Algorithm 2: Decryption Algorithm

Input : $r_1,\dots ,r_6$: pseudo-random sequences     $(t,x^{\prime },y^{\prime },p^{\prime })$: an encrypted event     $M,N$: the width and height of the NVS     Output: $(t,x,y,p)$: an event

1$r_6^{\prime }=Sort\left(r_6\right)$; 2Generate ${\mathcal{S}}_{r_6}^{\prime }$ which maps the index of each element in the sorted sequence $r_6^{\prime }$ to the original sequence $r_6$; 3$p_{temp}=\lfloor \frac{{\mathcal{S}}_{r_6}^{\prime }(p^{\prime }\ast M+x^{\prime })}{M}\rfloor$; 4$x={\mathcal{S}}_{r_6}(p^{\prime }\ast M+x^{\prime })modM$; 5$r_5^{\prime }=Sort\left(r_5\right)$; 6Generate ${\mathcal{S}}_{r_5}^{\prime }$ which maps the index of each element in the sorted sequence of $r_5^{\prime }$ to the original sequence $r_5$; 7$p=\lfloor \frac{{\mathcal{S}}_{r_5}^{\prime }(p_{temp}\ast N+y^{\prime })}{N}\rfloor$; 8$y={\mathcal{S}}_{r_5}(p_{temp}\ast N+y^{\prime })modN$; 9$x=(x-r_4^y)modM$; 10$y=(y-r_3^x)modN$; 11$x=(x-r_2^y)modM$; 12$y=(y-r_1^x)modN$;