MFC封装的base64编码解码函数-蒲公英云

函数本就是网上下载的，经过我修改并测试通过以后，封装成C++代码了，并且使用了MFC的数据结构，懂C或C++的朋友可以拿过去稍作修改就可以用了。
提醒：作为C函数，在进行base64编码时一定要给够缓冲区，base64是将8比特数据封装到6比特里，所以缓冲区大小应该为原数据长度 x 4 / 3 + 3，同理，解码的时候则是原数据长度 x 3 / 4 +4，为什么+3、+4？这是因为整除会抛弃余数，余数最大值分别为：2、3，+3、4,多了1个字节，正好给结尾的字符0准备。

#include "StdAfx.h"
#include "base64.h"
// 进行1次base64编码，返回编码的源字符串字符个数，-1代表出错
static int base64_string( const unsigned char *src, size_t srclen, unsigned char *dest, size_t destlen, unsigned char *base64, int index )
{ 
    if ( NULL == src || NULL == dest || NULL == base64 || index < 0)
        return -1;
    if (srclen <= 0 || destlen < 5) { 
        return -1; // // dest多留1个给结尾的0
    }
    unsigned char one;
    unsigned char two;
    unsigned char thr;
    unsigned char fou;
    if ( srclen >= 3) 
    { 
        one = src[0] >> 2;
        two = ( ( src[0] & 0x03 ) << 4 ) | ( src[1] >> 4 );
        thr = ( ( src[1] & 0x0F ) << 2 ) | ( src[2] >> 6 );
        fou = src[2] & 0x3F;
        *( dest + index++ ) = base64[one];
        *( dest + index++ ) = base64[two];
        *( dest + index++ ) = base64[thr];
        *( dest + index   ) = base64[fou];
    }
    else if ( srclen == 2)
    { 
        one = src[0] >> 2;
        two = ( ( src[0] & 0x03) << 4 ) | ( src[1] >> 4 );
        thr = ( src[1] & 0x0F ) << 2;
        *( dest + index++ ) = base64[one];
        *( dest + index++ ) = base64[two];
        *( dest + index++ ) = base64[thr];
        *( dest + index   ) = '=';
    }
    else if ( srclen == 1 )
    { 
        one = src[0] >> 2;
        two = ( src[0] & 0x03 ) << 4;
        *( dest + index++ ) = base64[one];
        *( dest + index++ ) = base64[two];
        *( dest + index++ ) = '=';
        *( dest + index   ) = '=';
    }
    return srclen >= 3 ? 3 : srclen;
}
// base64编码，传入的dest空间必须>=src * 4 / 3 + 3才能保证安全
void base64_encode( const unsigned char *src, int srclen, unsigned char *dest, int destlen )
{ 
    if ( NULL == src || srclen == 0 || NULL == dest || destlen < srclen * 4 / 3 + 3)
        return;
    unsigned char base64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    int index = 0;
    while ( srclen > 0 )
    { 
        int step = base64_string( src, srclen, dest, destlen, base64, index );
        if (step == -1) { 
            break;
        }
        destlen -= 4; // 每次用掉4字节
        index += 4; // 那么索引每次必然增加4
        srclen -= step; // 每次减少3字节或2或者1，2或1都是结尾剩余字符个数
        src   += step; // 那么每次增加3或者2或者1
    }
    *( dest + index ) = '\0';
}
// 以上是base64编码的函数
// 下面是base64解码的函数
void create_array( unsigned char *array )
{ 
    int index  = 0;
    for ( index  = 0; index < 123; index += 1 )
    { 
        if ( index == 0x2B )
        { 
            *( array + index ) = 0x3E;
        }
        else if ( index == 0x2F )
        { 
            *( array + index ) = 0x3F;
        }
        else if ( index >= 0x30 && index <= 0x39 )
        { 
            *( array + index ) = 0x34 + index - 0x30;
        }
        else if ( index == 0x3D )
        { 
            /* 特殊字符'='，也可合并到else情况 */
            *( array + index ) = 0x00;
        }
        else if ( index >= 0x41 && index <= 0x5A )
