_{12}$ $a^3_{13}+c^1_{13}$ $a^3_{24}+c^1_{24}$ $a^3_{35}+c^1_{35}$ $a^2_{15}+b^1_{15}$ $a^2_{25}+b^1_{25}$ $a^2_{23}+b^1_{23}$ $a^2_{34}+b^1_{34}$ $a^2_{45}+b^1_{45}$ $a^3_{15}+c^1_{15}$ $a^3_{25}+c^1_{25}$ $a^3_{23}+c^1_{23}$ $a^3_{34}+c^1_{34}$ $a^3_{45}+c^1_{45}$ $b^2_{14}+c^2_{14}$ $b^2_{12}+c^2_{12}$ $b^2_{13}+c^2_{13}$ $b^2_{24}+c^2_{24}$ $b^2_{35}+c^2_{35}$ $b^2_{15}+c^2_{15}$ $b^2_{25}+c^2_{25}$ $b^2_{23}+c^2_{23}$ $b^2_{34}+c^2_{34}$ $b^2_{45}+c^2_{45}$ Step $3$ $a^4_{12}+b^2_{12}+c^2_{12}$ $a^4_{23}+b^2_{23}+c^2_{23}$ $a^4_{34}+b^2_{34}+c^2_{34}$ $a^4_{14}+b^2_{14}+c^2_{14}$ $a^4_{15}+b^2_{15}+c^2_{15}$ $a^4_{13}+b^2_{13}+c^2_{13}$ $a^4_{24}+b^2_{24}+c^2_{24}$ $a^4_{35}+b^2_{35}+c^2_{35}$ $a^4_{45}+b^2_{45}+c^2_{45}$ $a^4_{25}+b^2_{25}+c^2_{25}$
 Steps Tuple Number of Total symbols Number of Useful symbols Step 1 Single $N{{M}\choose{1}}\lambda$ $N\lambda$ Step 2 Pair $N{{M}\choose{2}}\lambda(t-1)$ $N{{M-1}\choose{2-1}}\lambda(t-1)$ Step 3 Triple $N{{M}\choose{3}}\lambda(t-1)^2$ $N{{M-1}\choose{3-1}}\lambda(t-1)^2$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step k k-tuple $N{{M}\choose{k}}\lambda(t-1)^{k-1}$ $N{{M-1}\choose{k-1}}\lambda(t-1)^{k-1}$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step M M-tuple $N{{M}\choose{M}}\lambda(t-1)^{M-1}$ $N{{M-1}\choose{M-1}}\lambda(t-1)^{M-1}$
 Steps Tuple Number of Total symbols Number of Useful symbols Step 1 Single $N{{M}\choose{1}}\lambda$ $N\lambda$ Step 2 Pair $N{{M}\choose{2}}\lambda(t-1)$ $N{{M-1}\choose{2-1}}\lambda(t-1)$ Step 3 Triple $N{{M}\choose{3}}\lambda(t-1)^2$ $N{{M-1}\choose{3-1}}\lambda(t-1)^2$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step k k-tuple $N{{M}\choose{k}}\lambda(t-1)^{k-1}$ $N{{M-1}\choose{k-1}}\lambda(t-1)^{k-1}$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step M M-tuple $N{{M}\choose{M}}\lambda(t-1)^{M-1}$ $N{{M-1}\choose{M-1}}\lambda(t-1)^{M-1}$
Queries to download message $A$
 $Serv^1$ $Serv^2$ $Serv^3$ $Serv^4$ $a^1_{12},a^1_{14}$ $a^1_{23},a^1_{24}$ $a^1_{13}$ $a^1_{34}$ $b^1_{12},b^1_{14}$ $b^1_{23},b^1_{24}$ $b^1_{13}$ $b^1_{34}$ $a^2_{13}+b^1_{13}$ $a^2_{12}+b^1_{12}$ $a^2_{23}+b^1_{23}$ $a^2_{14}+b^1_{14}$ $a^2_{34}+b^1_{34}$ $a^2_{24}+b^1_{24}$
 $Serv^1$ $Serv^2$ $Serv^3$ $Serv^4$ $a^1_{12},a^1_{14}$ $a^1_{23},a^1_{24}$ $a^1_{13}$ $a^1_{34}$ $b^1_{12},b^1_{14}$ $b^1_{23},b^1_{24}$ $b^1_{13}$ $b^1_{34}$ $a^2_{13}+b^1_{13}$ $a^2_{12}+b^1_{12}$ $a^2_{23}+b^1_{23}$ $a^2_{14}+b^1_{14}$ $a^2_{34}+b^1_{34}$ $a^2_{24}+b^1_{24}$
 Steps Tuple Number of Total symbols (all servers) Number of Useful symbols (all servers) Step 1 Single ${{{M}\choose{1}}[da+t(N-d)]}$ $da+t(N-d)$ Step 2 Pair ${{M}\choose{2}}\frac{d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)}{{{N}\choose{t}}^{0}}$ ${{M-1}\choose{2-1}}\frac{d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)}{{{N}\choose{t}}^0}$ Step 3 Triple ${{M}\choose{3}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^2}{{{N}\choose{t}}^{1}}$ ${{M-1}\choose{3-1}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^2}{{{N}\choose{t}}^1}$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step k k-tuple ${{M}\choose{k}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{k-1}}{{{N}\choose{t}}^{k-2}}$ ${{M-1}\choose{k-1}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{k-1}}{{{N}\choose{t}}^{k-2}}$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step M M-tuple ${{M}\choose{M}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{M-1}}{{{N}\choose{t}}^{M-2}}$ ${{M-1}\choose{M-1}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{M-1}}{{{N}\choose{t}}^{M-2}}$
 Steps Tuple Number of Total symbols (all servers) Number of Useful symbols (all servers) Step 1 Single ${{{M}\choose{1}}[da+t(N-d)]}$ $da+t(N-d)$ Step 2 Pair ${{M}\choose{2}}\frac{d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)}{{{N}\choose{t}}^{0}}$ ${{M-1}\choose{2-1}}\frac{d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)}{{{N}\choose{t}}^0}$ Step 3 Triple ${{M}\choose{3}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^2}{{{N}\choose{t}}^{1}}$ ${{M-1}\choose{3-1}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^2}{{{N}\choose{t}}^1}$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step k k-tuple ${{M}\choose{k}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{k-1}}{{{N}\choose{t}}^{k-2}}$ ${{M-1}\choose{k-1}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{k-1}}{{{N}\choose{t}}^{k-2}}$ $\vdots$ $\vdots$ $\vdots$ $\vdots$ Step M M-tuple ${{M}\choose{M}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{M-1}}{{{N}\choose{t}}^{M-2}}$ ${{M-1}\choose{M-1}}\frac{{[d({{N-1}\choose{t-1}}-a)+(N-d)({{N-1}\choose{t-1}}-t)]}^{M-1}}{{{N}\choose{t}}^{M-2}}$
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