Base64编码说明
  Base64编码要求把3个8位字节(3*8=24)转化为4个6位的字节(4*6=24),之后在6位的前面补两个0,形成8位一个字节的形式。 如果剩下的字符不足3个字节,则用0填充,输出字符使用'=',因此编码后输出的文本末尾可能会出现1或2个'='。

  为了保证所输出的编码位可读字符,Base64制定了一个编码表,以便进行统一转换。编码表的大小为2^6=64,这也是Base64名称的由来。

转码过程举例:
  3*8=4*6
  内存1个字符占8位
  转前: s 1 3
  先转成ascii:对应 115 49 51
  2进制: 01110011 00110001 00110011
  6个一组(4组) 011100110011000100110011
  然后才有后面的 011100 110011 000100 110011
  然后计算机是8位8位的存数 6不够,自动就补两个高位0了
  所有有了 高位补0
  科学计算器输入 00011100 00110011 00000100 00110011
  得到 28 51 4 51
  查对下照表 c z E z

Base64编码表

码值字符   码值字符   码值字符   码值字符
0 A 16 Q 32 g 48 w
1 B 17 R 33 h 49 x
2 C 18 S 34 i 50 y
3 D 19 T 35 j 51 z
4 E 20 U 36 k 52 0
5 F 21 V 37 l 53 1
6 G 22 W 38 m 54 2
7 H 23 X 39 n 55 3
8 I 24 Y 40 o 56 4
9 J 25 Z 41 p 57 5
10 K 26 a 42 q 58 6
11 L 27 b 43 r 59 7
12 M 28 c 44 s 60 8
13 N 29 d 45 t 61 9
14 O 30 e 46 u 62 +
15 P 31 f 47 v 63 /

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

C++实现:

base64.h

 1 #ifndef _BASE64_HH  
 2 #define _BASE64_HH  
 3 
 4 #ifdef __cplusplus  
 5 extern "C" {  
 6 #endif    //__cplusplus 
 7 
 8 unsigned char* base64Decode(char* pszInput, unsigned int& iReturnSize, bool trimTrailingZeros = true);
 9 
10 char* base64Encode(char const* origSigned, unsigned iOrigLength); 
11 
12 #ifdef __cplusplus  
13 }  
14 #endif  //__cplusplus 
15   
16 #endif    //_BASE64_HH

 

