MD5消息摘要算法，屬Hash算法一類。
主要運用於數據加密，一致性，信息摘要以及校驗用。
比如最常用的支付寶，會將多個參數連起來加上一段key，進行MD5，連同參數一起發送給服務端，服務端根據參數進行同樣操作，如果MD5碼相同即爲正確。
當然支付寶肯定不是這樣使用的，這裏只是舉個例子！
算法原理
以下所描述的消息長度、填充數據都以位(Bit)爲單位，字節序爲小端字節。
1、數據填充
對消息進行數據填充，使消息的長度對512取餘得448，設消息長度爲X，即滿足X % 512=448。根據此公式得出需要填充的數據長度。
填充方法：在消息後面進行填充，填充第一位爲1，其餘爲0。
2、添加消息長度
在第一步結果之後再填充上原消息的長度，可用來進行的存儲長度爲64位。如果消息長度大於264，則只使用其低64位的值，即（消息長度 對 264取模）。
在此步驟進行完畢後，最終消息長度就是512的整數倍。
3、數據處理
準備需要用到的數據：
- 4個常數： A = 0x67452301, B = 0x0EFCDAB89, C = 0x98BADCFE, D = 0x10325476;
- &是與,|是或,~是非,^是異或
- 4個函數：F(X,Y,Z)=(X & Y) | ((~X) & Z); G(X,Y,Z)=(X & Z) | (Y & (~Z)); H(X,Y,Z)=X ^ Y ^ Z; I(X,Y,Z)=Y ^ (X | (~Z));
把消息分以512位爲一分組進行處理，每一個分組進行4輪變換，以上面所說4個常數爲起始變量進行計算，重新輸出4個變量，以這4個變量再進行下一分組的運算，如果已經是最後一個分組，則這4個變量爲最後的結果，即MD5值。
假設處理後的原文長度是M
主循環次數 = M / 512
每個主循環中包含 512 / 32 * 4 = 64 次 子循環。
在主循環下面64次子循環中，F、G、H、I 交替使用，第一個16次使用F，第二個16次使用G，第三個16次使用H，第四個16次使用I。
在64次子循環中，每一次用到的常量都是不同的。
最後把循環加工最終產生的A，B，C，D四個值拼接在一起，轉換成字符串即可。
下面給出在C++上的實現代碼。
#ifndef md5_INCLUDED
# define md5_INCLUDED
typedef unsigned char md5_byte_t; /* 8-bit byte */
typedef unsigned int md5_word_t; /* 32-bit word */
/* Define the state of the MD5 Algorithm. */
typedef struct md5_state_s {
md5_word_t count[2]; /* message length in bits, lsw first */
md5_word_t abcd[4]; /* digest buffer */
md5_byte_t buf[64]; /* accumulate block */
} md5_state_t;
#ifdef __cplusplus
extern "C"
{
#endif
/* Initialize the algorithm. */
void md5_init(md5_state_t *pms);
/* Append a string to the message. */
void md5_append(md5_state_t *pms, const md5_byte_t *data, int nbytes);
/* Finish the message and return the digest. */
void md5_finish(md5_state_t *pms, md5_byte_t digest[16]);
#ifdef __cplusplus
} /* end extern "C" */
#endif
#endif /* md5_INCLUDED */
#include "md5.h"
#include <string.h>
#undef BYTE_ORDER /* 1 = big-endian, -1 = little-endian, 0 = unknown */
#ifdef ARCH_IS_BIG_ENDIAN
# define BYTE_ORDER (ARCH_IS_BIG_ENDIAN ? 1 : -1)
#else
# define BYTE_ORDER 0
#endif
#define T_MASK ((md5_word_t)~0)
#define T1 /* 0xd76aa478 */ (T_MASK ^ 0x28955b87)
#define T2 /* 0xe8c7b756 */ (T_MASK ^ 0x173848a9)
#define T3 0x242070db
#define T4 /* 0xc1bdceee */ (T_MASK ^ 0x3e423111)
#define T5 /* 0xf57c0faf */ (T_MASK ^ 0x0a83f050)
#define T6 0x4787c62a
#define T7 /* 0xa8304613 */ (T_MASK ^ 0x57cfb9ec)
#define T8 /* 0xfd469501 */ (T_MASK ^ 0x02b96afe)
#define T9 0x698098d8
#define T10 /* 0x8b44f7af */ (T_MASK ^ 0x74bb0850)
#define T11 /* 0xffff5bb1 */ (T_MASK ^ 0x0000a44e)
#define T12 /* 0x895cd7be */ (T_MASK ^ 0x76a32841)
#define T13 0x6b901122
#define T14 /* 0xfd987193 */ (T_MASK ^ 0x02678e6c)
#define T15 /* 0xa679438e */ (T_MASK ^ 0x5986bc71)
