1. Base64使用A--Z,a--z,0--9,+,/ 这64个字符.
    2. 编码原理:将3个字节转换成4个字节( (3 X 8) = 24 = (4 X 6) )先读入3个字节,每读一个字节,左移8位,再右移四次,每次6位,这样就有4个字节了.
    3. 解码原理:将4个字节转换成3个字节.先读入4个6位(用或运算),每次左移6位,再右移3次,每次8位.这样就还原了.

    Base64是一种非经常见的编码规范，其作用是将二进制序列转换为人类可读的ASCII字符序列，经常使用在需用通过文本协议（比方HTTP和SMTP）来传输二进制数据的情况下。Base64并非一种用于安全领域的加密解密算法（这类算法有DES等），虽然我们有时也听到使用Base64来加密解密的说法，但这里所说的加密与解密实际是指编码（encode）和解码（decode）的过程，其变换是非常easy的，只可以避免信息被直接识别。

    Base64採用了一种非常easy的编码转换：对于待编码数据，以3个字节为单位，依次取6位数据并在前面补上两个0形成新的8位编码，因为3*8=4*6，这样3个字节的输入会变成4个字节的输出，长度上添加?了1/3。

上面的处理还不能保证得到的字符都是可见字符，为了达到此目的，Base64制定了一个编码表，进行统一的转换。码表的大小为2^6=64，这也是Base64名称的由来。

    Base64编解码算法都非常easy，网上有非常多源代码，这里就不介绍了。

    另外另一点要注意的地方，前面提到编码是以3个字节为单位，当剩下的字符数量不足3个字节时，则应使用0进行填充，对应的，输出字符则使用‘=‘占位，因此编码后输出的文本末尾可能会出现1至2个‘=‘。

这是一种典型的编码转换的处理方法，相似的可能还有UTF16与UTF8之间的转换。 /*** /file base64.h*/#ifndef XYSSL_BASE64_H#define XYSSL_BASE64_H#define XYSSL_ERR_BASE64_BUFFER_TOO_SMALL -0x0010#define XYSSL_ERR_BASE64_INVALID_CHARACTER -0x0012#ifdef __cplusplusextern "C" {#endif/*** /brief Encode a buffer into base64 format** /param dst destination buffer* /param dlen size of the buffer* /param src source buffer* /param slen amount of data to be encoded** /return 0 if successful, or XYSSL_ERR_BASE64_BUFFER_TOO_SMALL.* *dlen is always updated to reflect the amount* of data that has (or would have) been written.** /note Call this function with *dlen = 0 to obtain the* required buffer size in *dlen*/int base64_encode(const unsigned char *src, int slen,unsigned char *dst, int *dlen);/*** /brief Decode a base64-formatted buffer** /param dst destination buffer* /param dlen size of the buffer* /param src source buffer* /param slen amount of data to be decoded** /return 0 if successful, XYSSL_ERR_BASE64_BUFFER_TOO_SMALL, or* XYSSL_ERR_BASE64_INVALID_DATA if the input data is not* correct. *dlen is always updated to reflect the amount* of data that has (or would have) been written.** /note Call this function with *dlen = 0 to obtain the* required buffer size in *dlen*/int base64_decode(const unsigned char *src, int slen,unsigned char *dst, int *dlen);/*** /brief Checkup routine** /return 0 if successful, or 1 if the test failed*/int base64_self_test( int verbose );#ifdef __cplusplus}#endifclass CBase64 {public: CBase64(); virtual ~CBase64(); static BOOL Encrypt(const unsigned char *pSrc,int iSlen,unsigned char *pDst,int *iDlen,CString &strErrorInfo); static BOOL Decrypt(const unsigned char *pSrc,int iSlen,unsigned char *pDst,int *iDlen,CString &strErrorInfo);private:};#endif /* base64.h */

