MD5即Message-Digest Algorithm 5（信息-摘要算法5），用于确保信息传输完整一致。是计算机广泛使用的杂凑算法之一（又译摘要算法、哈希算法），主流编程语言普遍已有MD5实现。
将数据（如汉字）运算为另一固定长度值，是杂凑算法的基础原理，MD5的前身有MD2、MD3和MD4。
MD5的作用是让大容量信息在用数字签名软件签署私人密钥前被"压缩"成一种保密的格式（就是把一个任意长度的字节串变换成一定长的十六进制数字串）。
对MD5算法简要的叙述可以为：MD5以512位分组来处理输入的信息，且每一分组又被划分为16个32位子分组，经过了一系列的处理后，算法的输出由四个32位分组组成，将这四个32位分组级联后将生成一个128位散列值。
在MD5算法中，首先需要对信息进行填充，使其位长对512求余的结果等于448。因此，信息的位长（Bits Length）将被扩展至N*512+448，N为一个非负整数，N可以是零。填充的方法如下，在信息的后面填充一个1和无数个0，直到满足上面的条件时才停止用0对信息的填充。然后，在这个结果后面附加一个以64位二进制表示的填充前信息长度。经过这两步的处理，信息的位长=N*512+448+64=(N+1）*512，即长度恰好是512的整数倍。这样做的原因是为满足后面处理中对信息长度的要求。总体流程如下图所示，表示第i个分组，每次的运算都由前一轮的128位结果值和第i块512bit值进行运算。初始的128位值为初试链接变量，这些参数用于第一轮的运算，以大端字节序来表示，他们分别为：A=0x01234567，B=0x89ABCDEF，C=0xFEDCBA98，D=0x76543210。
C++代码实现
1. 《MD5.h》
#pragma once /* POINTER defines a generic pointer type */
typedef unsigned char *POINTER; /* UINT2 defines a two byte word */
typedef unsigned short int UINT2; /* UINT4 defines a four byte word */
typedef unsigned long int UINT4; class cMd5
{
private:
unsigned int m_Content[16];
unsigned int m_ContentLen;
unsigned int m_TotalLen;
static unsigned int m_T[64];
unsigned int m_A, m_B, m_C, m_D; private:
inline void FF(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s);
inline void GG(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s);
inline void HH(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s);
inline void II(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s);
inline unsigned int roateLeft(unsigned int a, unsigned int c);
void transform(); /* MD5 context. */
typedef struct _MD5_CTX{
UINT4 state[4]; /* state (ABCD) */
UINT4 count[2]; /* bit ,2^64(Low before) */
unsigned char buffer[64]; /* input buffer */
} MD5_CTX; void MD5Init (MD5_CTX *);
void MD5Update (MD5_CTX *, unsigned char *, unsigned int);
void MD5Final (unsigned char [16], MD5_CTX *);
void MD5String(char *string, long buflen, char digest[]); static void MD5Transform (UINT4 [4], unsigned char [64]);
static void Encode (unsigned char *, UINT4 *, unsigned int);
static void Decode (UINT4 *, unsigned char *, unsigned int);
static void MD5_memcpy (POINTER, POINTER, unsigned int);
static void MD5_memset (POINTER, int, unsigned int); public:
cMd5();
virtual ~cMd5();
//initialize
void beginCal();
//Show that no longer input data, will add a filling
void endCal();
//Is initialized, input data, in endCal call before, can continue to add data
void addData(unsigned char* data, unsigned int size);
//End after operation, get the code
unsigned char* getCode();
void clearMemoryHelper(char* p);
};
2. 《MD5.CPP》
#include "stdafx.h"
#include "Md5.h"
#include <string.h>
#include <stdio.h> unsigned int cMd5::m_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
}; cMd5::cMd5()
{ }
cMd5::~cMd5()
{ } void cMd5::FF(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s)
{
a = b + roateLeft(a + ((b & c) | ((~b) & d)) + m_Content[k] + m_T[i], s);
}
void cMd5::GG(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s)
{
a = b + roateLeft(a + ((b & d) | (c & (~d))) + m_Content[k] + m_T[i], s);
}
void cMd5::HH(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s)
{
a = b + roateLeft(a + (b ^ c ^ d) + m_Content[k] + m_T[i], s);
}
void cMd5::II(unsigned int& a, unsigned int b, unsigned int c, unsigned int d, int k, int i, int s)
{
a = b + roateLeft(a + (c ^ (b | (~d))) + m_Content[k] + m_T[i], s);
}
unsigned int cMd5::roateLeft(unsigned int a, unsigned int c)
{
return ( (a << c) | (a >> (32-c)) );
}
void cMd5::transform()
{
unsigned int AA, BB, CC, DD;
AA = m_A;
BB = m_B;
CC = m_C;
DD = m_D;
//1 round