        { 
            *( array + index ) = index - 0x41;
        }
        else if ( index >= 0x61 && index <= 0x7A ) { 
            *( array + index ) = 0x1A + index - 0x61;
        }
        else
        { 
            *( array + index ) = 0x00;
        }
    }
    *( array + 123 ) = '\0';
}
int decode_string( const unsigned char *src, size_t srclen, unsigned char *dest, size_t destlen, unsigned char *array, int index )
{ 
    if ( NULL == src || NULL == dest || NULL == array || index < 0)
        return -1;
    if (srclen < 4 || destlen < 4) { 
        return -1; // // dest多留1个给结尾的0
    }
    int step = 0;
    unsigned char one;
    unsigned char two;
    unsigned char thr;
    if ( src[3] == '=' && src[2] == '=' )
    { 
        one = ( array[src[0]] << 2 ) | ( array[src[1]] >> 4 );
        *( dest + index ) = one;
        step = 1;
    }
    else if ( src[3] == '=' )
    { 
        one = ( array[src[0]] << 2 ) | ( array[src[1]] >> 4 );
        two = ( ( array[src[1]] & 0x0F ) << 4 ) | ( array[src[2]] >> 2 );
        *( dest + index++ ) = one;
        *( dest + index   ) = two;
        step = 2;
    }
    else
    { 
        one = ( array[src[0]] << 2 ) | ( array[src[1]] >> 4 );
        two = ( ( array[src[1]] & 0x0F ) << 4 ) | ( array[src[2]] >> 2 );
        thr = ( array[src[2]] << 6 ) | array[src[3]];
        *( dest + index++ ) = one;
        *( dest + index++ ) = two;
        *( dest + index   ) = thr;
        step = 3;
    }
    return step;
}
// base64解码，传入的dest空间=src空间 * 3 / 4 + 4即可满足要求
void base64_decode( const unsigned char *src, size_t srclen, unsigned char *dest, size_t destlen )
{ 
    if ( NULL == src || NULL == dest || srclen < 0 || destlen < srclen * 3 / 4 + 4)
        return;
    static unsigned char array[124] = { '\0'};
    if (array[0x2B] == '\0') {  // 需要初始化
        create_array( array );
    }
    int index = 0;
    while ( srclen )
    { 
        int step = decode_string( src, srclen, dest, destlen, array, index );
        if (step == -1) { 
            break;
        }
        srclen -= 4; // 每次解码4个字节
        src    += 4; // 源字符串前进4字节
        destlen -= step; // 每次消耗step（可能是3字节，也可能是2字节或1字节）
        index   += step; // 目标字符串每次前进step字节
    }
    *( dest + index ) = '\0';
}
// 下面是导出给MFC用的
CString Base64Encode(const unsigned char *src, size_t srclen)
{ 
    CString dest;
    srclen = srclen > 0 ? srclen : strlen((const char*)src);
    size_t destlen = srclen * 4 / 3 + 3;
    base64_encode(src, srclen, (unsigned char*)dest.GetBuffer(destlen), destlen);
    dest.ReleaseBuffer();
    return dest;
}
CString Base64Decode(const unsigned char *src, size_t srclen)
{ 
    CString dest;
    srclen = srclen > 0 ? srclen : strlen((const char*)src);
    size_t destlen = srclen * 3 / 4 + 4;
    base64_decode(src, srclen, (unsigned char*)dest.GetBuffer(destlen), destlen);
    dest.ReleaseBuffer();
    return dest;
}
#ifndef __BASE64_20200920_H__
#define __BASE64_20200920_H__
__declspec(dllexport) CString Base64Encode(const unsigned char *src, size_t srclen = 0);
__declspec(dllexport) CString Base64Decode(const unsigned char *src, size_t srclen = 0);
#endif