#define T16 0x49b40821
#define T17 /* 0xf61e2562 */ (T_MASK ^ 0x09e1da9d)
#define T18 /* 0xc040b340 */ (T_MASK ^ 0x3fbf4cbf)
#define T19 0x265e5a51
#define T20 /* 0xe9b6c7aa */ (T_MASK ^ 0x16493855)
#define T21 /* 0xd62f105d */ (T_MASK ^ 0x29d0efa2)
#define T22 0x02441453
#define T23 /* 0xd8a1e681 */ (T_MASK ^ 0x275e197e)
#define T24 /* 0xe7d3fbc8 */ (T_MASK ^ 0x182c0437)
#define T25 0x21e1cde6
#define T26 /* 0xc33707d6 */ (T_MASK ^ 0x3cc8f829)
#define T27 /* 0xf4d50d87 */ (T_MASK ^ 0x0b2af278)
#define T28 0x455a14ed
#define T29 /* 0xa9e3e905 */ (T_MASK ^ 0x561c16fa)
#define T30 /* 0xfcefa3f8 */ (T_MASK ^ 0x03105c07)
#define T31 0x676f02d9
#define T32 /* 0x8d2a4c8a */ (T_MASK ^ 0x72d5b375)
#define T33 /* 0xfffa3942 */ (T_MASK ^ 0x0005c6bd)
#define T34 /* 0x8771f681 */ (T_MASK ^ 0x788e097e)
#define T35 0x6d9d6122
#define T36 /* 0xfde5380c */ (T_MASK ^ 0x021ac7f3)
#define T37 /* 0xa4beea44 */ (T_MASK ^ 0x5b4115bb)
#define T38 0x4bdecfa9
#define T39 /* 0xf6bb4b60 */ (T_MASK ^ 0x0944b49f)
#define T40 /* 0xbebfbc70 */ (T_MASK ^ 0x4140438f)
#define T41 0x289b7ec6
#define T42 /* 0xeaa127fa */ (T_MASK ^ 0x155ed805)
#define T43 /* 0xd4ef3085 */ (T_MASK ^ 0x2b10cf7a)
#define T44 0x04881d05
#define T45 /* 0xd9d4d039 */ (T_MASK ^ 0x262b2fc6)
#define T46 /* 0xe6db99e5 */ (T_MASK ^ 0x1924661a)
#define T47 0x1fa27cf8
#define T48 /* 0xc4ac5665 */ (T_MASK ^ 0x3b53a99a)
#define T49 /* 0xf4292244 */ (T_MASK ^ 0x0bd6ddbb)
#define T50 0x432aff97
#define T51 /* 0xab9423a7 */ (T_MASK ^ 0x546bdc58)
#define T52 /* 0xfc93a039 */ (T_MASK ^ 0x036c5fc6)
#define T53 0x655b59c3
#define T54 /* 0x8f0ccc92 */ (T_MASK ^ 0x70f3336d)
#define T55 /* 0xffeff47d */ (T_MASK ^ 0x00100b82)
#define T56 /* 0x85845dd1 */ (T_MASK ^ 0x7a7ba22e)
#define T57 0x6fa87e4f
#define T58 /* 0xfe2ce6e0 */ (T_MASK ^ 0x01d3191f)
#define T59 /* 0xa3014314 */ (T_MASK ^ 0x5cfebceb)
#define T60 0x4e0811a1
#define T61 /* 0xf7537e82 */ (T_MASK ^ 0x08ac817d)
#define T62 /* 0xbd3af235 */ (T_MASK ^ 0x42c50dca)
#define T63 0x2ad7d2bb
#define T64 /* 0xeb86d391 */ (T_MASK ^ 0x14792c6e)
static void
md5_process(md5_state_t *pms, const md5_byte_t *data /*[64]*/)
{
md5_word_t
a = pms->abcd[0], b = pms->abcd[1],
c = pms->abcd[2], d = pms->abcd[3];
md5_word_t t;
#if BYTE_ORDER > 0
/* Define storage only for big-endian CPUs. */
md5_word_t X[16];
#else
/* Define storage for little-endian or both types of CPUs. */
md5_word_t xbuf[16];
const md5_word_t *X;
#endif
{
#if BYTE_ORDER == 0
/*
* Determine dynamically whether this is a big-endian or
* little-endian machine, since we can use a more efficient
* algorithm on the latter.