FF(AA, BB, CC, DD, 0, 0, 7);
FF(DD, AA, BB, CC, 1, 1, 12);
FF(CC, DD, AA, BB, 2, 2, 17);
FF(BB, CC, DD, AA, 3, 3, 22);
FF(AA, BB, CC, DD, 4, 4, 7);
FF(DD, AA, BB, CC, 5, 5, 12);
FF(CC, DD, AA, BB, 6, 6, 17);
FF(BB, CC, DD, AA, 7, 7, 22);
FF(AA, BB, CC, DD, 8, 8, 7);
FF(DD, AA, BB, CC, 9, 9, 12);
FF(CC, DD, AA, BB, 10, 10, 17);
FF(BB, CC, DD, AA, 11, 11, 22);
FF(AA, BB, CC, DD, 12, 12, 7);
FF(DD, AA, BB, CC, 13, 13, 12);
FF(CC, DD, AA, BB, 14, 14, 17);
FF(BB, CC, DD, AA, 15, 15, 22);
//2 round
GG(AA, BB, CC, DD, 1, 16, 5);
GG(DD, AA, BB, CC, 6, 17, 9);
GG(CC, DD, AA, BB, 11, 18, 14);
GG(BB, CC, DD, AA, 0, 19, 20);
GG(AA, BB, CC, DD, 5, 20, 5);
GG(DD, AA, BB, CC, 10, 21, 9);
GG(CC, DD, AA, BB, 15, 22, 14);
GG(BB, CC, DD, AA, 4, 23, 20);
GG(AA, BB, CC, DD, 9, 24, 5);
GG(DD, AA, BB, CC, 14, 25, 9);
GG(CC, DD, AA, BB, 3, 26, 14);
GG(BB, CC, DD, AA, 8, 27, 20);
GG(AA, BB, CC, DD, 13, 28, 5);
GG(DD, AA, BB, CC, 2, 29, 9);
GG(CC, DD, AA, BB, 7, 30, 14);
GG(BB, CC, DD, AA, 12, 31, 20);
//3 round
HH(AA, BB, CC, DD, 5, 32, 4);
HH(DD, AA, BB, CC, 8, 33, 11);
HH(CC, DD, AA, BB, 11, 34, 16);
HH(BB, CC, DD, AA, 14, 35, 23);
HH(AA, BB, CC, DD, 1, 36, 4);
HH(DD, AA, BB, CC, 4, 37, 11);
HH(CC, DD, AA, BB, 7, 38, 16);
HH(BB, CC, DD, AA, 10, 39, 23);
HH(AA, BB, CC, DD, 13, 40, 4);
HH(DD, AA, BB, CC, 0, 41, 11);
HH(CC, DD, AA, BB, 3, 42, 16);
HH(BB, CC, DD, AA, 6, 43, 23);
HH(AA, BB, CC, DD, 9, 44, 4);
HH(DD, AA, BB, CC, 12, 45, 11);
HH(CC, DD, AA, BB, 15, 46, 16);
HH(BB, CC, DD, AA, 2, 47, 23);
//4 round
II(AA, BB, CC, DD, 0, 48, 6);
II(DD, AA, BB, CC, 7, 49, 10);
II(CC, DD, AA, BB, 14, 50, 15);
II(BB, CC, DD, AA, 5, 51, 21);
II(AA, BB, CC, DD, 12, 52, 6);
II(DD, AA, BB, CC, 3, 53, 10);
II(CC, DD, AA, BB, 10, 54, 15);
II(BB, CC, DD, AA, 1, 55, 21);
II(AA, BB, CC, DD, 8, 56, 6);
II(DD, AA, BB, CC, 15, 57, 10);
II(CC, DD, AA, BB, 6, 58, 15);
II(BB, CC, DD, AA, 13, 59, 21);
II(AA, BB, CC, DD, 4, 60, 6);
II(DD, AA, BB, CC, 11, 61, 10);
II(CC, DD, AA, BB, 2, 62, 15);
II(BB, CC, DD, AA, 9, 63, 21);
m_A = m_A + AA;
m_B = m_B + BB;
m_C = m_C + CC;
m_D = m_D + DD;
}
//initialize
void cMd5::beginCal()
{
memset(m_Content, 0, 64);
m_ContentLen = 0;
m_TotalLen = 0;
m_A = 0x67452301;
m_B = 0xEFCDAB89;
m_C = 0x98BADCFE;
m_D = 0x10325476;
}
void cMd5::endCal()
{
memset((char*)m_Content + m_ContentLen, 0x80, 1);
if (m_ContentLen < 56){
memset((char*)m_Content + m_ContentLen + 1, 0, 55 - m_ContentLen);
}
else{
memset((char*)m_Content + m_ContentLen + 1, 0, 63 - m_ContentLen);
transform();
memset((char*)m_Content, 0, 56);
}
unsigned int h, l;
h = 0;
l = m_TotalLen;
h |= ((l & 0x80000000) >> 31);
h <<= 1;
l <<= 1;
h |= ((l & 0x80000000) >> 31);
h <<= 1;
l <<= 1;
h |= ((l & 0x80000000) >> 31);
h <<= 1;
l <<= 1;
memcpy(m_Content + 14, &l, 4);
memcpy(m_Content + 15, &h, 4);
transform();
}
void cMd5::addData(unsigned char* data, unsigned int size)
{
unsigned int i = 0;
unsigned int remain = 64 - m_ContentLen;
if (remain > size)
remain = size;
memcpy(((char*)m_Content) + m_ContentLen, data, remain);
i += remain;
m_ContentLen += remain;
m_TotalLen += remain;
if (m_ContentLen < 64)
return;
transform();
while (i + 64 <= size)
{
memcpy(m_Content, data + i, 64);
transform();
i += 64;
m_TotalLen += 64;
}
m_ContentLen = size - i;
m_TotalLen += m_ContentLen;
memcpy(m_Content, data + i, m_ContentLen);
}
unsigned char* cMd5::getCode()
{
unsigned char * code = new unsigned char[16];
memcpy(code, &m_A, 4);
memcpy(code + 4, &m_B, 4);
memcpy(code + 8, &m_C, 4);
memcpy(code + 12, &m_D, 4);
return code;
}
void cMd5::clearMemoryHelper(char* p)
{
delete []p;
}