md5哈希算法c实现

MD5是一种常用的哈希算法，它将任意长度的消息压缩成一个128位的消息摘要（或称为指纹）。下面是一个简单的C语言实现：

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

#define LEFTROTATE(x, c) (((x) << (c)) | ((x) >> (32 - (c))))

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

    uint32_t h0, h1, h2, h3;
    uint8_t *msg = NULL;

    // 初始化常量
    uint32_t k[] = {
        0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5,
        0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
        0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3,
        0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
        0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc,
        0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
        0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7,
        0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
        0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
        0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
        0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3,
        0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
        0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5,
        0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
        0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208,
        0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
    };

    // 初始化变量
    uint32_t a, b, c, d, i, j;

    // 初始化消息
    size_t new_len = ((((initial_len + 8) / 64) + 1) * 64) - 8;
    msg = calloc(new_len + 64, 1);
    memcpy(msg, initial_msg, initial_len);
    msg[initial_len] = 128;

    uint32_t bits_len = 8 * initial_len;
    memcpy(msg + new_len, &bits_len, 4);

    // 初始化哈希值
    h0 = 0x67452301;
    h1 = 0xefcdab89;
    h2 = 0x98badcfe;
    h3 = 0x10325476;

    // 处理消息
    for (i = 0; i < new_len; i += 64) {
        a = h0;
        b = h1;
        c = h2;
        d = h3;

        uint32_t *chunk = (uint32_t *)(msg + i);
        uint32_t *w = (uint32_t *)calloc(64, 1);

        for (j = 0; j < 16; j++) {
            w[j] = chunk[j];
        }

        for (j = 16; j < 64; j++) {
            uint32_t s0 = LEFTROTATE(w[j - 15], 7) ^ LEFTROTATE(w[j - 15], 18) ^ (w[j - 15] >> 3);
            uint32_t s1 = LEFTROTATE(w[j - 2], 17) ^ LEFTROTATE(w[j - 2], 19) ^ (w[j - 2] >> 10);
            w[j] = w[j - 16] + s0 + w[j - 7] + s1;
        }

        for (j = 0; j < 64; j++) {
            uint32_t s0 = LEFTROTATE(a, 12) ^ LEFTROTATE(a, 25) ^ LEFTROTATE(a, 27);
            uint32_t maj = (a & b) ^ (a & c) ^ (b & c);
            uint32_t t2 = s0 + maj;
            uint32_t s1 = LEFTROTATE(d, 5) ^ LEFTROTATE(d, 13) ^ LEFTROTATE(d, 22);
            uint32_t ch = (d & b) ^ ((~d) & c);
            uint32_t t1 = h[j] + s1 + ch + k[j] + w[j];
            d = c;
            c = b;
            b = a;
            a = t1 + t2;
        }

        h0 += a;
        h1 += b;
        h2 += c;
        h3 += d;

        free(w);
    }

    // 生成摘要
    memcpy(digest, &h0, 4);
    memcpy(digest + 4, &h1, 4);
    memcpy(digest + 8, &h2, 4);
    memcpy(digest + 12, &h3, 4);

    free(msg);
}

int main(int argc, char **argv) {
    uint8_t digest[16];
    char *msg = "hello world";
    md5((uint8_t *)msg, strlen(msg), digest);

    printf("MD5(\"%s\") = ", msg);
    for (int i = 0; i < 16; i++) {
        printf("%02x", digest[i]);
    }
    printf("\n");

    return 0;
}

这个实现使用了标准的MD5算法，包括初始化常量、初始化变量、消息处理和生成摘要等步骤。你可以将任意长度的消息传递给`md5`函数，它将返回一个16字节的摘要。