Decodicacion Base64 en C ...

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

//T W F u
#define _T ( tmp[0+offset] & 0x3F)
#define _W ( tmp[1+offset] & 0x3F)
#define _F ( tmp[2+offset] & 0x3F)
#define _u ( tmp[3+offset] & 0x3F)

//M a n
#define _M ((_T<<2) | (_W>>4))
#define _a ((_W <<4) | (_F >>2))
#define _n (((_F&0x03)<<6) | _u)


int main (){

    int offset=0;//Gestiona por bloques de 4
    int pto=0;//Gestiona por bloques de 3 es el destino
    int menos=0;//elimina = del final
    char *decodificador="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    char *Codificado="TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGludWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=";

/*
Man is distinguished, not only by his reason, but by this singular passion from other animals,
which is a lust of the mind, that by a perseverance of delight in the continued and indefatigable
generation of knowledge, exceeds the short vehemence of any carnal pleasure.
*/

    //char *Codificado="TWFu";//Man
    //char *Codificado="TWFuTWFuTWFuTWFu";//Man

    char *tmp= (char*) calloc (strlen (Codificado)+2,sizeof (char*));
    if (!tmp){printf ("Error calloc \n");return 0;}


    tmp=memcpy (tmp,Codificado,strlen(Codificado)-menos);


    char *claro= (char*) calloc (strlen (Codificado)+4,sizeof (char*));
    if (!claro){printf ("Error calloc \n");return 0;}


    //printf ("%s",tmp);


    //Decodificacion de la base 64
    for (int i=0;i<strlen(Codificado);i++){//Recorre el string
        //printf (" Caracter =%c ",Codificado[i]);
        for(int pos=0;pos < (strlen (decodificador));pos++){//Busca el caracter
        //printf (" Caracter =%c  conpara=%c\n",Codificado[i],decodificador[pos]);
            if (tmp[i]==decodificador[pos]){//Lo ha encontrado el caracter
                //printf (" , %d \n",pos);
                tmp[i]=(unsigned char)pos;
                break;
            }
        }
    }

    //Elimina los ==
    if (tmp[strlen(tmp)-1]=='='){tmp[strlen(tmp)-1]='\0';}
    if (tmp[strlen(tmp)-2]=='='){tmp[strlen(tmp)-2]='\0';}

    //Texto limpio original salta de 4 en     4 el destino de 3 en tres
    for (offset=0; offset<(strlen (tmp)); offset+=4 ,pto+=3){
        claro[0+pto]=_M;
        claro[1+pto]=_a;
        claro[2+pto]=_n;
    }


    printf ("%s",claro);
    if (tmp) {free (tmp);}
    return 0;
}