How to encode and decode base64 strings in C?

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5

I need to encode (and decode) a JSON to be transmitted from the web to a microcontroller. The% w_that I am sending is encrypted with JSON , but cyphertext contains unprintable characters and I have some problems with this. For example, when I try to receive this data in the microcontroller the decryption does not take place correctly. However, if I encrypt and decrypt a string inside the microcontroller, the process normally occurs.

I believe the problem is character encoding because there are nonprinting characters that are not transmitted. I need to do the following steps in exactly this order:

  • Encrypt text using AES 128
  • Encode cyphertext in base64 to be transmitted via the Web
  • Receiving the data in the microcontroller
  • Decrypt cyphertext in base64, for its natural representation
  • Decrypting cyphertext using AES 128

    • My question is: how to encode and decode strings AES 128 into base64 ? Is there a library for this?

    Edit: Is there any optimized way to do this? I found an example that contains the algorithm that decodes, but it uses a huge array to do this.

        
    asked by anonymous 01.09.2016 / 04:55

    1 answer

    5

    Avelino, do not judge a code by its arrays!

    The example you found is based on the file ap_base64.c of the project APR - Apache Portable Runtime ( see here the original code ), which in turn, is very widespread and used in several other projects. It was implemented in pure C and does not depend on any other external libraries.

    This huge matrix full of magic numbers you mentioned is capable of delivering a considerable increase in data processing performance in base64 , which makes it quite feasible.

    My suggestion is: Do not be prejudiced with this code, it is fast, portable and very efficient.

    The code itself is rewritten so that it works only with ASCII:

    Header base64.h : 

    /*
    base64.h
*/

#ifndef __BASE64_H__
#define __BASE64_H__

int base64decode_len( const char * bufcoded );
int base64decode( char * bufplain, const char * bufcoded );

int base64encode_len( int len );
int base64encode( char * encoded, const char * string, int len );

#endif

/* fim-de-arquivo */

    Implementation base64.c : 

    /*
    base64.c
*/

#include <string.h>

#include "base64.h"


static int base64decode_binary( unsigned char * bufplain, const char * bufcoded );
static int base64encode_binary( char * encoded, const unsigned char * string, int len );


static const char basis_64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

static const unsigned char pr2six[256] =
{
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64,
    64,  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, 64, 64, 64, 64, 64,
    64, 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, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64
};


static int base64decode_binary( unsigned char * bufplain, const char * bufcoded )
{
    int nbytesdecoded;
    register const unsigned char *bufin;
    register unsigned char *bufout;
    register int nprbytes;

    bufin = (const unsigned char *) bufcoded;

    while (pr2six[*(bufin++)] <= 63);

    nprbytes = (bufin - (const unsigned char *) bufcoded) - 1;
    nbytesdecoded = ((nprbytes + 3) / 4) * 3;

    bufout = (unsigned char *) bufplain;
    bufin = (const unsigned char *) bufcoded;

    while (nprbytes > 4)
    {
        *(bufout++) = (unsigned char) (pr2six[*bufin] << 2 | pr2six[bufin[1]] >> 4);
        *(bufout++) = (unsigned char) (pr2six[bufin[1]] << 4 | pr2six[bufin[2]] >> 2);
        *(bufout++) = (unsigned char) (pr2six[bufin[2]] << 6 | pr2six[bufin[3]]);

        bufin += 4;
        nprbytes -= 4;
    }

    /* Note: (nprbytes == 1) would be an error, so just ingore that case */
    if (nprbytes > 1)
    {
        *(bufout++) = (unsigned char) (pr2six[*bufin] << 2 | pr2six[bufin[1]] >> 4);
    }

    if (nprbytes > 2)
    {
        *(bufout++) = (unsigned char) (pr2six[bufin[1]] << 4 | pr2six[bufin[2]] >> 2);
    }

    if (nprbytes > 3)
    {
        *(bufout++) = (unsigned char) (pr2six[bufin[2]] << 6 | pr2six[bufin[3]]);
    }

    *(bufout++) = '
    /*
    base64.h
*/

#ifndef __BASE64_H__
#define __BASE64_H__

int base64decode_len( const char * bufcoded );
int base64decode( char * bufplain, const char * bufcoded );

int base64encode_len( int len );
int base64encode( char * encoded, const char * string, int len );

#endif

/* fim-de-arquivo */

    ';
    nbytesdecoded -= (4 - nprbytes) & 3;

    return nbytesdecoded;
}


static int base64encode_binary(char *encoded, const unsigned char *string, int len )
{
    int i;
    char *p;

    p = encoded;

    for (i = 0; i < len - 2; i += 3)
    {
        *p++ = basis_64[(string[i] >> 2) & 0x3F];
        *p++ = basis_64[((string[i] & 0x3) << 4) | ((int) (string[i + 1] & 0xF0) >> 4)];
        *p++ = basis_64[((string[i + 1] & 0xF) << 2) | ((int) (string[i + 2] & 0xC0) >> 6)];
        *p++ = basis_64[string[i + 2] & 0x3F];
    }

    if (i < len)
    {
        *p++ = basis_64[(string[i] >> 2) & 0x3F];

        if (i == (len - 1))
        {
            *p++ = basis_64[((string[i] & 0x3) << 4)];
            *p++ = '=';
        }
        else
        {
            *p++ = basis_64[((string[i] & 0x3) << 4) | ((int) (string[i + 1] & 0xF0) >> 4)];
            *p++ = basis_64[((string[i + 1] & 0xF) << 2)];
        }