*/
static const int w = 1;
if (*((const md5_byte_t *)&w)) /* dynamic little-endian */
#endif
#if BYTE_ORDER <= 0 /* little-endian */
{
/*
* On little-endian machines, we can process properly aligned
* data without copying it.
*/
if (!((data - (const md5_byte_t *)0) & 3)) {
/* data are properly aligned */
X = (const md5_word_t *)data;
} else {
/* not aligned */
memcpy(xbuf, data, 64);
X = xbuf;
}
}
#endif
#if BYTE_ORDER == 0
else /* dynamic big-endian */
#endif
#if BYTE_ORDER >= 0 /* big-endian */
{
/*
* On big-endian machines, we must arrange the bytes in the
* right order.
*/
const md5_byte_t *xp = data;
int i;
# if BYTE_ORDER == 0
X = xbuf; /* (dynamic only) */
# else
# define xbuf X /* (static only) */
# endif
for (i = 0; i < 16; ++i, xp += 4)
xbuf[i] = xp[0] + (xp[1] << 8) + (xp[2] << 16) + (xp[3] << 24);
}
#endif
}
#define ROTATE_LEFT(x, n) (((x) << (n)) | ((x) >> (32 - (n))))
/* Round 1. */
/* Let [abcd k s i] denote the operation
a = b + ((a + F(b,c,d) + X[k] + T[i]) <<< s). */
#define F(x, y, z) (((x) & (y)) | (~(x) & (z)))
#define SET(a, b, c, d, k, s, Ti)\
t = a + F(b,c,d) + X[k] + Ti;\
a = ROTATE_LEFT(t, s) + b
/* Do the following 16 operations. */
SET(a, b, c, d, 0, 7, T1);
SET(d, a, b, c, 1, 12, T2);
SET(c, d, a, b, 2, 17, T3);
SET(b, c, d, a, 3, 22, T4);
SET(a, b, c, d, 4, 7, T5);
SET(d, a, b, c, 5, 12, T6);
SET(c, d, a, b, 6, 17, T7);
SET(b, c, d, a, 7, 22, T8);
SET(a, b, c, d, 8, 7, T9);
SET(d, a, b, c, 9, 12, T10);
SET(c, d, a, b, 10, 17, T11);
SET(b, c, d, a, 11, 22, T12);
SET(a, b, c, d, 12, 7, T13);
SET(d, a, b, c, 13, 12, T14);
SET(c, d, a, b, 14, 17, T15);
SET(b, c, d, a, 15, 22, T16);
#undef SET
/* Round 2. */
/* Let [abcd k s i] denote the operation
a = b + ((a + G(b,c,d) + X[k] + T[i]) <<< s). */
#define G(x, y, z) (((x) & (z)) | ((y) & ~(z)))
#define SET(a, b, c, d, k, s, Ti)\
t = a + G(b,c,d) + X[k] + Ti;\
a = ROTATE_LEFT(t, s) + b
/* Do the following 16 operations. */
SET(a, b, c, d, 1, 5, T17);
SET(d, a, b, c, 6, 9, T18);
SET(c, d, a, b, 11, 14, T19);
SET(b, c, d, a, 0, 20, T20);
SET(a, b, c, d, 5, 5, T21);
SET(d, a, b, c, 10, 9, T22);
SET(c, d, a, b, 15, 14, T23);
SET(b, c, d, a, 4, 20, T24);
SET(a, b, c, d, 9, 5, T25);
SET(d, a, b, c, 14, 9, T26);
SET(c, d, a, b, 3, 14, T27);
SET(b, c, d, a, 8, 20, T28);
SET(a, b, c, d, 13, 5, T29);
SET(d, a, b, c, 2, 9, T30);
SET(c, d, a, b, 7, 14, T31);
SET(b, c, d, a, 12, 20, T32);
#undef SET
/* Round 3. */
/* Let [abcd k s t] denote the operation
a = b + ((a + H(b,c,d) + X[k] + T[i]) <<< s). */
#define H(x, y, z) ((x) ^ (y) ^ (z))
#define SET(a, b, c, d, k, s, Ti)\
t = a + H(b,c,d) + X[k] + Ti;\
a = ROTATE_LEFT(t, s) + b
/* Do the following 16 operations. */
SET(a, b, c, d, 5, 4, T33);
SET(d, a, b, c, 8, 11, T34);
SET(c, d, a, b, 11, 16, T35);
SET(b, c, d, a, 14, 23, T36);
SET(a, b, c, d, 1, 4, T37);
SET(d, a, b, c, 4, 11, T38);
SET(c, d, a, b, 7, 16, T39);
SET(b, c, d, a, 10, 23, T40);
SET(a, b, c, d, 13, 4, T41);
SET(d, a, b, c, 0, 11, T42);
SET(c, d, a, b, 3, 16, T43);
SET(b, c, d, a, 6, 23, T44);
SET(a, b, c, d, 9, 4, T45);
SET(d, a, b, c, 12, 11, T46);
