feat: add ImageDataDecoder — LSB steganography decoder

Reads hidden messages from encoded PNG images by extracting
LSB data from channels indicated by the pixel 0 header.

ImageDataDecoder/ImageDataDecoder.cpp

@@ -0,0 +1,98 @@
+#define _CRT_SECURE_NO_WARNINGS
+#include <iostream>
+#include <string>
+#include <stdexcept>
+#include <cstdlib>
+
+#define STB_IMAGE_IMPLEMENTATION
+#include "stb_image.h"
+
+int width, height, channels;
+std::string inputPath, message;
+bool useR, useG, useB;
+unsigned char* data;
+
+void loadImage() {
+    data = stbi_load(inputPath.c_str(), &width, &height, &channels, 0);
+    if (!data) {
+        throw std::runtime_error("Failed to load image: " + inputPath);
+    }
+
+    if (channels < 3) {
+        stbi_image_free(data);
+        throw std::runtime_error("Image must have at least 3 channels (RGB). Found: " + std::to_string(channels));
+    }
+}
+
+void getInputPath() {
+    std::cout << "Enter image file path: ";
+    std::getline(std::cin, inputPath);
+    if (inputPath.size() >= 2 && inputPath.front() == '"' && inputPath.back() == '"') {
+        inputPath = inputPath.substr(1, inputPath.size() - 2);
+    }
+}
+
+void readChannelHeaders() {
+    useR = data[0] & 1;
+    useG = data[1] & 1;
+    useB = data[2] & 1;
+
+    if (!useR && !useG && !useB) {
+        stbi_image_free(data);
+        throw std::runtime_error("No active channels found in header pixel. Image may not contain a hidden message.");
+    }
+
+    std::cout << "Active channels: " << (useR ? "R" : "") << (useG ? "G" : "") << (useB ? "B" : "") << "\n";
+}
+
+bool processBit(int channelOffset, int base, unsigned char& currentByte, int& bitCount) {
+    int bit = data[base + channelOffset] & 1;
+    currentByte = (currentByte << 1) | bit;
+    bitCount++;
+    if (bitCount == 8) {
+        if (currentByte == '\0') return true; // null terminator found
+        message.push_back(static_cast<char>(currentByte));
+        currentByte = 0;
+        bitCount = 0;
+    }
+    return false;
+}
+
+void readMessage() {
+    int totalPixels = width * height;
+    unsigned char currentByte = 0;
+    int bitCount = 0;
+
+    for (int pixel = 1; pixel < totalPixels; pixel++) {
+        int base = pixel * channels;
+
+        if (useR && processBit(0, base, currentByte, bitCount)) return;
+        if (useG && processBit(1, base, currentByte, bitCount)) return;
+        if (useB && processBit(2, base, currentByte, bitCount)) return;
+    }
+}
+
+int main() {
+    try {
+        getInputPath();
+        loadImage();
+        readChannelHeaders();
+        readMessage();
+
+        stbi_image_free(data);
+
+        if (message.empty()) {
+            std::cout << "No message found (or message is empty).\n";
+        } else {
+            std::cout << "Hidden message (" << message.size() << " bytes):\n";
+            std::cout << message << "\n";
+        }
+
+    } catch (const std::exception& e) {
+        std::cerr << "Error: " << e.what() << "\n";
+        std::cin.get();
+        return EXIT_FAILURE;
+    }
+    std::cin.get();
+    return EXIT_SUCCESS;
+}