//source file: base64.cpp/** RFC 1521 base64 encoding/decoding** Copyright (C) 2006-2007 Christophe Devine** This library is free software; you can redistribute it and/or* modify it under the terms of the GNU Lesser General Public* License, version 2.1 as published by the Free Software Foundation.** This library is distributed in the hope that it will be useful,* but WITHOUT ANY WARRANTY; without even the implied warranty of* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU* Lesser General Public License for more details.** You should have received a copy of the GNU Lesser General Public* License along with this library; if not, write to the Free Software* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,* MA 02110-1301 USA*/#include "stdafx.h"#include "base64.h"static const unsigned char base64_enc_map[64] ={‘A‘, ‘B‘, ‘C‘, ‘D‘, ‘E‘, ‘F‘, ‘G‘, ‘H‘, ‘I‘, ‘J‘,‘K‘, ‘L‘, ‘M‘, ‘N‘, ‘O‘, ‘P‘, ‘Q‘, ‘R‘, ‘S‘, ‘T‘,‘U‘, ‘V‘, ‘W‘, ‘X‘, ‘Y‘, ‘Z‘, ‘a‘, ‘b‘, ‘c‘, ‘d‘,‘e‘, ‘f‘, ‘g‘, ‘h‘, ‘i‘, ‘j‘, ‘k‘, ‘l‘, ‘m‘, ‘n‘,‘o‘, ‘p‘, ‘q‘, ‘r‘, ‘s‘, ‘t‘, ‘u‘, ‘v‘, ‘w‘, ‘x‘,‘y‘, ‘z‘, ‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘,‘8‘, ‘9‘, ‘+‘, ‘/‘};static const unsigned char base64_dec_map[128] ={127, 127, 127, 127, 127, 127, 127, 127, 127, 127,127, 127, 127, 127, 127, 127, 127, 127, 127, 127,127, 127, 127, 127, 127, 127, 127, 127, 127, 127,127, 127, 127, 127, 127, 127, 127, 127, 127, 127,127, 127, 127, 62, 127, 127, 127, 63, 52, 53,54, 55, 56, 57, 58, 59, 60, 61, 127, 127,127, 64, 127, 127, 127, 0, 1, 2, 3, 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14,15, 16, 17, 18, 19, 20, 21, 22, 23, 24,25, 127, 127, 127, 127, 127, 127, 26, 27, 28,29, 30, 31, 32, 33, 34, 35, 36, 37, 38,39, 40, 41, 42, 43, 44, 45, 46, 47, 48,49, 50, 51, 127, 127, 127, 127, 127};/** Encode a buffer into base64 format*/int base64_encode(const unsigned char *src, int slen,unsigned char *dst, int *dlen){ int i, n; int C1, C2, C3; unsigned char *p; if( slen == 0 ) return( 0 ); n = (slen << 3) / 6; switch( (slen << 3) - (n * 6) ) { case 2: n += 3; break; case 4: n += 2; break; default: break; } if( *dlen < n + 1 ) { *dlen = n + 1; return( XYSSL_ERR_BASE64_BUFFER_TOO_SMALL ); } n = (slen / 3) * 3; for( i = 0, p = dst; i < n; i += 3 ) { C1 = *src++; C2 = *src++; C3 = *src++; *p++ = base64_enc_map[(C1 >> 2) & 0x3F]; *p++ = base64_enc_map[(((C1 & 3) << 4) + (C2 >> 4)) & 0x3F]; *p++ = base64_enc_map[(((C2 & 15) << 2) + (C3 >> 6)) & 0x3F]; *p++ = base64_enc_map[C3 & 0x3F]; } if( i < slen ) { C1 = *src++; C2 = ((i + 1) < slen) ? *src++ : 0; *p++ = base64_enc_map[(C1 >> 2) & 0x3F]; *p++ = base64_enc_map[(((C1 & 3) << 4) + (C2 >> 4)) & 0x3F]; if( (i + 1) < slen ) *p++ = base64_enc_map[((C2 & 15) << 2) & 0x3F]; else *p++ = ‘=‘; *p++ = ‘=‘; } *dlen =static_cast<int>(p - dst); *p = 0; return( 0 );}/** Decode a base64-formatted buffer*/int base64_decode(const unsigned char *src, int slen,unsigned char *dst, int *dlen){ int i, j, n; unsigned long x; unsigned char *p; for( i = j = n = 0; i < slen; i++ ) { if( ( slen - i ) >= 2 && *(src+i) == ‘/r‘ && *(src+i+1) == ‘/n‘ ) continue; if(*(src+i) == ‘/n‘ ) continue; if(*(src+i) == ‘=‘ && ++j > 2 ) return( XYSSL_ERR_BASE64_INVALID_CHARACTER ); if(*(src+i) > 127 || base64_dec_map[*(src+i)] == 127 ) return( XYSSL_ERR_BASE64_INVALID_CHARACTER ); if( base64_dec_map[*(src+i)] < 64 && j != 0 ) return( XYSSL_ERR_BASE64_INVALID_CHARACTER ); n++; } if( n == 0 ) return( 0 ); n = ((n * 6) + 7) >> 3; if( *dlen < n ) { *dlen = n; return( XYSSL_ERR_BASE64_BUFFER_TOO_SMALL ); } for( j = 3, n = x = 0, p = dst; i > 0; i--, src++ ) { if( *src =http://www.mamicode.com/= ‘/r‘ || *src == ‘/n‘ )""); int iRet=base64_encode(pSrc,iSlen,pDst,iDlen); if(iRet==XYSSL_ERR_BASE64_BUFFER_TOO_SMALL) strErrorInfo=_T("分配的缓冲区太小!"); else if(iRet==XYSSL_ERR_BASE64_INVALID_CHARACTER) strErrorInfo=_T("无效数据!"); return iRet==0; }BOOL CBase64::Decrypt(const unsigned char *pSrc,int iSlen,unsigned char *pDst,int *iDlen,CString &strErrorInfo){ strErrorInfo=_T(""); int iRet=base64_decode(pSrc,iSlen,pDst,iDlen); if(iRet==XYSSL_ERR_BASE64_BUFFER_TOO_SMALL) strErrorInfo=_T("分配的缓冲区太小!"); else if(iRet==XYSSL_ERR_BASE64_INVALID_CHARACTER) strErrorInfo=_T("无效数据!"); return iRet==0; }