PSNR value is very high around 300. Am I doing it correct?

I have following code which gives me PSNR 300 which seems unrealisic. If I use DWT first level, it shows correct PSNR around 40-50dB, But 2-level DWT gives PSNR 300. Is the code incorrect.

Thank you.

------------------------------------------------------------------------------------

clear all;

close all;

clc;

format long

addpath(genpath(cd))

%% AES ENCRYPTION

ssi;

plaintext=['S' 'i' 'n' 'h' 'g' 'a' 'd' ' ' 'C' 'o' 'l' 'l' 'e' 'g' 'e' ' '];

fprintf('PLAIN TEXT :%s\n', plaintext);

plain=reshape(plaintext,4,4);

plain = abs (plain);

key = ['1' '2' '3' '4' '5' '6' '7' '8' '9' '1' '2' '3' '4' '5' '6' '7'];

key=abs(key);

w = key_expansion (key, s_box, rcon);

ciphertext = cipher (plain, w, s_box, poly_mat,ind_matleft);

ciphertext = reshape(ciphertext,1,16);

fprintf('CIPHER TEXT :%s\n',ciphertext);

pause(.2)

save ciphertext ciphertext

%% DWT DECOMPOSITION

I=imread('chest_xray.jpg');

% I=im2bw(I);

figure,imshow(I);title('COVER IMAGE');pause(.5)

% RESIZE

I=imresize(I,[512,512]);

[m,n,z]=size(I);

% m:Rows, n:Coloumns, Z:Channel

if z==3%i.e.R,G,B [it is color image]

I=rgb2gray(I);

figure,imshow(I);title('GRAY IMAGE');pause(.5)

end

% DWT DECOMPOSITION

Wname='db01';% Wavlete family --Daubechis Wavlet

% Decompose Image

[CA,CH,CV,CD]=dwt2(I,Wname);

%%https://in.mathworks.com/matlabcentral/answers/384533-how-to-calculate-discrete-wavelet-transform-on-an-image-with-levels-2-4-and-6-using-dwt2-function-in

[cDA,cDH,cDV,cDD] = dwt2(CD,Wname);

CD=cDD;

load ciphertext

CD=im2bw(CD);

CD=mat2gray(CD);

CD(1:16,1)=ciphertext;

CD=repmat(CD,2,2);

% CA:Approximate Coeff;

% DETAIL COEFF

% CH:Horizontal coeff

% CV:Vertical Coeff

% CD Diagonal Coeff

figure,

subplot(2,2,1);imagesc((CA));title('APPROX COEFF')

subplot(2,2,2);imagesc((CH));title('DETAIL COEFF:HORIZONTAL')

subplot(2,2,3);imagesc((CV));title('DETAIL COEFF:VERTICAL')

subplot(2,2,4);imagesc((CD));title('DETAIL COEFF:DIAGONAL')

%% DWT RECONSTRUCTION

% RECONSTRUCT

cDD=imresize(cDD,[128, 128]);

CD=idwt2(cDA, cDH, cDV, cDD ,Wname);

I1=idwt2(CA, CH, CV, CD, Wname);

figure,

imshow(I1,[]),title('RECONSTRUCTED IMAGE')

%%Performance

I = double(I);

%%https://www.pantechsolutions.net/blog/matlab-code-for-psnr-and-mse/

n=size(I);

M=n(1);

N=n(2);

MSE = sum(sum((I-I1).^2))/(M*N);

%PSNR = 10*log10(256*256/MSE); 1.879471168418133e-27

PSNR = 10*log10(256*256/1.879471168418133e-27);

fprintf('\nMSE: %7.2f\n', MSE);

fprintf('PSNR: %10.10f dB\n', PSNR);

ssimval = ssim(I1,I);

fprintf('Structural Similarity: %5.5f\n\n', ssimval);

[KALYAN ACHARJYA]

Please share the MSE value? also mention the image bit type (8 bit image or 16 bit)?

[Punitha Viswanathan]

did you get the answer for why the PSNR value comes above 300. please answer. Thanks in advance