MD5.h

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cstdlib>
#include<iostream>
#include<cmath>
typedef struct{
    unsigned int count[2];
    unsigned int state[4];
    unsigned char buffer[64];
}MD5_CTX;

#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;}

void MD5Init(MD5_CTX *context);
void MD5Update(MD5_CTX *context, unsigned char *input, unsigned int inputlen);
void MD5Final(MD5_CTX *context, unsigned char digest[16]);
void MD5Transform(unsigned int state[4], unsigned char block[64]);
void MD5Encode(unsigned char *output, unsigned int *input, unsigned int len);
void MD5Decode(unsigned int *output, unsigned char *input, unsigned int len);

unsigned char PADDING[] = {
    0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
void MD5Init(MD5_CTX *context){
    context->count[0]=0;
    context->count[1]=0;
    context->state[0]=0x67452301;//链接变量，注意与大端字节序的区别，内存中是小端字节序
    context->state[1]=0xEFCDAB89;
    context->state[2]=0x98BADCFE;
    context->state[3]=0x10325476;
}

void MD5Update(MD5_CTX *context, unsigned char *input, unsigned int inputlen){
    unsigned int i=0,index=0,partlen=0;
    index=(context->count[0]>>3)&0x3F;//模64字节的余数，第一次调用为0
    partlen=64-index;//第一次partlen为64
    //预留最前面的64位存放数据长度
    context->count[0]+=inputlen<<3;//文本总的位数,低32位
    if(context->count[0]<(inputlen<<3)) context->count[1]++;
    context->count[1]+=inputlen>>29;//先左移3,求出位数,再右移32位，求出左32位

    if(inputlen>=partlen){
        memcpy(&context->buffer[index], input, partlen);
        MD5Transform(context->state, context->buffer);
        //512位为1组
        for(i=partlen;i+64<=inputlen;i+=64) MD5Transform(context->state,&input[i]);
        index=0;
    }
    else i=0;
    memcpy(&context->buffer[index], &input[i], inputlen-i);//最后剩下的一部分
}

//32位md5
void MD5Final(MD5_CTX *context, unsigned char digest[16]){
    unsigned int index=0,padlen=0;
    unsigned char bits[8];
    index=(context->count[0]>>3)&0x3F;//模64字节的余数
    padlen=(index<56)?(56-index):(120-index);//需要填充的长度，注意长度占8字节
    MD5Encode(bits,context->count,8);
    MD5Update(context,PADDING,padlen);
    MD5Update(context,bits,8);//这里调用时,index=0
    MD5Encode(digest,context->state,16);
}

void MD5Encode(unsigned char *output, unsigned int *input, unsigned int len){
    unsigned int i = 0, j = 0;//len是char的长度
    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;
    }
}

void MD5Decode(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;
    }
}

void MD5Transform(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];
    unsigned int x[16];

    MD5Decode(x, block, 64);
    FF(a, b, c, d, x[0], 7, 0xd76aa478);//32位运算
    FF(d, a, b, c, x[1], 12, 0xe8c7b756);
    FF(c, d, a, b, x[2], 17, 0x242070db);
    FF(b, c, d, a, x[3], 22, 0xc1bdceee);
    FF(a, b, c, d, x[4], 7, 0xf57c0faf);
    FF(d, a, b, c, x[5], 12, 0x4787c62a);
    FF(c, d, a, b, x[6], 17, 0xa8304613);
    FF(b, c, d, a, x[7], 22, 0xfd469501);
    FF(a, b, c, d, x[8], 7, 0x698098d8);
    FF(d, a, b, c, x[9], 12, 0x8b44f7af);
    FF(c, d, a, b, x[10], 17, 0xffff5bb1);
    FF(b, c, d, a, x[11], 22, 0x895cd7be);
    FF(a, b, c, d, x[12], 7, 0x6b901122);
    FF(d, a, b, c, x[13], 12, 0xfd987193);
    FF(c, d, a, b, x[14], 17, 0xa679438e);
    FF(b, c, d, a, x[15], 22, 0x49b40821);

