根據定義，UUID（Universally Unique IDentifier，也稱GUID）在時間和空間都是唯一的。為保證空間的唯一性，每個UUID使用了一個48位的值來記錄，一般是計算機的網卡地址。為保證時間上的唯一性，每個UUID具有一個60位的時間戳（timestamp）。這個時間戳表示自公元1582年（絕對不是1852，這是《COM技術內幕》，1999年3月第1版第89頁中的一個錯誤）10月15號00：00：00：00以來的時間，是以100納秒為單位的時間間隔。1納秒（ns）=10-9秒（s）。UUID算法可以保證至大約公元3400年仍然唯一。UUID的C語言結構定義如下：
以軟件算法實現UUID非常有現實意義。參考RFC4122文檔和其他一些開源代碼，我寫了一個WIN32下的UUID實現C語言程序——UUID32.c。程序符合RFC4122標準。程序不但實現創建UUID和UUID String，還可以對UUID進行字符和時間上的比較。還可以從UUID從提取時間戳（精度到秒）。頭文件uuid32.h定義如下：
/*
* md5.c - Copyright 1997 Lachlan Roche
* - Modified by cheungmine, 2007-9-15
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <memory.h>
#include "md5.h"
#define MD5STR_LEN 32
/*=====================================================================
The remaining code is the reference MD5 code (md5c.c) from rfc1321
MD5C.C - RSA Data Security, Inc., MD5 message-digest algorithm
Copyright (C) 1991-2, RSA Data Security, Inc. Created 1991. All
rights reserved.
License to copy and use this software is granted provided that it
is identified as the "RSA Data Security, Inc. MD5 Message-Digest
Algorithm" in all material mentioning or referencing this software
or this function.
License is also granted to make and use derivative works provided
that such works are identified as "derived from the RSA Data
Security, Inc. MD5 Message-Digest Algorithm" in all material
mentioning or referencing the derived work.
RSA Data Security, Inc. makes no representations concerning either
the merchantability of this software or the suitability of this
software for any particular purpose. It is provided "as is"
without express or implied warranty of any kind.
These notices must be retained in any copies of any part of this
documentation and/or software.
=====================================================================*/
/* Constants for _MD5Transform routine. */
#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
static void _MD5Transform(unsigned int[4], const unsigned char[64]);
static void _Encode(unsigned char *, unsigned int *, unsigned int);
static void _Decode(unsigned int *, const unsigned char *, unsigned int);
static unsigned char PADDING[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
};
/* F, G, H and I are basic MD5 functions. */
#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)))
/* ROTATE_LEFT rotates x left n bits. */
#define ROTATE_LEFT(x, n) (((x) << (n)) | ((x) >> (32-(n))))
/* FF, GG, HH, and II transformations for rounds 1, 2, 3, and 4.
Rotation is separate from addition to prevent recomputation. */
#define FF(a, b, c, d, x, s, ac) {
(a) += F ((b), (c), (d)) + (x) + (unsigned int)(ac);
(a) = ROTATE_LEFT ((a), (s));
(a) += (b);
}
#define GG(a, b, c, d, x, s, ac) {
(a) += G ((b), (c), (d)) + (x) + (unsigned int)(ac);
(a) = ROTATE_LEFT ((a), (s));
(a) += (b);
}
#define HH(a, b, c, d, x, s, ac) {
(a) += H ((b), (c), (d)) + (x) + (unsigned int)(ac);
(a) = ROTATE_LEFT ((a), (s));
(a) += (b);
}
#define II(a, b, c, d, x, s, ac) {
(a) += I ((b), (c), (d)) + (x) + (unsigned int)(ac);
(a) = ROTATE_LEFT ((a), (s));
(a) += (b);
}
/* MD5 initialization. Begins an MD5 operation, writing a new context. */
void MD5_init(MD5_CTX * context)
{
context->count[0] = context->count[1] = 0;
/* Load magic initialization constants. */
context->state[0] = 0x67452301;
context->state[1] = 0xefcdab89;