SET(c, d, a, b, 15, 16, T47);
SET(b, c, d, a, 2, 23, T48);
#undef SET
/* Round 4. */
/* Let [abcd k s t] denote the operation
a = b + ((a + I(b,c,d) + X[k] + T[i]) <<< s). */
#define I(x, y, z) ((y) ^ ((x) | ~(z)))
#define SET(a, b, c, d, k, s, Ti)\
t = a + I(b,c,d) + X[k] + Ti;\
a = ROTATE_LEFT(t, s) + b
/* Do the following 16 operations. */
SET(a, b, c, d, 0, 6, T49);
SET(d, a, b, c, 7, 10, T50);
SET(c, d, a, b, 14, 15, T51);
SET(b, c, d, a, 5, 21, T52);
SET(a, b, c, d, 12, 6, T53);
SET(d, a, b, c, 3, 10, T54);
SET(c, d, a, b, 10, 15, T55);
SET(b, c, d, a, 1, 21, T56);
SET(a, b, c, d, 8, 6, T57);
SET(d, a, b, c, 15, 10, T58);
SET(c, d, a, b, 6, 15, T59);
SET(b, c, d, a, 13, 21, T60);
SET(a, b, c, d, 4, 6, T61);
SET(d, a, b, c, 11, 10, T62);
SET(c, d, a, b, 2, 15, T63);
SET(b, c, d, a, 9, 21, T64);
#undef SET
/* Then perform the following additions. (That is increment each
of the four registers by the value it had before this block
was started.) */
pms->abcd[0] += a;
pms->abcd[1] += b;
pms->abcd[2] += c;
pms->abcd[3] += d;
}
void
md5_init(md5_state_t *pms)
{
pms->count[0] = pms->count[1] = 0;
pms->abcd[0] = 0x67452301;
pms->abcd[1] = /*0xefcdab89*/ T_MASK ^ 0x10325476;
pms->abcd[2] = /*0x98badcfe*/ T_MASK ^ 0x67452301;
pms->abcd[3] = 0x10325476;
}
void
md5_append(md5_state_t *pms, const md5_byte_t *data, int nbytes)
{
const md5_byte_t *p = data;
int left = nbytes;
int offset = (pms->count[0] >> 3) & 63;
md5_word_t nbits = (md5_word_t)(nbytes << 3);
if (nbytes <= 0)
return;
/* Update the message length. */
pms->count[1] += nbytes >> 29;
pms->count[0] += nbits;
if (pms->count[0] < nbits)
pms->count[1]++;
/* Process an initial partial block. */
if (offset) {
int copy = (offset + nbytes > 64 ? 64 - offset : nbytes);
memcpy(pms->buf + offset, p, copy);
if (offset + copy < 64)
return;
p += copy;
left -= copy;
md5_process(pms, pms->buf);
}
/* Process full blocks. */
for (; left >= 64; p += 64, left -= 64)
md5_process(pms, p);
/* Process a final partial block. */
if (left)
memcpy(pms->buf, p, left);
}
void
md5_finish(md5_state_t *pms, md5_byte_t digest[16])
{
static const md5_byte_t pad[64] = {
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
};
md5_byte_t data[8];
int i;
/* Save the length before padding. */
for (i = 0; i < 8; ++i)
data[i] = (md5_byte_t)(pms->count[i >> 2] >> ((i & 3) << 3));
/* Pad to 56 bytes mod 64. */
md5_append(pms, pad, ((55 - (pms->count[0] >> 3)) & 63) + 1);
/* Append the length. */
md5_append(pms, data, 8);
for (i = 0; i < 16; ++i)
digest[i] = (md5_byte_t)(pms->abcd[i >> 2] >> ((i & 3) << 3));
}
調用:
#include "md5.h"
std::string getDataMD5Hash(const std::string &dataStr)
{
static const unsigned int MD5_DIGEST_LENGTH = 16;
if (dataStr.empty())
return std::string();
md5_state_t state;
md5_byte_t digest[MD5_DIGEST_LENGTH];
char hexOutput[(MD5_DIGEST_LENGTH << 1) + 1] = { 0 };
md5_init(&state);
md5_append(&state, (const md5_byte_t *)dataStr.c_str(), (int)dataStr.size());
md5_finish(&state, digest);
for (int di = 0; di < 16; ++di)
sprintf(hexOutput + di * 2, "%02x", digest[di]);
return hexOutput;
}