#include <string.h>

#include "md5.h"

#define F(x, y, z) ((x & y) | (~x & z))
#define G(x, y, z) ((x & z) | (y & ~z))
#define H(x, y, z) (x ^ y ^ z)
#define I(x, y, z) (y ^ (x | ~z))

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

#define FF(a, b, c, d, x, s, ac)  \
    {                             \
        a += F(b, c, d) + x + ac; \
        a = ROTATE_LEFT(a, s);    \
        a += b;                   \
    }

#define GG(a, b, c, d, x, s, ac)  \
    {                             \
        a += G(b, c, d) + x + ac; \
        a = ROTATE_LEFT(a, s);    \
        a += b;                   \
    }

#define HH(a, b, c, d, x, s, ac)  \
    {                             \
        a += H(b, c, d) + x + ac; \
        a = ROTATE_LEFT(a, s);    \
        a += b;                   \
    }

#define II(a, b, c, d, x, s, ac)  \
    {                             \
        a += I(b, c, d) + x + ac; \
        a = ROTATE_LEFT(a, s);    \
        a += b;                   \
    }

typedef struct
{
    unsigned int count[2];
    unsigned int state[4];
    unsigned char buffer[64];
} md5_ctx_t;

static md5_ctx_t md5;

static unsigned char g_md5_padding[] = {
    0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0x00, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0x00, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0x00, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

static unsigned int x[64] = {0};


static void md5_encode(unsigned char *output, unsigned int *input, unsigned int len)
{
    unsigned int i = 0, j = 0;

    while (j < len)
    {
        output[j] = input[i] & 0xFF;
        output[j + 1] = (input[i] >> 8) & 0xFF;
        output[j + 2] = (input[i] >> 16) & 0xFF;
        output[j + 3] = (input[i] >> 24) & 0xFF;
        i ++;
        j += 4;
    }
}

static void md5_decode(unsigned int *output, unsigned char *input, unsigned int len)
{
    unsigned int i = 0, j = 0;

    while (j < len)
    {
        output[i] = (input[j]) |
                    (input[j + 1] << 8) |
                    (input[j + 2] << 16) |
                    (input[j + 3] << 24);
        i++;
        j += 4;
    }
}

static void md5_transform(unsigned int state[4], unsigned char block[64])
{
    unsigned int a = state[0];
    unsigned int b = state[1];
    unsigned int c = state[2];
    unsigned int d = state[3];

    md5_decode(x, block, 64);
    FF(a, b, c, d, x[0],   7, 0xd76aa478);  /* 1 */
    FF(d, a, b, c, x[1],  12, 0xe8c7b756);  /* 2 */
    FF(c, d, a, b, x[2],  17, 0x242070db);  /* 3 */
    FF(b, c, d, a, x[3],  22, 0xc1bdceee);  /* 4 */
    FF(a, b, c, d, x[4],   7, 0xf57c0faf);  /* 5 */
    FF(d, a, b, c, x[5],  12, 0x4787c62a);  /* 6 */
    FF(c, d, a, b, x[6],  17, 0xa8304613);  /* 7 */
    FF(b, c, d, a, x[7],  22, 0xfd469501);  /* 8 */
    FF(a, b, c, d, x[8],   7, 0x698098d8);  /* 9 */
    FF(d, a, b, c, x[9],  12, 0x8b44f7af);  /* 10 */
    FF(c, d, a, b, x[10], 17, 0xffff5bb1);  /* 11 */
    FF(b, c, d, a, x[11], 22, 0x895cd7be);  /* 12 */
    FF(a, b, c, d, x[12],  7, 0x6b901122);  /* 13 */
    FF(d, a, b, c, x[13], 12, 0xfd987193);  /* 14 */
    FF(c, d, a, b, x[14], 17, 0xa679438e);  /* 15 */
    FF(b, c, d, a, x[15], 22, 0x49b40821);  /* 16 */
 
    /* Round 2 */
    GG(a, b, c, d, x[1],   5, 0xf61e2562);  /* 17 */
    GG(d, a, b, c, x[6],   9, 0xc040b340);  /* 18 */
    GG(c, d, a, b, x[11], 14, 0x265e5a51);  /* 19 */
    GG(b, c, d, a, x[0],  20, 0xe9b6c7aa);  /* 20 */
    GG(a, b, c, d, x[5],   5, 0xd62f105d);  /* 21 */
    GG(d, a, b, c, x[10],  9, 0x2441453);   /* 22 */
    GG(c, d, a, b, x[15], 14, 0xd8a1e681);  /* 23 */
    GG(b, c, d, a, x[4],  20, 0xe7d3fbc8);  /* 24 */
    GG(a, b, c, d, x[9],   5, 0x21e1cde6);  /* 25 */
    GG(d, a, b, c, x[14],  9, 0xc33707d6);  /* 26 */
    GG(c, d, a, b, x[3],  14, 0xf4d50d87);  /* 27 */
    GG(b, c, d, a, x[8],  20, 0x455a14ed);  /* 28 */
    GG(a, b, c, d, x[13],  5, 0xa9e3e905);  /* 29 */
    GG(d, a, b, c, x[2],   9, 0xfcefa3f8);  /* 30 */
    GG(c, d, a, b, x[7],  14, 0x676f02d9);  /* 31 */
    GG(b, c, d, a, x[12], 20, 0x8d2a4c8a);  /* 32 */

