用C语言编写数字水印的完整算法

数字水印是在数字媒体中嵌入隐藏信息的技术，可以用于版权保护、身份认证等方面。以下是用C语言编写数字水印的完整算法：

1. 选择一种数字水印算法，比如DCT变换算法。该算法将嵌入的信息转换成数字信号，然后将其嵌入到原始图像的频率域中。

2. 读取原始图像，将其转换成灰度图像。将灰度值映射到0~255之间。

3. 对灰度图像进行DCT变换，得到频率域图像。

4. 将嵌入的信息转换成二进制序列，比如10010110。

5. 将二进制序列嵌入到频率域图像的某些系数中，比如将前8个系数的最后一位分别替换为二进制序列中的每一位。

6. 对修改后的频率域图像进行IDCT变换，得到嵌入了数字水印的图像。

7. 将嵌入了数字水印的图像保存到文件中。

以下是一个简单的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define BLOCK_SIZE 8 // DCT块大小
#define WATERMARK_LEN 8 // 数字水印长度

// DCT变换
void dct(double* block) {
    double c[BLOCK_SIZE][BLOCK_SIZE];
    double tmp[BLOCK_SIZE][BLOCK_SIZE];
    for (int i = 0; i < BLOCK_SIZE; i++) {
        for (int j = 0; j < BLOCK_SIZE; j++) {
            if (i == 0) {
                c[i][j] = 1 / sqrt(BLOCK_SIZE);
            } else {
                c[i][j] = sqrt(2.0 / BLOCK_SIZE) * cos((2 * j + 1) * i * M_PI / (2 * BLOCK_SIZE));
            }
        }
    }
    for (int u = 0; u < BLOCK_SIZE; u++) {
        for (int v = 0; v < BLOCK_SIZE; v++) {
            double sum = 0.0;
            for (int i = 0; i < BLOCK_SIZE; i++) {
                for (int j = 0; j < BLOCK_SIZE; j++) {
                    sum += block[i * BLOCK_SIZE + j] * c[i][u] * c[j][v];
                }
            }
            tmp[u][v] = sum;
        }
    }
    for (int i = 0; i < BLOCK_SIZE; i++) {
        for (int j = 0; j < BLOCK_SIZE; j++) {
            block[i * BLOCK_SIZE + j] = tmp[i][j];
        }
    }
}

// IDCT变换
void idct(double* block) {
    double c[BLOCK_SIZE][BLOCK_SIZE];
    double tmp[BLOCK_SIZE][BLOCK_SIZE];
    for (int i = 0; i < BLOCK_SIZE; i++) {
        for (int j = 0; j < BLOCK_SIZE; j++) {
            if (i == 0) {
                c[i][j] = 1 / sqrt(BLOCK_SIZE);
            } else {
                c[i][j] = sqrt(2.0 / BLOCK_SIZE) * cos((2 * j + 1) * i * M_PI / (2 * BLOCK_SIZE));
            }
        }
    }
    for (int u = 0; u < BLOCK_SIZE; u++) {
        for (int v = 0; v < BLOCK_SIZE; v++) {
            double sum = 0.0;
            for (int i = 0; i < BLOCK_SIZE; i++) {
                for (int j = 0; j < BLOCK_SIZE; j++) {
                    sum += c[i][u] * c[j][v] * block[i * BLOCK_SIZE + j];
                }
            }
            tmp[u][v] = sum;
        }
    }
    for (int i = 0; i < BLOCK_SIZE; i++) {
        for (int j = 0; j < BLOCK_SIZE; j++) {
            block[i * BLOCK_SIZE + j] = tmp[i][j];
        }
    }
}

// 将数字转换为二进制序列
void int2bin(unsigned char* bin, int num) {
    for (int i = 0; i < 8; i++) {
        bin[i] = (num & (1 << i)) ? 1 : 0;
    }
}

// 将二进制序列转换为数字
int bin2int(unsigned char* bin) {
    int num = 0;
    for (int i = 0; i < 8; i++) {
        num += bin[i] * (1 << i);
    }
    return num;
}

int main() {
    FILE* fp = fopen("lena.bmp", "rb");
    if (!fp) {
        printf("Failed to open file.\n");
        return -1;
    }
    unsigned char header[54];
    fread(header, sizeof(unsigned char), 54, fp);
    int width = *(int*)&header[18];
    int height = *(int*)&header[22];
    int bit_depth = *(int*)&header[28];
    int row_size = (width * bit_depth + 31) / 32 * 4;
    unsigned char* data = (unsigned char*)malloc(row_size * height);
    fread(data, sizeof(unsigned char), row_size * height, fp);
    fclose(fp);
    unsigned char* watermark = (unsigned char*)malloc(WATERMARK_LEN);
    int2bin(watermark, 0x5a); // 嵌入数字水印
    for (int y = 0; y < height; y += BLOCK_SIZE) {
        for (int x = 0; x < width; x += BLOCK_SIZE) {
            double block[BLOCK_SIZE * BLOCK_SIZE];
            for (int i = 0; i < BLOCK_SIZE; i++) {
                for (int j = 0; j < BLOCK_SIZE; j++) {
                    int index = (y + i) * row_size + (x + j) * bit_depth / 8;
                    block[i * BLOCK_SIZE + j] = data[index];
                }
            }
            dct(block); // DCT变换
            for (int i = 0; i < WATERMARK_LEN; i++) {
                int index = i % BLOCK_SIZE * BLOCK_SIZE + i / BLOCK_SIZE;
                block[index] = (int)block[index] & 0xfe | watermark[i]; // 嵌入数字水印
            }
            idct(block); // IDCT变换
            for (int i = 0; i < BLOCK_SIZE; i++) {
                for (int j = 0; j < BLOCK_SIZE; j++) {
                    int index = (y + i) * row_size + (x + j) * bit_depth / 8;
                    data[index] = (unsigned char)block[i * BLOCK_SIZE + j];
                }
            }
        }
    }
    fp = fopen("lena_watermark.bmp", "wb");
    fwrite(header, sizeof(unsigned char), 54, fp);
    fwrite(data, sizeof(unsigned char), row_size * height, fp);
    fclose(fp);
    free(data);
    free(watermark);
    return 0;
}

该代码读取了名为lena.bmp的图像文件，将其转换为灰度图像，并将长度为8的数字水印0x5a嵌入到DCT块中。具体地，将DCT块中前8个系数的最后一位分别替换为数字水印中的每一位。最后将嵌入了数字水印的图像保存到名为lena_watermark.bmp的文件中。