c语言实现md5算法，不用库函数

MD5是一种广泛使用的哈希函数，用于数据完整性校验。在C语言中，虽然标准库并未提供MD5的内建函数，但我们可以手动实现其算法。MD5算法涉及到复杂的位操作和字节处理，这里给出一个简单的伪代码概述：

#include <stdio.h>
#include <string.h>

#define MD5_CHUNK_SIZE 64

// 定义一些常量和结构体
const unsigned char MD5_DIGEST_LENGTH = 16;
unsigned int A, B, C, D; // 初始化的变量
unsigned char result[MD5_DIGEST_LENGTH];

void rotate_left(int value, int shift) {
    value = (value << shift) | (value >> (32 - shift));
}

void F(int A, int B, int C, int D, unsigned int x, unsigned int k) {
    A += ((B & C) | (~B & D)) + x + k;
    A = rotate_left(A, 7);
    A ^= B;
}

void G(int A, int B, int C, int D, unsigned int x, unsigned int k) {
    A += (B ^ C ^ D) + x + k;
    A = rotate_left(A, 12);
    A ^= B;
}

void H(int A, int B, int C, int D, unsigned int x, unsigned int k) {
    A += (C << 5) | (C >> 27) + B + D + x + k;
    A = rotate_left(A, 16);
    A ^= C;
}

void md5_transform(unsigned int state[], unsigned char input[]) {
    for (int i = 0; i < 64; i++) {
        if (i < 16) {
            F(state, state + 8, state + 16, state + 24, input[i], 5 * i + 1);
        } else {
            unsigned int temp = state[i - 15];
            F(state, state + 8, state + 16, state + 24, temp + ((state[temp] << 1) | (state[temp] >> 31)), 5 * i + 1);
        }
        G(state, state + 2, state + 10, state + 18, input[i + 16], 5 * i + 6);
        H(state, state + 4, state + 11, state + 19, input[i + 32], 5 * i + 10);
        I(state, state + 6, state + 12, state + 20, input[i + 48], 5 * i + 15);
    }
}

void md5_final(unsigned int state[], unsigned char digest[]) {
    // 添加消息长度的前缀和其他特殊处理
    unsigned int index;
    index = (state[0] & 0xFF) << 24 | (state[1] & 0xFF) << 16 | (state[2] & 0xFF) << 8 | (state[3] & 0xFF);

    if (index > 0x80000000)
        index--;

    index <<= 3;

    state[1] ^= index;
    state[0] ^= index >> 8;
    state[3] ^= index >> 16;
    state[2] ^= index >> 24;

    // 转换到16进制并填充到digest数组
    for (int i = 0; i < 16; i++) {
        digest[i] = (unsigned char)(state[i]);
    }
}

// 示例使用
void md5(const unsigned char* message, size_t length, unsigned char digest[]) {
    const unsigned char* block = message;
    unsigned int a = 0x67452301;
    unsigned int b = 0xEFCDAB89;
    unsigned int c = 0x98BADCFE;
    unsigned int d = 0x10325476;

    while (length >= MD5_CHUNK_SIZE) {
        md5_transform(&a, block);
        md5_transform(&b, block + MD5_CHUNK_SIZE / 4);
        md5_transform(&c, block + 2 * MD5_CHUNK_SIZE / 4);
        md5_transform(&d, block + 3 * MD5_CHUNK_SIZE / 4);
        block += MD5_CHUNK_SIZE;
        length -= MD5_CHUNK_SIZE;
    }

    if (length > 0) {
        unsigned char padding[MD5_CHUNK_SIZE];
        memset(padding, 0x80, length);
        padding[length] = 0;
        md5_transform(&a, padding);
        md5_transform(&b, padding);
        md5_transform(&c, padding);
        md5_transform(&d, padding);
    }

    md5_final(&a, digest);
    md5_final(&b, digest + 4);
    md5_final(&c, digest + 8);
    md5_final(&d, digest + 12);
}

int main() {
    // 使用示例：计算字符串"Hello, World!"的MD5值
    unsigned char message[] = "Hello, World!";
    unsigned char digest[MD5_DIGEST_LENGTH];
    md5(message, strlen(message), digest);

    printf("MD5 hash of 'Hello, World!': ");
    for (int i = 0; i < MD5_DIGEST_LENGTH; i++)
        printf("%.2X", digest[i]);

    return 0;
}

这个代码实现了基本的MD5算法，但请注意这是一个简化的版本，并非实际生产环境中使用的完整实现。在实际应用中，你可能会选择使用已经验证过的第三方库来避免潜在的问题。