Steganography
is a technique that hides secret information inside another document or image
and it is extracted by different person.
In this post, we will see how to
hide text message or image inside an image without tampering the original or
cover image. Learn about bit plane slicing to know more about the background,
EXAMPLE 1: Hide Secret
Message inside an image
DECRYPTION:
It is
straight-forward reverse process. The secret message is obtained by extracting
the least significant bits from the encrypted matrix
MATLAB CODE:
%ENCRYPTION
clear all
%READ THE INPUT IMAGE
A = imread('cameraman.tif');
%
Output = A;
%MESSAGE TO BE HIDDEN
INSIDE THE IMAGE
Str = 'Image
Processing! Let us learn together. Visit angeljohnsy.blogspot.com';
%NUMBER OF CHARACTERS IN
THE MESSAGE
numLetters = numel(Str);
%REPRESENT THE LENGTH OF
THE MESSAGE IN BINARY FORMAT
Bi_Total = de2bi(numLetters,8);
Bi_Msg = de2bi(double(Str),8)';
Encrypt_msg = Bi_Msg(:)';
%FIND THE LENGTH OF THE
MESSAGE IN THE BINARY FORMAT
num_En_msg = numel(Encrypt_msg);
%DEFINE THE INTERVAL
BETWEEN EACH VALUE
dt=floor(numel(A)/num_En_msg);
%BINARY VALUE OF THE
INTERVAL
Bi_dt = de2bi(dt,8);
% FIRST 8 BITS - INTERVAL
% SECOND 8 BITS - TOTAL
LENGTH OF THE BINARY VALUES
Info_data = [Bi_dt,Bi_Total];
Output(1:16) = bitset(A(1:16),1,Info_data);
%REPLACE THE LEAST
SIGNIFICANT BIT IN THE IMAGE
Start_pt =17;
End_pt = (num_En_msg*dt)+16;
Output(Start_pt:dt:End_pt) =
bitset(A(Start_pt:dt:End_pt),1,Encrypt_msg);
figure,subplot(121),imshow(A);title('Original
Image');
subplot(122),imshow(Output);title('Image with
the hidden message');
%SHOW THE DIFFERENCE BETWEEN
THE ORIGINAL AND THE ENCRYPTED IMAGE
figure,imagesc(double(A)-double(Output));colormap(jet);colorbar;
%WRITE THE ENCRYPTED IMAGE
IN THE PNG FORMAT
imwrite(Output,'Encrypt1.png');
display('Message is
hidden inside the image!');
EXPLANATION:
A message is hidden inside the
image by converting the message into binary format and placing it in the Least
Significant Bit of the image pixels. The encrypted image is saved as ‘PNG’
image file in order to have lossless compression. With the lossless compression
technique, the pixel values will not be altered so the information embedded in
the LSB will not be lost.
The maximum absolute difference in
value between the original and the encrypted image is one. This is obvious due
to the fact that we are dealing only with the LSB bit of the image.
We can place the bits of the
secret message in a specific pattern or with regular intervals instead of replacing
the pixels adjacent to each other. The image shown below indicates that the
bits are placed with specific interval values.
The number of characters in the
secret message should be less than 10% of the cover image. This is due to the
fact that for each character we need 8 bits and to place these 8 bits we need 8
pixels.
MATLAB CODE:
%DECRYPT THE MESSAGE
clear all
clc
%READ THE ENCRYPTED IMAGE
A = imread('Encrypt1.png');
%FIRST 8 BITS REPRESENT THE
INTERVAL
Dt = bitget(A(1:8),1);
Dt = double(bi2de(Dt));
%LENGTH OF THE TEXT MESSAGE
Len = bitget(A(9:16),1);
Len = double(bi2de(Len));
%FETCH THE LEAST
SIGNIFICANT BIT FROM THE ENCRYPTD IMAGE
Start_pt = 17;
End_pt = (Len*Dt*8)+16;
Decrypt_msg =
bitget(A(Start_pt:Dt:End_pt),1);
%CONVERT THE IMAGE INTO
DECIMAL
Decrypt_msg = reshape(Decrypt_msg,8,[])';
Dec_Char = bi2de(Decrypt_msg)';
display('Text Message
inside the image:');
display(char(Dec_Char));
EXPLANATION:
Decryption is the reverse process.
First the interval between the pixels is fetched, followed by the length of the
message. Based on the length of the
message, the bits are extracted and the final message is obtained.
EXAMPLE 2: Hide Secret image inside an image
