Untitled

Implementing DCT-based (Discrete Cosine Transform) steganography for PNG images involves several steps. Here’s a simplified outline of how you can embed a secret message into an image using DCT in C:

### Steps to Implement DCT-based Steganography

1. **Convert Image to Frequency Domain using DCT**:
   - Split the image into 8x8 blocks.
   - Apply DCT to each block to transform it into the frequency domain.

2. **Embed the Secret Message**:
   - Embed bits of the secret message in the DCT coefficients. This is typically done by modifying the least significant bits (LSBs) of the DCT coefficients.

3. **Inverse DCT to Transform Back to the Spatial Domain**:
   - After embedding the message, apply the inverse DCT to each block to transform the image back to the spatial domain.

4. **Save the Modified Image**:
   - Save the modified image to a new PNG file.

### Tools Required
- `libpng`: For handling PNG images.
- `libjpeg` (optional): If you want to use JPEG-specific libraries for DCT operations.

### Example Code Outline
This is a simplified version to give you an idea. A full implementation will require a lot of error handling, boundary checking, and optimizations.

#### 1. **DCT Implementation**
You'll need a function to perform the DCT. This example uses a simple 8x8 block DCT. A full JPEG-style DCT is more complex and requires more code.

```c
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <png.h>

#define BLOCK_SIZE 8
#define PI 3.14159265358979323846

void perform_dct(double block[BLOCK_SIZE][BLOCK_SIZE]) {
    double temp[BLOCK_SIZE][BLOCK_SIZE];
    double alpha, beta, sum;
    int u, v, x, y;

    for (u = 0; u < BLOCK_SIZE; ++u) {
        for (v = 0; v < BLOCK_SIZE; ++v) {
            sum = 0.0;
            for (x = 0; x < BLOCK_SIZE; ++x) {
                for (y = 0; y < BLOCK_SIZE; ++y) {
                    sum += block[x][y] * 
                           cos((2 * x + 1) * u * PI / 16.0) * 
                           cos((2 * y + 1) * v * PI / 16.0);
                }
            }

            alpha = (u == 0) ? 1 / sqrt(2.0) : 1.0;
            beta = (v == 0) ? 1 / sqrt(2.0) : 1.0;
            temp[u][v] = 0.25 * alpha * beta * sum;
        }
    }

    // Copy temp back to block
    for (x = 0; x < BLOCK_SIZE; ++x) {
        for (y = 0; y < BLOCK_SIZE; ++y) {
            block[x][y] = temp[x][y];
        }
    }
}
```

#### 2. **Embedding the Secret Message**
You can modify the DCT coefficients to embed your message. This is a basic way to do it using LSB modification:

```c
void embed_message(double dct_block[BLOCK_SIZE][BLOCK_SIZE], const char *message) {
    int bit;
    size_t message_length = strlen(message);
    size_t message_index = 0;
    int bit_index = 0;

    for (int u = 0; u < BLOCK_SIZE; ++u) {
        for (int v = 0; v < BLOCK_SIZE; ++v) {
            if (message_index < message_length) {
                bit = (message[message_index] >> bit_index) & 1;
                if (bit) {
                    dct_block[u][v] = dct_block[u][v] + 0.5; // Modify coefficient slightly
                } else {
                    dct_block[u][v] = dct_block[u][v] - 0.5; // Modify coefficient slightly
                }
                bit_index++;
                if (bit_index == 8) {
                    bit_index = 0;
                    message_index++;
                }
            }
        }
    }
}
```

#### 3. **Image Processing**
To work with the PNG image data, you'll use `libpng` to read the image into memory, perform DCT, embed the message, and write it back.

```c
void process_image(const char *input_file, const char *output_file, const char *message) {
    FILE *fp = fopen(input_file, "rb");
    if (!fp) {
        perror("File opening failed");
        return;
    }

    png_structp png = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (!png) {
        fclose(fp);
        return;
    }

    png_infop info = png_create_info_struct(png);
    if (!info) {
        png_destroy_read_struct(&png, NULL, NULL);
        fclose(fp);
        return;
    }

    if (setjmp(png_jmpbuf(png))) {
        png_destroy_read_struct(&png, &info, NULL);
        fclose(fp);
        return;
    }

    png_init_io(png, fp);
    png_read_info(png, info);

    int width = png_get_image_width(png, info);
    int height = png_get_image_height(png, info);
    png_bytep row_pointers[height];
    for (int y = 0; y < height; y++) {
        row_pointers[y] = (png_bytep) malloc(png_get_rowbytes(png, info));
    }
    png_read_image(png, row_pointers);

    fclose(fp);

    // Process each 8x8 block
    for (int i = 0; i < height; i += BLOCK_SIZE) {
        for (int j = 0; j < width; j += BLOCK_SIZE) {
            double block[BLOCK_SIZE][BLOCK_SIZE];
            for (int x = 0; x < BLOCK_SIZE; ++x) {
                for (int y = 0; y < BLOCK_SIZE; ++y) {
                    if (i + x < height && j + y < width) {
                        block[x][y] = row_pointers[i + x][j + y];
                    } else {
                        block[x][y] = 0; // Zero-padding for edge cases
                    }
                }
            }
            perform_dct(block);
            embed_message(block, message);
            // Apply inverse DCT and update row_pointers here if necessary
        }
    }

    // Write modified image to output_file
    FILE *out_fp = fopen(output_file, "wb");
    if (!out_fp) {
        perror("File opening failed");
        return;
    }

    png_structp png_out = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (!png_out) {
        fclose(out_fp);
        return;
    }

    png_infop info_out = png_create_info_struct(png_out);
    if (!info_out) {
        png_destroy_write_struct(&png_out, NULL);
        fclose(out_fp);
        return;
    }

    if (setjmp(png_jmpbuf(png_out))) {
        png_destroy_write_struct(&png_out, &info_out);
        fclose(out_fp);
        return;
    }

    png_init_io(png_out, out_fp);
    png_set_IHDR(png_out, info_out, width, height,
                 png_get_bit_depth(png, info),
                 png_get_color_type(png, info),
                 PNG_INTERLACE_NONE,
                 PNG_COMPRESSION_TYPE_DEFAULT,
                 PNG_FILTER_TYPE_DEFAULT);

    png_write_info(png_out, info_out);
    png_write_image(png_out, row_pointers);
    png_write_end(png_out, NULL);

    fclose(out_fp);

    // Free memory
    for (int y = 0; y < height; y++) {
        free(row_pointers[y]);
    }
    png_destroy_read_struct(&png, &info, NULL);
    png_destroy_write_struct(&png_out, &info_out);
}
```

#### 4. **Putting It All Together**
Create a `main` function to execute this:

```c
int main() {
    const char *input_file = "input.png";
    const char *output_file = "output.png";
    const char *message = "Secret message";

    process_image(input_file, output_file, message);

    return 0;
}
```

### Notes:
- **DCT Complexity**: Implementing a full JPEG-like DCT is complex. The above code is a simplified version and is not optimized.
- **Error Handling**: More comprehensive error handling is needed for robustness.
- **Security**: DCT-based steganography is not secure against attacks. For sensitive information, consider cryptographic methods before embedding.

### Compilation
Ensure you link against `libpng` when compiling:

```bash
gcc dct_steganography.c -lpng -lm -o dct_steganography
```

This implementation gives a basic understanding of how to perform DCT-based steganography on PNG images. For real-world applications, consider using libraries specifically designed for JPEG image processing to handle DCT more efficiently.