Image encryption with standard chaotic map

I have code for chaotic encryption and decryption of image but after decrypting it doesn`t return original image.

Can anyone pls suggest?

Answer 1

[Milan Bansal]

Hi Asha D.

I understand that you are facing an issue with chaotic encryption and decryption of an image where the decrypted image does not return to the original image. The problem appears to be related to discrepancies in how the encryption and decryption processes handle the bit representations of the image data.

Ensure Consistent Bit Depth: The "encrypt" function uses a 9-bit representation "dec2bin(image,9)", which is not standard for image data and likely an error. Image data typically uses 8 bits per color channel. This discrepancy must be corrected to ensure that the decryption process accurately reverses the encryption. Please make the following modifications in the "encrypt" function as shown below to resolve the issue.

clc;clear;
I=imread('Lena.jpg');
% % I=I(1:2:end,1:2:end,1:3); %zoom out
[eI,key]=encrypt(I);
dI=decrypt(eI,key);
figure
subplot(2,2,1);imshow(I);title('origin image');
subplot(2,2,2);imshow(eI);title('encrypted image');
subplot(2,2,3);imshow(dI);title('decrypted image');

function [encrypted,key] = encrypt(image,key)
if nargin==1
    key = 10*rand(1,4); 
end

x=key(1);y=key(2);k=key(3);h=key(4);
[cols,rows,chans] = size(image); 

kk = 2 + exp( mod(h + 10 * sin(k) + k ,2*pi));
hh = 2 + exp( mod(h + 10 * sin(k) ,2*pi));
xx = mod(x + hh * sin(y) + kk * y ,2*pi);
yy = mod(x + hh * sin(y) ,2*pi); 

bitArray = dec2bin(image,8);  % Modification
bitArray = [bitArray(:)]; 

N = size(bitArray,1);
posX = zeros(1, 2*N); posY = zeros(1, 2*N); 
posX(1)=xx ; posY(1) = yy; 

for i = 1 : 2*N-1 
    posY(i+1) = mod(posX(i) + kk * sin(posY(i)) ,2*pi);
    posX(i+1) = mod(posY(i+1) + hh * posX(i) ,2*pi);
end
posX = ceil(posX*N/(2*pi)); posY = ceil(posY*N/(2*pi)); 
for j = [posX ; posY] 
    tmp = bitArray(j(1));
    bitArray(j(1)) = bitArray(j(2));
    bitArray(j(2)) = tmp;
end

bitArray = reshape(bitArray,N/8,8);  % Modification
Array = bin2dec(bitArray);
encrypted = uint8(reshape(Array,cols,rows,chans)); 
end

Please refer to the following documentation links to learn more about "dec2bin" and "bin2dec" functions.

• https://www.mathworks.com/help/matlab/ref/dec2bin.html
• https://www.mathworks.com/help/matlab/ref/bin2dec.html

Hope this helps!