以下代码是自己用C++语言对MD5算法的实现，编程工具是VC2013。虽然代码的封装性、安全性和效率远远比不上RFC文档中的标准实现或者OpenSSL的源码（这些代码研究了很久依旧没看懂，它们的效率虽高，但可读性不高，不适合于我这种基础比较薄弱的童鞋阅读），但算法思路比较直观，清晰易懂，方便让基础比较薄弱的童鞋阅读。本人属于C++初学者，编写的代码可能不够简洁，不够美观，很多地方也没有考虑周到，希望大家见谅，也希望能有高手能指正批评。
注意：每处理64个字符就进行一次MD5操作，MD5算法本身不算太难，主要是输入消息的预处理过程比较麻烦。
#pragma warning(disable : 4996)//VC2013增加了安全机制，如果不加这句会出现编译错误
#include<iostream>
#include<string>
#include<stdio.h>
#include<stdlib.h>
#include <bitset>
using namespace std;
typedef unsigned int UNINT;//为unsigned int重命名，方便编程
#define S11 7
#define S12 12
#define S13 17
#define S14 22
#define S21 5
#define S22 9
#define S23 14
#define S24 20
#define S31 4
#define S32 11
#define S33 16
#define S34 23
#define S41 6
#define S42 10
#define S43 15
#define S44 21
UNINT hash_state[4];
UNINT word_32[16];
char block[64];
//初始化向量（A,B,C,D）
void Init()
{
hash_state[0] = 0x67452301;
hash_state[1] = 0xefcdab89;
hash_state[2] = 0x98badcfe;
hash_state[3] = 0x10325476;
}
//将长度为64的字符数组转换成长度为16的整型数组，即将四个字符数组单元转换为一个整型数组
int char64_to_word32(int n){
int i;
for (i = 0; i < n; i++){
word_32[i] = (block[4 * i + 0] << 24) ^ (block[4 * i + 1] << 16) ^ (block[4 * i + 2] << 8) ^ block[4 * i + 3];
}
return i;
}
//新建一个分组存放消息长度
void newblock_length(UNINT l){
for (int i = 0; i < 15; i++){
word_32[i] = 0;
}
//将消息长度存放在最后4个字节
word_32[15] = l;
}
UNINT rotate_left(UNINT x, UNINT n)
{
return ((x << n) | (x >> (32 - n)));
}
UNINT F(UNINT x, UNINT y, UNINT z)
{
return ((x & y) | (~x & z));
}
UNINT G(UNINT x, UNINT y, UNINT z)
{
return ((x & z) | (y & ~z));
}
UNINT H(UNINT x, UNINT y, UNINT z)
{
return (x ^ y ^ z);
}
UNINT I(UNINT x, UNINT y, UNINT z)
{
return (y ^ (x | ~z));
}
void FF(UNINT* a, UNINT b, UNINT c, UNINT d, UNINT x, UNINT s, UNINT ac)
{
*a += F(b, c, d) + x + ac;
*a = rotate_left(*a, s);
*a += b;
}
void GG(UNINT* a, UNINT b, UNINT c, UNINT d, UNINT x, UNINT s, UNINT ac)
{
*a += G(b, c, d) + x + ac;
*a = rotate_left(*a, s);
*a += b;
}
void HH(UNINT* a, UNINT b, UNINT c, UNINT d, UNINT x, UNINT s, UNINT ac)
{
*a += H(b, c, d) + x + ac;
*a = rotate_left(*a, s);
*a += b;
}
void II(UNINT* a, UNINT b, UNINT c, UNINT d, UNINT x, UNINT s, UNINT ac)
{
*a += I(b, c, d) + x + ac;
*a = rotate_left(*a, s);
*a += b;
}
void MD5_Transform(UNINT* x)
{
UNINT a, b, c, d;
a = hash_state[0];
b = hash_state[1];
c = hash_state[2];
d = hash_state[3];
// Round 1
FF(&a, b, c, d, x[0], S11, 0xd76aa478);
FF(&d, a, b, c, x[1], S12, 0xe8c7b756);
FF(&c, d, a, b, x[2], S13, 0x242070db);
FF(&b, c, d, a, x[3], S14, 0xc1bdceee);
