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以下代碼是自己用C++語言對MD5算法的實現，編程工具是VC2013。雖然代碼的封裝性、安全性和效率遠遠比不上RFC文檔中的標準實現或者OpenSSL的源碼（這些代碼研究了很久依舊沒看懂，它們的效率雖高，但可讀性不高，不適合于我這種基礎比較薄弱的童鞋閱讀），但算法思路比較直觀，清晰易懂，方便讓基礎比較薄弱的童鞋閱讀。本人屬于C++初學者，編寫的代碼可能不夠簡潔，不夠美觀，很多地方也沒有考慮周到，希望大家見諒，也希望能有高手能指正批評。
注意：每處理64個字符就進行一次MD5操作，MD5算法本身不算太難，主要是輸入消息的預處理過程比較麻煩。
#pragma warning(disable : 4996)//VC2013增加了安全機制，如果不加這句會出現編譯錯誤
#include<iostream>
#include<string>
#include<stdio.h>
#include<stdlib.h>
#include <bitset>
using namespace std;
typedef unsigned int UNINT;//為unsigned int重命名，方便編程
#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;
}
//將長度為64的字符數組轉換成長度為16的整型數組，即將四個字符數組單元轉換為一個整型數組
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;
}
//新建一個分組存放消息長度
void newblock_length(UNINT l){
for (int i = 0; i < 15; i++){
word_32[i] = 0;
}
//將消息長度存放在最后4個字節
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();//計算輸入消息的長度
counter_update = counter;
block_pos = 0;
//對輸入消息進行分組，每組64bytes
while (counter_update / 64>0){
str.copy(block, 64, 64 * block_pos);//將起始位置為64*j的后續64個字符拷貝到block數組
char64_to_word32(16);//將64個字符轉換成16個字
//MD5
MD5_Transform(word_32);
block_pos++;
counter_update -= 64;
}
if (counter_update % 64 == 0){
newblock_length(counter);
//MD5
MD5_Transform(word_32);
}
else{
//對剩余不足64bytes的分組進行處理
str.copy(block, counter_update, 64 * block_pos);
//如果最后一個分組剩余不足4bytes空間來存放原始消息長度，則用0將該分組填充滿，然后再新建一個分組存放消息長度
if (counter_update > 60){
//將block數組前面4k個字符轉換成k個整型數
word_pos = char64_to_word32(counter_update / 4);
//更新block最后剩余的字符數
counter_update -= 4 * word_pos;
//根據剩余字符數模4的不同結果進行不同的處理
switch (counter_update){
case 0:{
break;
}
//如果最后剩余一個字符，則將其左移到最高8位，后面24位補0
case 1:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ 0x000000;
word_pos++;
break;
}
//如果最后剩余兩個字符，則將其左移到最高16位，后面16位補0
case 2:{
word_32[word_pos] = (block[4 * word_pos] << 24) ^ (block[4 * word_pos + 1] << 16) ^ 0x0000;
word_pos++;
break;
}
//如果最后剩余三個字符，則將其左移到最高24位，后面8位補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;
}
}
//對word_36數組的剩余單元填充
while (word_pos < 16){
word_32[word_pos] = 0;
word_pos++;
}
//MD5
MD5_Transform(word_32);
//新建一個數組存放消息長度
newblock_length(counter);
//MD5
MD5_Transform(word_32);
}
//如果最后一個分組剩余超過4bytes的空間，則用最后4bytes存放消息長度，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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