steg_encode.m

function [ output ] = steg_encode( cover, key, message )
%STEG_ENCODE Embeds a message into a cover image.
%   Uses a custom steganography solution to embed a message into a cover
%   image. The message is converted to binary and hidden in the luminance
%   values of the cover image. The data is encoded usign a linear block
%   code and scattered redunantly throughout the image.

n = 7; % Block code bits
k = 3; % Block code parity bits
capacity = 1026; % The number of bits which can be hidden
addpath('./lib');

% Convert message to binary
data = dec2bin(message);
data = arrayfun(@(x) padarray((data(x, :) - '0'), [0, 8 - numel(data(x, :))], 0, 'pre'), 1:size(data, 1), 'UniformOutput', false);
data = cell2mat(data);
data = padarray(data, [0, capacity - numel(data)], 0, 'post');
data = transpose(data);
data = reshape(data, [k, numel(data) / k]);
data = transpose(data);

% Convert the image to HSL
hsl_img = cover;
for col = 1:size(hsl_img, 2)
    for row = 1:size(hsl_img, 1)
        pixel = double(hsl_img(row, col, :)) / 255;
        hsl_img(row, col, :) = rgb2hsl(pixel) * 255;
    end
end

% Linear encode binary message
pol = cyclpoly(n,k);
parmat = cyclgen(n,pol);
genmat = gen2par(parmat);
data = arrayfun(@(x) encode(data(x, :), n, k, 'linear/binary', genmat), 1:size(data, 1), 'UniformOutput', false);
data = cell2mat(data);

% Embed data in luminance
lum = hsl_img(:, :, 3);
bits = padarray(data, [0, numel(lum) - numel(data)], 'circular', 'post');
rng(key);
perm = randperm(length(bits));
data = zeros(1, length(bits));
for index = 1:length(data)
    data(index) = bits(perm(index));
end
data = reshape(data, [size(lum, 1), size(lum, 2)]);
lum = bitset(lum, 1, data);

% Set the modified luminance in the cover image
output = cover;
for col = 1:size(output, 2)
    for row = 1:size(output, 1)
        hsl = double(hsl_img(row, col, :)) / 255;
        hsl(3) = double(lum(row, col)) / 255;
        output(row, col, :) = hsl2rgb(hsl) * 255;
    end
end
end