FF(&a, b, c, d, x[4], S11, 0xf57c0faf);
FF(&d, a, b, c, x[5], S12, 0x4787c62a);
FF(&c, d, a, b, x[6], S13, 0xa8304613);
FF(&b, c, d, a, x[7], S14, 0xfd469501);
FF(&a, b, c, d, x[8], S11, 0x698098d8);
FF(&d, a, b, c, x[9], S12, 0x8b44f7af);
FF(&c, d, a, b, x[10], S13, 0xffff5bb1);
FF(&b, c, d, a, x[11], S14, 0x895cd7be);
FF(&a, b, c, d, x[12], S11, 0x6b901122);
FF(&d, a, b, c, x[13], S12, 0xfd987193);
FF(&c, d, a, b, x[14], S13, 0xa679438e);
FF(&b, c, d, a, x[15], S14, 0x49b40821);
// Round 2
GG(&a, b, c, d, x[1], S21, 0xf61e2562);
GG(&d, a, b, c, x[6], S22, 0xc040b340);
GG(&c, d, a, b, x[11], S23, 0x265e5a51);
GG(&b, c, d, a, x[0], S24, 0xe9b6c7aa);
GG(&a, b, c, d, x[5], S21, 0xd62f105d);
GG(&d, a, b, c, x[10], S22, 0x2441453);
GG(&c, d, a, b, x[15], S23, 0xd8a1e681);
GG(&b, c, d, a, x[4], S24, 0xe7d3fbc8);
GG(&a, b, c, d, x[9], S21, 0x21e1cde6);
GG(&d, a, b, c, x[14], S22, 0xc33707d6);
GG(&c, d, a, b, x[3], S23, 0xf4d50d87);
GG(&b, c, d, a, x[8], S24, 0x455a14ed);
GG(&a, b, c, d, x[13], S21, 0xa9e3e905);
GG(&d, a, b, c, x[2], S22, 0xfcefa3f8);
GG(&c, d, a, b, x[7], S23, 0x676f02d9);
GG(&b, c, d, a, x[12], S24, 0x8d2a4c8a);
// Round 3
HH(&a, b, c, d, x[5], S31, 0xfffa3942);
HH(&d, a, b, c, x[8], S32, 0x8771f681);
HH(&c, d, a, b, x[11], S33, 0x6d9d6122);
HH(&b, c, d, a, x[14], S34, 0xfde5380c);
HH(&a, b, c, d, x[1], S31, 0xa4beea44);
HH(&d, a, b, c, x[4], S32, 0x4bdecfa9);
HH(&c, d, a, b, x[7], S33, 0xf6bb4b60);
HH(&b, c, d, a, x[10], S34, 0xbebfbc70);
HH(&a, b, c, d, x[13], S31, 0x289b7ec6);
HH(&d, a, b, c, x[0], S32, 0xeaa127fa);
HH(&c, d, a, b, x[3], S33, 0xd4ef3085);
HH(&b, c, d, a, x[6], S34, 0x4881d05);
HH(&a, b, c, d, x[9], S31, 0xd9d4d039);
HH(&d, a, b, c, x[12], S32, 0xe6db99e5);
HH(&c, d, a, b, x[15], S33, 0x1fa27cf8);
HH(&b, c, d, a, x[2], S34, 0xc4ac5665);
// Round 4
II(&a, b, c, d, x[0], S41, 0xf4292244);
II(&d, a, b, c, x[7], S42, 0x432aff97);
II(&c, d, a, b, x[14], S43, 0xab9423a7);
II(&b, c, d, a, x[5], S44, 0xfc93a039);
II(&a, b, c, d, x[12], S41, 0x655b59c3);
II(&d, a, b, c, x[3], S42, 0x8f0ccc92);
II(&c, d, a, b, x[10], S43, 0xffeff47d);
II(&b, c, d, a, x[1], S44, 0x85845dd1);
II(&a, b, c, d, x[8], S41, 0x6fa87e4f);
II(&d, a, b, c, x[15], S42, 0xfe2ce6e0);
II(&c, d, a, b, x[6], S43, 0xa3014314);
II(&b, c, d, a, x[13], S44, 0x4e0811a1);
II(&a, b, c, d, x[4], S41, 0xf7537e82);
II(&d, a, b, c, x[11], S42, 0xbd3af235);
II(&c, d, a, b, x[2], S43, 0x2ad7d2bb);
II(&b, c, d, a, x[9], S44, 0xeb86d391);
hash_state[0] += a;
