C语言MD5算法程序

以下是一个简单的C语言实现MD5算法的程序：

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

#define ROTLEFT(x, n) (((x) << (n)) | ((x) >> (32-(n))))

void md5(uint8_t *initial_msg, size_t initial_len, uint8_t *digest) {

    //定义常量
    uint32_t h0, h1, h2, h3;
    h0 = 0x67452301;
    h1 = 0xEFCDAB89;
    h2 = 0x98BADCFE;
    h3 = 0x10325476;

    //计算消息长度
    uint8_t *msg = NULL;
    size_t new_len, offset;
    uint32_t w[16];
    int i;
    for (new_len = initial_len + 1; new_len % 64 != 56; new_len++);
    msg = (uint8_t*)calloc(new_len + 8, 1);
    memcpy(msg, initial_msg, initial_len);
    msg[initial_len] = 0x80;
    for (offset = initial_len + 1; offset < new_len; offset++);
    uint64_t bits_len = 8 * initial_len;
    memcpy(msg + offset, &bits_len, 8);

    //主循环
    for (offset = 0; offset < new_len; offset += (512 / 8)) {
        //初始化 a, b, c, d
        uint32_t a, b, c, d;
        a = h0;
        b = h1;
        c = h2;
        d = h3;

        //将消息分块，每块512位
        for (i = 0; i < 16; i++) {
            w[i] = ((uint32_t)msg[offset + i * 4]) |
                (((uint32_t)msg[offset + i * 4 + 1]) << 8) |
                (((uint32_t)msg[offset + i * 4 + 2]) << 16) |
                (((uint32_t)msg[offset + i * 4 + 3]) << 24);
        }

        //计算四轮循环
        uint32_t f, g;
        for (i = 0; i < 64; i++) {
            if (i < 16) {
                f = (b & c) | ((~b) & d);
                g = i;
            }
            else if (i < 32) {
                f = (d & b) | ((~d) & c);
                g = (5 * i + 1) % 16;
            }
            else if (i < 48) {
                f = b ^ c ^ d;
                g = (3 * i + 5) % 16;
            }
            else {
                f = c ^ (b | (~d));
                g = (7 * i) % 16;
            }

            uint32_t tmp = d;
            d = c;
            c = b;
            b = b + ROTLEFT((a + f + w[g] + 0x5A827999), 5);
            a = tmp;
        }

        //更新哈希值
        h0 += a;
        h1 += b;
        h2 += c;
        h3 += d;
    }

    //拼接哈希值
    memcpy(digest, &h0, sizeof(h0));
    memcpy(digest + sizeof(h0), &h1, sizeof(h1));
    memcpy(digest + sizeof(h0) + sizeof(h1), &h2, sizeof(h2));
    memcpy(digest + sizeof(h0) + sizeof(h1) + sizeof(h2), &h3, sizeof(h3));

    //释放内存
    free(msg);
}

int main(int argc, char **argv) {

    //测试用例
    uint8_t digest[16];
    md5((uint8_t*)"hello world", strlen("hello world"), digest);

    //输出结果
    int i;
    for (i = 0; i < 16; i++) {
        printf("%02x", digest[i]);
    }
    printf("\n");

    return 0;
}

这个程序实现了一个简单的MD5哈希函数，可以将任意长度的输入消息转换为一个128位（16字节）的哈希值。在主函数中，我们对字符串“hello world”进行哈希，然后输出哈希结果。