base64.cpp

  1 #include "base64.h"  
  2 #include <string.h>  
  3 
  4 static char base64DecodeTable[65];  
  5 //初始化编码对照表
  6 static const char base64Char[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";  
  7 
  8 char* strDupSize(char const* str)   
  9 {  
 10     if (str == NULL) return NULL;  
 11     size_t len = strlen(str) + 1;  
 12     char* copy = new char[len];  
 13   
 14     return copy;  
 15 }
 16 
 17 //初始化解码对照表
 18 static void initBase64DecodeTable()  
 19 {   
 20   for (int i = 0; i < 65; ++i) 
 21       base64DecodeTable[i] = (char)0x80;   // default value: invalid  
 22   
 23   for (int i = 'A'; i <= 'Z'; ++i) 
 24       base64DecodeTable[i] = 0 + (i - 'A');  
 25   for (int i = 'a'; i <= 'z'; ++i) 
 26       base64DecodeTable[i] = 26 + (i - 'a');  
 27   for (int i = '0'; i <= '9'; ++i) 
 28       base64DecodeTable[i] = 52 + (i - '0');  
 29   base64DecodeTable[(unsigned char)'+'] = 62;  
 30   base64DecodeTable[(unsigned char)'/'] = 63;  
 31   base64DecodeTable[(unsigned char)'='] = 64;  //0x40
 32 }  
 33 
 34 // 解码
 35 unsigned char* base64Decode(char* pszInput, unsigned int& iReturnSize, bool trimTrailingZeros)   
 36 {  
 37     static bool haveInitedBase64DecodeTable = false;  
 38     if (!haveInitedBase64DecodeTable)  
 39     {  
 40         initBase64DecodeTable();  
 41         haveInitedBase64DecodeTable = true;  
 42     }  
 43 
 44     unsigned char* pszOutput = new unsigned char[strlen(pszInput)];
 45     int k = 0;  
 46     /*******************************************************************************************
 47      *技巧:先减去3 ---in case "pszInput" is not a multiple of 4 bytes (although it should be) *
 48      *      个人觉得这里不用做处理,因为编码时候已经做过处理,密文肯定为4的整数倍               *
 49      *******************************************************************************************/
 50     int const jMax = strlen(pszInput) - 3;  
 51 
 52     //将密文四个字节为一组,每组解码为三个字节
 53     for (int j = 0; j < jMax; j += 4)   
 54     {  
 55         char inTmp[4], outTmp[4];  
 56         for (int i = 0; i < 4; ++i)   
 57         {  
 58             inTmp[i] = pszInput[i+j];  
 59             outTmp[i] = base64DecodeTable[(unsigned char)inTmp[i]];  
 60             if ((outTmp[i]&0x40) != 0)    //如果遇到'=',将其视为结束字符
 61                 outTmp[i] = 0x00; 
 62         }  
 63 
 64         pszOutput[k++] = (outTmp[0]<<2) | (outTmp[1]>>4);  
 65         pszOutput[k++] = (outTmp[1]<<4) | (outTmp[2]>>2);  
 66         pszOutput[k++] = (outTmp[2]<<6) | outTmp[3];  
 67     }  
 68 
 69     if (trimTrailingZeros)   
 70     {  
 71         while (k > 0 && pszOutput[k-1] == '\0') 
 72             --k;  
 73     }  
 74     iReturnSize = k;  
 75     unsigned char* result = new unsigned char[iReturnSize];  
 76     memset(result, 0, iReturnSize);
 77     memmove(result, pszOutput, iReturnSize);  
 78     delete[] pszOutput;  
 79     return result;  
 80 }  
 81 
 82 // 编码
 83 char* base64Encode(char const* origSigned, unsigned iOrigLength)   
 84 {  
 85   unsigned char const* orig = (unsigned char const*)origSigned;  
 86   if (orig == NULL) 
 87       return NULL;
 88 
 89   unsigned const iNumResultBytes = 4*(iOrigLength/3 + (iOrigLength%3 ? 1 : 0));
 90   char* result = new char[iNumResultBytes + 1]; 
 91   
 92   //源码三个字节为一组,编码后将三个字节变为四个字节
 93   unsigned i;  
 94   for (i = 0; i < iOrigLength/3; ++i)   
 95   {  
 96     // 取源码的第一个字节的前六位,前面加两个零生成第一个字节
 97     result[4*i+0] = base64Char[(orig[3*i]>>2) & 0x3F];    
 98     // 取源码的第一个字节的后两位和第二个字节的前四位,前面补两个零生成第二个字节
 99     result[4*i+1] = base64Char[((orig[3*i]<<4) | (orig[3*i+1]>>4)) & 0x3F];
100     // 取源码的第二个字节的后四位和第三个字节的前两位,前面补两个零生成第三个字节
101     result[4*i+2] = base64Char[((orig[3*i+1]<<2) | (orig[3*i+2]>>6)) & 0x3F];
102     // 取源码的第三个字节的后六位,前面补两个零生成第四个字节
103     result[4*i+3] = base64Char[orig[3*i+2]&0x3F];                                
104   }
105   //不足三个字节的最后补'='字符,补足三个字节
106   if (iOrigLength%3)
107   {
108     result[4*i+0] = base64Char[(orig[3*i]>>2) & 0x3F];
109     if (iOrigLength%3 == 2)
110     {
111       result[4*i+1] = base64Char[((orig[3*i]<<4) | (orig[3*i+1]>>4))&0x3F];  
112       result[4*i+2] = base64Char[(orig[3*i+1]<<2)&0x3C];  
113     }
114     else
115     {
116       result[4*i+1] = base64Char[((orig[3*i])<<4)&0x3F];
117       result[4*i+2] = '=';
118     }
119     result[4*i+3] = '=';
120   }
121 
122   result[iNumResultBytes] = '\0';  
123   return result;  
124 }  

测试代码main.cpp

 1 #include "base64.h"
 2 #include <iostream>
 3 
 4 using namespace std;
 5 
 6 int main()
 7 {
 8     cout << "请输入要转换的字符串(支持中文):" << endl;
 9     char szStrOrigin[256];
10     scanf("%s", szStrOrigin);
11     unsigned int iReturnSize;
12     cout << "编码之前字符串:" << szStrOrigin << endl;
13     char *pEncodeStr = base64Encode(szStrOrigin, strlen(szStrOrigin));
14     cout << "编码之后字符串:" <<  pEncodeStr << endl;
15     unsigned char *pDecodeStr = base64Decode(pEncodeStr, iReturnSize);
16     cout << "解码之后字符串:" << pDecodeStr << endl;
17     system("pause");
18 }

运行结果:

 

 

posted on 2015-03-31 09:05  可笑痴狂  阅读(1983)  评论(0编辑  收藏  举报