hash_state[1] += b;
hash_state[2] += c;
hash_state[3] += d;
}
int main()
{
UNINT word_pos, block_pos, counter, counter_update;
string str ;
Init();//初始化向量（A,B,C,D）
cin >> str;
counter = str.length();//计算输入消息的长度
counter_update = counter;
block_pos = 0;
//对输入消息进行分组，每组64bytes
while (counter_update / 64>0){
str.copy(block, 64, 64 * block_pos);//将起始位置为64*j的后续64个字符拷贝到block数组
char64_to_word32(16);//将64个字符转换成16个字
//MD5
MD5_Transform(word_32);
block_pos++;
counter_update -= 64;
}
if (counter_update % 64 == 0){
newblock_length(counter);
//MD5
MD5_Transform(word_32);
}
else{
//对剩余不足64bytes的分组进行处理
str.copy(block, counter_update, 64 * block_pos);
//如果最后一个分组剩余不足4bytes空间来存放原始消息长度，则用0将该分组填充满，然后再新建一个分组存放消息长度
if (counter_update > 60){
//将block数组前面4k个字符转换成k个整型数
word_pos = char64_to_word32(counter_update / 4);
//更新block最后剩余的字符数
counter_update -= 4 * word_pos;
//根据剩余字符数模4的不同结果进行不同的处理
switch (counter_update){
case 0:{
break;
}
//如果最后剩余一个字符，则将其左移到最高8位，后面24位补0
case 1:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ 0x000000;
word_pos++;
break;
}
//如果最后剩余两个字符，则将其左移到最高16位，后面16位补0
case 2:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ (block[4 * word_pos + 1] << 16) ^ 0x0000;
word_pos++;
break;
}
//如果最后剩余三个字符，则将其左移到最高24位，后面8位补0
case 3:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ (block[4 * word_pos + 1] << 16) ^ (block[4 * word_pos + 2] << 8) ^ 0x00;
word_pos++;
break;
}
}
//对word_36数组的剩余单元填充
while (word_pos < 16){
word_32[word_pos] = 0;
word_pos++;
}
//MD5
MD5_Transform(word_32);
//新建一个数组存放消息长度
newblock_length(counter);
//MD5
MD5_Transform(word_32);
}
//如果最后一个分组剩余超过4bytes的空间，则用最后4bytes存放消息长度，其他位置用0填充
else{
word_pos = char64_to_word32(counter_update / 4);
counter_update -= 4 * word_pos;
switch (counter_update){
case 0:{
break;
}
case 1:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ 0x000000;
word_pos++;
break;
}
case 2:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ (block[4 * word_pos + 1] << 16) ^ 0x0000;
word_pos++;
break;
}
case 3:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ (block[4 * word_pos + 1] << 16) ^ (block[4 * word_pos + 2] << 8) ^ 0x00;
word_pos++;
break;
}
}
while (word_pos < 15){
word_32[word_pos] = 0;
word_pos++;
}
word_32[15] = counter;
//MD5
MD5_Transform(word_32);
}
}
cout << hex << hash_state[0] << endl;
cout << hex << hash_state[1] << endl;
cout << hex << hash_state[2] << endl;
cout << hex << hash_state[3] << endl;
system("pause");
return 0;
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