Base64加密解密原理以及代码实现

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是一种很常见的编码规范，其作用是将二进制序列转换为人类可读的ASCII字符序列，常用在需用通过文本协议（比如HTTP和SMTP）来传输二进制数据的情况下。Base64并不是一种用于安全领域的加密解密算法（这类算法有DES等），尽管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，其变换是非常简单的，仅仅能够避免信息被直接识别。

   

    Base64采用了一种很简单的编码转换：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，由于3*8=4*6，这样3个字节的输入会变成4个字节的输出，长度上增加了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编码表

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

    Base64编解码算法都很简单，网上有很多源码，这里就不介绍了。

 

    另外还有一点要注意的地方，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，相应的，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，类似的可能还有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 __cplusplus
 extern "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
 }
 #endif

 class 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 == '/r' || *src == '/n' )
         continue;

         j -= ( base64_dec_map[*src] == 64 );
         x = (x << 6) | ( base64_dec_map[*src] & 0x3F );

         if( ++n == 4 )
         {
             n = 0;
             if( j > 0 ) *p++ = (unsigned char)( x >> 16 );
             if( j > 1 ) *p++ = (unsigned char)( x >> 8 );
             if( j > 2 ) *p++ = (unsigned char)( x );
         }
     }

     *dlen =static_cast<int>(p - dst);

     return( 0 );
 }


 BOOL CBase64::Encrypt(const unsigned char *pSrc,int iSlen,unsigned char *pDst,int *iDlen,CString &strErrorInfo)
 {
     strErrorInfo=_T("");
     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;
 }