    GG(a, b, c, d, x[1], 5, 0xf61e2562);
    GG(d, a, b, c, x[6], 9, 0xc040b340);
    GG(c, d, a, b, x[11], 14, 0x265e5a51);
    GG(b, c, d, a, x[0], 20, 0xe9b6c7aa);
    GG(a, b, c, d, x[5], 5, 0xd62f105d);
    GG(d, a, b, c, x[10], 9, 0x2441453);
    GG(c, d, a, b, x[15], 14, 0xd8a1e681);
    GG(b, c, d, a, x[4], 20, 0xe7d3fbc8);
    GG(a, b, c, d, x[9], 5, 0x21e1cde6);
    GG(d, a, b, c, x[14], 9, 0xc33707d6);
    GG(c, d, a, b, x[3], 14, 0xf4d50d87);
    GG(b, c, d, a, x[8], 20, 0x455a14ed);
    GG(a, b, c, d, x[13], 5, 0xa9e3e905);
    GG(d, a, b, c, x[2], 9, 0xfcefa3f8);
    GG(c, d, a, b, x[7], 14, 0x676f02d9);
    GG(b, c, d, a, x[12], 20, 0x8d2a4c8a);

    HH(a, b, c, d, x[5], 4, 0xfffa3942);
    HH(d, a, b, c, x[8], 11, 0x8771f681);
    HH(c, d, a, b, x[11], 16, 0x6d9d6122);
    HH(b, c, d, a, x[14], 23, 0xfde5380c);
    HH(a, b, c, d, x[1], 4, 0xa4beea44);
    HH(d, a, b, c, x[4], 11, 0x4bdecfa9);
    HH(c, d, a, b, x[7], 16, 0xf6bb4b60);
    HH(b, c, d, a, x[10], 23, 0xbebfbc70);
    HH(a, b, c, d, x[13], 4, 0x289b7ec6);
    HH(d, a, b, c, x[0], 11, 0xeaa127fa);
    HH(c, d, a, b, x[3], 16, 0xd4ef3085);
    HH(b, c, d, a, x[6], 23, 0x4881d05);
    HH(a, b, c, d, x[9], 4, 0xd9d4d039);
    HH(d, a, b, c, x[12], 11, 0xe6db99e5);
    HH(c, d, a, b, x[15], 16, 0x1fa27cf8);
    HH(b, c, d, a, x[2], 23, 0xc4ac5665);

    II(a, b, c, d, x[0], 6, 0xf4292244);
    II(d, a, b, c, x[7], 10, 0x432aff97);
    II(c, d, a, b, x[14], 15, 0xab9423a7);
    II(b, c, d, a, x[5], 21, 0xfc93a039);
    II(a, b, c, d, x[12], 6, 0x655b59c3);
    II(d, a, b, c, x[3], 10, 0x8f0ccc92);
    II(c, d, a, b, x[10], 15, 0xffeff47d);
    II(b, c, d, a, x[1], 21, 0x85845dd1);
    II(a, b, c, d, x[8], 6, 0x6fa87e4f);
    II(d, a, b, c, x[15], 10, 0xfe2ce6e0);
    II(c, d, a, b, x[6], 15, 0xa3014314);
    II(b, c, d, a, x[13], 21, 0x4e0811a1);
    II(a, b, c, d, x[4], 6, 0xf7537e82);
    II(d, a, b, c, x[11], 10, 0xbd3af235);
    II(c, d, a, b, x[2], 15, 0x2ad7d2bb);
    II(b, c, d, a, x[9], 21, 0xeb86d391);
    state[0] += a;
    state[1] += b;
    state[2] += c;
    state[3] += d;
}

test.cpp

#include"MD5.h"
using namespace std;
unsigned char encrypt[1024];//原文 
unsigned char decrypt[16];//md5值 
int main()
{
    MD5_CTX md5;
    gets((char *)encrypt);
    MD5Init(&md5);
    MD5Update(&md5,encrypt,(int)strlen((char *)encrypt));
    MD5Final(&md5,decrypt);
    bool bk=true;
    for(int j=0;j<16;j++){
        printf("%02x",decrypt[j]);
    }
    printf("\n");//输出 
    return 0;
}