context->state[2] = 0x98badcfe;
context->state[3] = 0x10325476;
}
/* MD5 block update operation. Continues an MD5 message-digest operation,
processing another message block, and updating the context. */
void MD5_update(MD5_CTX * context, const unsigned char *input, unsigned int inputLen)
{
unsigned int i, index, partLen;
/* Compute number of bytes mod 64 */
index = (unsigned int) ((context->count[0] >> 3) & 0x3F);
/* Update number of bits */
if ((context->count[0] += ((unsigned int) inputLen << 3)) < ((unsigned int) inputLen << 3))
context->count[1]++;
context->count[1] += ((unsigned int) inputLen >> 29);
partLen = 64 - index;
/* Transform as many times as possible. */
if (inputLen >= partLen) {
memcpy((void *) &context->buffer[index], (void *) input, partLen);
_MD5Transform(context->state, context->buffer);
for (i = partLen; i + 63 < inputLen; i += 64)
_MD5Transform(context->state, &input[i]);
index = 0;
}
else
i = 0;
/* Buffer remaining input */
memcpy((void *) &context->buffer[index], (void *) &input[i], inputLen - i);
}
/* MD5 finalization. Ends an MD5 message-digest operation, writing the message digest and zeroizing the context. */
void MD5_fini(unsigned char digest[16], MD5_CTX * context)
{
unsigned char bits[8];
unsigned int index, padLen;
/* Save number of bits */
_Encode(bits, context->count, 8);
/* Pad out to 56 mod 64. */
index = (unsigned int) ((context->count[0] >> 3) & 0x3f);
padLen = (index < 56) ? (56 - index) : (120 - index);
MD5_update(context, PADDING, padLen);
/* Append length (before padding) */
MD5_update(context, bits, 8);
/* Store state in digest */
_Encode(digest, context->state, 16);
/* Zeroize sensitive information.*/
memset((void *) context, 0, sizeof (*context));
}
#pragma warning(push) /* C4996 */
#pragma warning( disable : 4996 )
char* MD5_sign (const unsigned char *str, unsigned int len)
{
int i;
MD5_CTX md5;
static char md5_str[MD5STR_LEN+1];
char hash[16], tmp[3];
md5_str[0] = 0;
MD5_init(&md5);
MD5_update (&md5, str, len);
MD5_fini (hash, &md5);
for ( i = 0 ; i < 16 ; i++ )
{
_itoa((unsigned char)hash[i], tmp , 16);
if (tmp[1] == 0){
tmp[2]=0; tmp[1]=tmp[0]; tmp[0]='0';
}
strcat(md5_str, tmp);
}
return md5_str;
}
#pragma warning(pop) /* C4996 */
/* MD5 basic transformation. Transforms state based on block. */
static void _MD5Transform(unsigned int state[4], const unsigned char block[64])
{
unsigned int a = state[0],
b = state[1],
c = state[2],
d = state[3],
x[16];
_Decode(x, block, 64);
/* Round 1 */
FF(a, b, c, d, x[0], S11, 0xd76aa478); /* 1 */
FF(d, a, b, c, x[1], S12, 0xe8c7b756); /* 2 */
FF(c, d, a, b, x[2], S13, 0x242070db); /* 3 */
FF(b, c, d, a, x[3], S14, 0xc1bdceee); /* 4 */
FF(a, b, c, d, x[4], S11, 0xf57c0faf); /* 5 */
FF(d, a, b, c, x[5], S12, 0x4787c62a); /* 6 */
FF(c, d, a, b, x[6], S13, 0xa8304613); /* 7 */
FF(b, c, d, a, x[7], S14, 0xfd469501); /* 8 */
FF(a, b, c, d, x[8], S11, 0x698098d8); /* 9 */
FF(d, a, b, c, x[9], S12, 0x8b44f7af); /* 10 */
FF(c, d, a, b, x[10], S13, 0xffff5bb1); /* 11 */
FF(b, c, d, a, x[11], S14, 0x895cd7be); /* 12 */
FF(a, b, c, d, x[12], S11, 0x6b901122); /* 13 */
FF(d, a, b, c, x[13], S12, 0xfd987193); /* 14 */
FF(c, d, a, b, x[14], S13, 0xa679438e); /* 15 */
FF(b, c, d, a, x[15], S14, 0x49b40821); /* 16 */
/* Round 2 */
GG(a, b, c, d, x[1], S21, 0xf61e2562); /* 17 */
