MD5（Message Digest Algorithm 5）是由Ronald Rivest于1991年设计的密码散列函数，可将任意长度数据转换为128位（16字节）的固定长度散列值。曾广泛应用于数据完整性校验和密码存储领域。

加密流程
A[输入数据] --> B[填充bit]
B --> C[添加长度]
C --> D[分块处理 512bit/块]
D --> E[初始化MD缓冲区]
E --> F[四轮主循环]
F --> G[输出128位散列值]

重点流程
B 数据填充部分 先补一个1，后面全部补0 填充后的位长度必须满足 L≡448(mod512) 在填充后的消息后面，再附加一个64位 的值，用来记录原始消息的长度 填充后的 448 位 + 长度 64 位 = 512 位
E 初始化的部分 这个重点要记一下
    h0 = 0x67452301;
    h1 = 0xEFCDAB89;
    h2 = 0x98BADCFE;
    h3 = 0x10325476;

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

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

// 1. 每一步对应的左移位数数组 (RFC 1321 标准)
const uint32_t S[] = {
    7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22,
    5,  9, 14, 20, 5,  9, 14, 20, 5,  9, 14, 20, 5,  9, 14, 20,
    4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23,
    6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21
};

const uint32_t T[64] = {
    0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee, 0xf57c0faf, 0x4787c62a, 0xa8304613, 0xfd469501,
    0x698098d8, 0x8b44f7af, 0xffff5bb1, 0x895cd7be, 0x6b901122, 0xfd987193, 0xa679438e, 0x49b40821,
    0xf61e2562, 0xc040b340, 0x265e5a51, 0xe9b6c7aa, 0xd62f105d, 0x02441453, 0xd8a1e681, 0xe7d3fbc8,
    0x21e1cde6, 0xc33707d6, 0xf4d50d87, 0x455a14ed, 0xa9e3e905, 0xfcefa3f8, 0x676f02d9, 0x8d2a4c8a,
    0xfffa3942, 0x8771f681, 0x6d9d6122, 0xfde5380c, 0xa4beea44, 0x4bdecfa9, 0xf6bb4b60, 0xbebfbc70,
    0x289b7ec6, 0xeaa127fa, 0xd4ef3085, 0x04881d05, 0xd9d4d039, 0xe6db99e5, 0x1fa27cf8, 0xc4ac5665,
    0xf4292244, 0x432aff97, 0xab9423a7, 0xfc93a039, 0x655b59c3, 0x8f0ccc92, 0xffeff47d, 0x85845dd1,
    0x6fa87e4f, 0xfe2ce6e0, 0xa3014314, 0x4e0811a1, 0xf7537e82, 0xbd3af235, 0x2ad7d2bb, 0xeb86d391
};

void md5(const uint8_t *initial_msg, size_t initial_len, uint8_t *digest) {
    uint32_t h0 = 0x67452301, h1 = 0xefcdab89, h2 = 0x98badcfe, h3 = 0x10325476;
    
    // 2. 修正填充逻辑
    size_t new_len = (((initial_len + 8) / 64) + 1) * 64;
    uint8_t *msg = (uint8_t*)calloc(1, new_len); // 使用 calloc 自动清零
    memcpy(msg, initial_msg, initial_len);
    msg[initial_len] = 0x80;

    uint64_t bits_len = (uint64_t)initial_len * 8;
    memcpy(msg + new_len - 8, &bits_len, 8);

    for (size_t offset = 0; offset < new_len; offset += 64) {
        uint32_t *w = (uint32_t *)(msg + offset);
        uint32_t a = h0, b = h1, c = h2, d = h3;

        for (uint32_t i = 0; i < 64; i++) {
            uint32_t f, g;
            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;
            }

            // 3. 修正核心步进逻辑
            uint32_t temp = d;
            d = c;
            c = b;
            // 核心计算公式：b = b + rotate_left(a + f + T[i] + w[g], S[i])
            b = b + LEFT_ROTATE((a + f + T[i] + w[g]), S[i]); 
            a = temp;
        }
        h0 += a; h1 += b; h2 += c; h3 += d;
    }
    
    free(msg);
    memcpy(digest, &h0, 4);
    memcpy(digest + 4, &h1, 4);
    memcpy(digest + 8, &h2, 4);
    memcpy(digest + 12, &h3, 4);
}

int main() {
    // 测试 "Hello MD5!"
    // 结果应为: 2955f190623a8e9e1c3a64f4340d1656
    char *msg = "Hello MD5!";
    uint8_t digest[16];
    
    md5((uint8_t*)msg, strlen(msg), digest);
    
    printf("MD5: ");
    for (int i = 0; i < 16; i++) printf("%02x", digest[i]);
    printf("\n");
    
    return 0;
}