    /* Round 3 */
    HH(a, b, c, d, x[5],   4, 0xfffa3942);  /* 33 */
    HH(d, a, b, c, x[8],  11, 0x8771f681);  /* 34 */
    HH(c, d, a, b, x[11], 16, 0x6d9d6122);  /* 35 */
    HH(b, c, d, a, x[14], 23, 0xfde5380c);  /* 36 */
    HH(a, b, c, d, x[1],   4, 0xa4beea44);  /* 37 */
    HH(d, a, b, c, x[4],  11, 0x4bdecfa9);  /* 38 */
    HH(c, d, a, b, x[7],  16, 0xf6bb4b60);  /* 39 */
    HH(b, c, d, a, x[10], 23, 0xbebfbc70);  /* 40 */
    HH(a, b, c, d, x[13],  4, 0x289b7ec6);  /* 41 */
    HH(d, a, b, c, x[0],  11, 0xeaa127fa);  /* 42 */
    HH(c, d, a, b, x[3],  16, 0xd4ef3085);  /* 43 */
    HH(b, c, d, a, x[6],  23, 0x4881d05);   /* 44 */
    HH(a, b, c, d, x[9],   4, 0xd9d4d039);  /* 45 */
    HH(d, a, b, c, x[12], 11, 0xe6db99e5);  /* 46 */
    HH(c, d, a, b, x[15], 16, 0x1fa27cf8);  /* 47 */
    HH(b, c, d, a, x[2],  23, 0xc4ac5665);  /* 48 */

    /* Round 4 */
    II(a, b, c, d, x[0],   6, 0xf4292244);   /* 49 */
    II(d, a, b, c, x[7],  10, 0x432aff97);   /* 50 */
    II(c, d, a, b, x[14], 15, 0xab9423a7);   /* 51 */
    II(b, c, d, a, x[5],  21, 0xfc93a039);   /* 52 */
    II(a, b, c, d, x[12],  6, 0x655b59c3);   /* 53 */
    II(d, a, b, c, x[3],  10, 0x8f0ccc92);   /* 54 */
    II(c, d, a, b, x[10], 15, 0xffeff47d);   /* 55 */
    II(b, c, d, a, x[1],  21, 0x85845dd1);   /* 56 */
    II(a, b, c, d, x[8],   6, 0x6fa87e4f);   /* 57 */
    II(d, a, b, c, x[15], 10, 0xfe2ce6e0);   /* 58 */
    II(c, d, a, b, x[6],  15, 0xa3014314);   /* 59 */
    II(b, c, d, a, x[13], 21, 0x4e0811a1);   /* 60 */
    II(a, b, c, d, x[4],   6, 0xf7537e82);   /* 61 */
    II(d, a, b, c, x[11], 10, 0xbd3af235);   /* 62 */
    II(c, d, a, b, x[2],  15, 0x2ad7d2bb);   /* 63 */
    II(b, c, d, a, x[9],  21, 0xeb86d391);   /* 64 */
    state[0] += a;
    state[1] += b;
    state[2] += c;
    state[3] += d;
}

static void md5_init(md5_ctx_t *context)
{
    context->count[0] = 0;
    context->count[1] = 0;
    context->state[0] = 0x67452301;
    context->state[1] = 0xEFCDAB89;
    context->state[2] = 0x98BADCFE;
    context->state[3] = 0x10325476;
}

static void md5_update(md5_ctx_t *context, unsigned char *input, unsigned int inputlen)
{
    unsigned int i = 0, index = 0, partlen = 0;

    index = (context->count[0] >> 3) & 0x3F;
    partlen = 64 - index;
    context->count[0] += inputlen << 3;
    if (context->count[0] < (inputlen << 3))
        context->count[1]++;
    context->count[1] += inputlen >> 29;

    if (inputlen >= partlen)
    {
        memcpy(&context->buffer[index], input, partlen);
        md5_transform(context->state, context->buffer);
        for (i = partlen; i + 64 <= inputlen; i += 64)
            md5_transform(context->state, &input[i]);
        index = 0;
    }
    else
    {
        i = 0;
    }
    memcpy(&context->buffer[index], &input[i], inputlen - i);
}

static void md5_start(md5_ctx_t *context, unsigned char *input, unsigned int len)
{
    md5_update(context, input, len);
}

static void md5_process(md5_ctx_t *context)
{
    unsigned int index = 0, padlen = 0;
    unsigned char bits[8];

    index = (context->count[0] >> 3) & 0x3F;
    padlen = (index < 56) ? (56 - index) : (120 - index);
    md5_encode(bits, context->count, 8);
    md5_update(context, g_md5_padding, padlen);
    md5_update(context, bits, 8);
}

static void md5_final(md5_ctx_t *context, unsigned char *digest)
{
    md5_encode(digest, context->state, 16);
}

void md5_sign(unsigned char *input, unsigned int input_len, unsigned char *output)
{
    md5_init(&md5);

    md5_start(&md5, input, input_len);
    md5_process(&md5);
    md5_final(&md5, output);
}

#ifndef _MD5_H
#define _MD5_H

void md5_sign(unsigned char *input, unsigned int input_len, unsigned char *output);

#endif

使用例子伪代码

char buff[16];
md5_sign("xjq",3,buff);