Algorithm 1: Encryption Algorithm

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

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