Algorithm 4 Image-decryption algorithm.

Input: Ciphertext image C, the secret key sequences $\{X,Y\}$, the initial values of the intermediate key $Pre$ and k. Output: Plaintext image P. 1:$[M,N]=size\left(P\right)$;                                 ▹ Get image size 2:$num=1$;                        ▹ Position number of the secret key sequence 3:for $i=1:M$ do 4:   for $j=1:N$ do 5:     $i_{next}=mod(floor(X\left(num\right)\ast {10}^6),M)+1$; 6:     $j_{next}=mod(floor(Y\left(num\right)\ast {10}^6),N)+1$; 7:     $P(i,j)=mod(bitxor(C(i_{next},j_{next}),Pre)-k,256)$; 8:     $Pre=C(i_{next},j_{next})$; 9:     $num=num+1$; 10:   end for 11:end for