        *p++ = '=';
    }

    *p++ = '
    /*
    base64.c
*/

#include <string.h>

#include "base64.h"


static int base64decode_binary( unsigned char * bufplain, const char * bufcoded );
static int base64encode_binary( char * encoded, const unsigned char * string, int len );


static const char basis_64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

static const unsigned char pr2six[256] =
{
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64,
    64,  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, 64, 64, 64, 64, 64,
    64, 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, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
    64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64
};


static int base64decode_binary( unsigned char * bufplain, const char * bufcoded )
{
    int nbytesdecoded;
    register const unsigned char *bufin;
    register unsigned char *bufout;
    register int nprbytes;

    bufin = (const unsigned char *) bufcoded;

    while (pr2six[*(bufin++)] <= 63);

    nprbytes = (bufin - (const unsigned char *) bufcoded) - 1;
    nbytesdecoded = ((nprbytes + 3) / 4) * 3;

    bufout = (unsigned char *) bufplain;
    bufin = (const unsigned char *) bufcoded;

    while (nprbytes > 4)
    {
        *(bufout++) = (unsigned char) (pr2six[*bufin] << 2 | pr2six[bufin[1]] >> 4);
        *(bufout++) = (unsigned char) (pr2six[bufin[1]] << 4 | pr2six[bufin[2]] >> 2);
        *(bufout++) = (unsigned char) (pr2six[bufin[2]] << 6 | pr2six[bufin[3]]);

        bufin += 4;
        nprbytes -= 4;
    }

    /* Note: (nprbytes == 1) would be an error, so just ingore that case */
    if (nprbytes > 1)
    {
        *(bufout++) = (unsigned char) (pr2six[*bufin] << 2 | pr2six[bufin[1]] >> 4);
    }

    if (nprbytes > 2)
    {
        *(bufout++) = (unsigned char) (pr2six[bufin[1]] << 4 | pr2six[bufin[2]] >> 2);
    }

    if (nprbytes > 3)
    {
        *(bufout++) = (unsigned char) (pr2six[bufin[2]] << 6 | pr2six[bufin[3]]);
    }

    *(bufout++) = '%pre%';
    nbytesdecoded -= (4 - nprbytes) & 3;

    return nbytesdecoded;
}


static int base64encode_binary(char *encoded, const unsigned char *string, int len )
{
    int i;
    char *p;

    p = encoded;

    for (i = 0; i < len - 2; i += 3)
    {
        *p++ = basis_64[(string[i] >> 2) & 0x3F];
        *p++ = basis_64[((string[i] & 0x3) << 4) | ((int) (string[i + 1] & 0xF0) >> 4)];
        *p++ = basis_64[((string[i + 1] & 0xF) << 2) | ((int) (string[i + 2] & 0xC0) >> 6)];
        *p++ = basis_64[string[i + 2] & 0x3F];
    }

    if (i < len)
    {
        *p++ = basis_64[(string[i] >> 2) & 0x3F];

        if (i == (len - 1))
        {
            *p++ = basis_64[((string[i] & 0x3) << 4)];
            *p++ = '=';
        }
        else
        {
            *p++ = basis_64[((string[i] & 0x3) << 4) | ((int) (string[i + 1] & 0xF0) >> 4)];
            *p++ = basis_64[((string[i + 1] & 0xF) << 2)];
        }

        *p++ = '=';
    }

    *p++ = '%pre%';

    return p - encoded;
}


int base64decode_len( const char * bufcoded )
{
    int nbytesdecoded;
    register const unsigned char *bufin;
    register int nprbytes;

    bufin = (const unsigned char *) bufcoded;

    while (pr2six[*(bufin++)] <= 63);

    nprbytes = (bufin - (const unsigned char *) bufcoded) - 1;
    nbytesdecoded = ((nprbytes + 3) / 4) * 3;

    return nbytesdecoded + 1;
}


int base64decode( char * bufplain, const char * bufcoded )
{
    int len;
    len = base64decode_binary((unsigned char *) bufplain, bufcoded);
    return len;
}


int base64encode_len( int len )
{
    return ((len + 2) / 3 * 4) + 1;
}


int base64encode( char * encoded, const char * string, int len )
{
    return base64encode_binary(encoded, (const unsigned char *) string, len);
}

/* fim-de-arquivo */

    ';

    return p - encoded;
}


int base64decode_len( const char * bufcoded )
{
    int nbytesdecoded;
    register const unsigned char *bufin;
    register int nprbytes;

    bufin = (const unsigned char *) bufcoded;

    while (pr2six[*(bufin++)] <= 63);

    nprbytes = (bufin - (const unsigned char *) bufcoded) - 1;
    nbytesdecoded = ((nprbytes + 3) / 4) * 3;

    return nbytesdecoded + 1;
}


int base64decode( char * bufplain, const char * bufcoded )
{
    int len;
    len = base64decode_binary((unsigned char *) bufplain, bufcoded);
    return len;
}


int base64encode_len( int len )
{
    return ((len + 2) / 3 * 4) + 1;
}


int base64encode( char * encoded, const char * string, int len )
{
    return base64encode_binary(encoded, (const unsigned char *) string, len);
}

/* fim-de-arquivo */
        
    01.09.2016 / 15:01
    Branch x Checkout x Automatic Merging for master What is the purpose of string.maketrans?