GG(d, a, b, c, x[6], S22, 0xc040b340); /* 18 */
GG(c, d, a, b, x[11], S23, 0x265e5a51); /* 19 */
GG(b, c, d, a, x[0], S24, 0xe9b6c7aa); /* 20 */
GG(a, b, c, d, x[5], S21, 0xd62f105d); /* 21 */
GG(d, a, b, c, x[10], S22, 0x2441453); /* 22 */
GG(c, d, a, b, x[15], S23, 0xd8a1e681); /* 23 */
GG(b, c, d, a, x[4], S24, 0xe7d3fbc8); /* 24 */
GG(a, b, c, d, x[9], S21, 0x21e1cde6); /* 25 */
GG(d, a, b, c, x[14], S22, 0xc33707d6); /* 26 */
GG(c, d, a, b, x[3], S23, 0xf4d50d87); /* 27 */
GG(b, c, d, a, x[8], S24, 0x455a14ed); /* 28 */
GG(a, b, c, d, x[13], S21, 0xa9e3e905); /* 29 */
GG(d, a, b, c, x[2], S22, 0xfcefa3f8); /* 30 */
GG(c, d, a, b, x[7], S23, 0x676f02d9); /* 31 */
GG(b, c, d, a, x[12], S24, 0x8d2a4c8a); /* 32 */
/* Round 3 */
HH(a, b, c, d, x[5], S31, 0xfffa3942); /* 33 */
HH(d, a, b, c, x[8], S32, 0x8771f681); /* 34 */
HH(c, d, a, b, x[11], S33, 0x6d9d6122); /* 35 */
HH(b, c, d, a, x[14], S34, 0xfde5380c); /* 36 */
HH(a, b, c, d, x[1], S31, 0xa4beea44); /* 37 */
HH(d, a, b, c, x[4], S32, 0x4bdecfa9); /* 38 */
HH(c, d, a, b, x[7], S33, 0xf6bb4b60); /* 39 */
HH(b, c, d, a, x[10], S34, 0xbebfbc70); /* 40 */
HH(a, b, c, d, x[13], S31, 0x289b7ec6); /* 41 */
HH(d, a, b, c, x[0], S32, 0xeaa127fa); /* 42 */
HH(c, d, a, b, x[3], S33, 0xd4ef3085); /* 43 */
HH(b, c, d, a, x[6], S34, 0x4881d05); /* 44 */
HH(a, b, c, d, x[9], S31, 0xd9d4d039); /* 45 */
HH(d, a, b, c, x[12], S32, 0xe6db99e5); /* 46 */
HH(c, d, a, b, x[15], S33, 0x1fa27cf8); /* 47 */
HH(b, c, d, a, x[2], S34, 0xc4ac5665); /* 48 */
/* Round 4 */
II(a, b, c, d, x[0], S41, 0xf4292244); /* 49 */
II(d, a, b, c, x[7], S42, 0x432aff97); /* 50 */
II(c, d, a, b, x[14], S43, 0xab9423a7); /* 51 */
II(b, c, d, a, x[5], S44, 0xfc93a039); /* 52 */
II(a, b, c, d, x[12], S41, 0x655b59c3); /* 53 */
II(d, a, b, c, x[3], S42, 0x8f0ccc92); /* 54 */
II(c, d, a, b, x[10], S43, 0xffeff47d); /* 55 */
II(b, c, d, a, x[1], S44, 0x85845dd1); /* 56 */
II(a, b, c, d, x[8], S41, 0x6fa87e4f); /* 57 */
II(d, a, b, c, x[15], S42, 0xfe2ce6e0); /* 58 */
II(c, d, a, b, x[6], S43, 0xa3014314); /* 59 */
II(b, c, d, a, x[13], S44, 0x4e0811a1); /* 60 */
II(a, b, c, d, x[4], S41, 0xf7537e82); /* 61 */
II(d, a, b, c, x[11], S42, 0xbd3af235); /* 62 */
II(c, d, a, b, x[2], S43, 0x2ad7d2bb); /* 63 */
II(b, c, d, a, x[9], S44, 0xeb86d391); /* 64 */
state[0] += a;
state[1] += b;
state[2] += c;
state[3] += d;
/* Zeroize sensitive information. */
memset((void *) x, 0, sizeof (x));
}
/* Encodes input (unsigned int) into output (unsigned char). Assumes len is a multiple of 4. */
static void _Encode(unsigned char *output, unsigned int *input, unsigned int len)
{
unsigned int i, j;
for (i = 0, j = 0; j < len; i++, j += 4) {
output[j] = (unsigned char) (input[i] & 0xff);
output[j + 1] = (unsigned char) ((input[i] >> 8) & 0xff);
output[j + 2] = (unsigned char) ((input[i] >> 16) & 0xff);
output[j + 3] = (unsigned char) ((input[i] >> 24) & 0xff);
}
}
/* Decodes input (unsigned char) into output (unsigned int). Assumes len is a multiple of 4.*/
static void _Decode(unsigned int *output, const unsigned char *input, unsigned int len)
{
unsigned int i, j;
for (i = 0, j = 0; j < len; i++, j += 4) {
output[i] = ((unsigned int) input[j]) | (((unsigned int) input[j + 1]) << 8) |
(((unsigned int) input[j + 2]) << 16) | (((unsigned int) input[j + 3]) << 24);
}
}