C语言实现base64编解码

#ifndef  BASE64_H
#define BASE64_H


#include <string.h>


#define ERR_BASE64_BUFFER_TOO_SMALL               -0x002A  /**< Output buffer too small. */
#define ERR_BASE64_INVALID_CHARACTER              -0x002C  /**< Invalid character in input. */


#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 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( unsigned char *dst, size_t *dlen,
                   const unsigned char *src, size_t slen );


/**
 * \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, ERR_BASE64_BUFFER_TOO_SMALL, or
 *                 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( unsigned char *dst, size_t *dlen,
                   const unsigned char *src, size_t slen );


/**
 * \brief          Checkup routine
 *
 * \return         0 if successful, or 1 if the test failed
 */
int base64_self_test( int verbose );


#ifdef __cplusplus
}
#endif


#endif /* base64.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( unsigned char *dst, size_t *dlen,
                   const unsigned char *src, size_t slen )
{
    size_t 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( 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 = p - dst;
    *p = 0;


    return( 0 );
}


/*
 * Decode a base64-formatted buffer
 */
int base64_decode( unsigned char *dst, size_t *dlen,
                   const unsigned char *src, size_t slen )
{
    size_t 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( ERR_BASE64_INVALID_CHARACTER );


        if( src[i] > 127 || base64_dec_map[src[i]] == 127 )
            return( ERR_BASE64_INVALID_CHARACTER );


        if( base64_dec_map[src[i]] < 64 && j != 0 )
            return( ERR_BASE64_INVALID_CHARACTER );


        n++;
    }


    if( n == 0 )
        return( 0 );


    n = ((n * 6) + 7) >> 3;


    if( *dlen < n )
    {
        *dlen = n;
        return( 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 = p - dst;


    return( 0 );
}