mbedtls之base64算法的學習

Recently, I have been looking at the differences between the processing of base64 encode and decode by RTOS nuttx and mbedtls. Here is the processing of base64 algorithm by mbedtls.

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
};

#define BASE64_SIZE_T_MAX   ( (size_t) -1 ) /* SIZE_T_MAX is not standard */

/*
 * Encode a buffer into base64 format
 */
int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
                   const unsigned char *src, size_t slen )
{
    size_t i, n;
    int C1, C2, C3;
    unsigned char *p;

    if( slen == 0 )
    {
        *olen = 0;
        return( 0 );
    }

    n = slen / 3 + ( slen % 3 != 0 );

    if( n > ( BASE64_SIZE_T_MAX - 1 ) / 4 )
    {
        *olen = BASE64_SIZE_T_MAX;
        return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
    }

    n *= 4;

    if( ( dlen < n + 1 ) || ( NULL == dst ) )
    {
    	/*This will be the buf size that you need to malloc when 	
    	your dst is NULL*/
        *olen = n + 1;
        return( MBEDTLS_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++ = '=';
    }

    *olen = p - dst;
    *p = 0;

    return( 0 );
}

/*
 * Decode a base64-formatted buffer
 */
int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
                   const unsigned char *src, size_t slen )
{
    size_t i, n;
    uint32_t j, x;
    unsigned char *p;

    /* First pass: check for validity and get output length */
    for( i = n = j = 0; i < slen; i++ )
    {
        /* Skip spaces before checking for EOL */
        x = 0;
        while( i < slen && src[i] == ' ' )
        {
            ++i;
            ++x;
        }

        /* Spaces at end of buffer are OK */
        if( i == slen )
            break;

        if( ( slen - i ) >= 2 &&
            src[i] == '\r' && src[i + 1] == '\n' )
            continue;

        if( src[i] == '\n' )
            continue;

        /* Space inside a line is an error */
        if( x != 0 )
            return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );

        if( src[i] == '=' && ++j > 2 )
            return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );

        if( src[i] > 127 || base64_dec_map[src[i]] == 127 )
            return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );

        if( base64_dec_map[src[i]] < 64 && j != 0 )
            return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );

        n++;
    }

    if( n == 0 )
    {
        *olen = 0;
        return( 0 );
    }

    /* The following expression is to calculate the following formula without
     * risk of integer overflow in n:
     *     n = ( ( n * 6 ) + 7 ) >> 3;
     */
    n = ( 6 * ( n >> 3 ) ) + ( ( 6 * ( n & 0x7 ) + 7 ) >> 3 );
    n -= j;

    if( dst == NULL || dlen < n )
    {
    	/*This will be the buf size that you need to malloc when 	
    	your dst is NULL*/
        *olen = n;
        return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
    }

   for( j = 3, n = x = 0, p = dst; i > 0; i--, src++ )
   {
        if( *src == '\r' || *src == '\n' || *src == ' ' )
            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       );
        }
    }

    *olen = p - dst;

    return( 0 );
}

When you need to estimate the size of the DST buf you need， you first need to call the mbedtls_base64_encode(NULL, 0, &len, src, slen) function in such a way that you will get the size of the buf needed.
Next you need to apply for a block of memory and then call mbedtls_base64_encode again to encode the SRC data.

The same is true for decode’s process.