#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main()
{
    unsigned char f[12] ="B25678C57F3E";
    //unsigned char* ff=(unsigned char*)malloc(17*sizeof(unsigned char));
    unsigned char ff[16];
    int k=sizeof(f)/sizeof(f[0]);
    int kk=0;
    int i=0;
    for (i=0; i<k;i++) {
    if ((i==2)||(i==4)||(i==6)||(i==8)||(i==10)) {ff[i+kk]=":";
    kk++; }
    else ff[i+kk]=f[i];
    }
    printf("%i\n",k);
    printf(f);
    printf("\n");
    printf(ff);
    return 0;
}

    int j=0;
    for (j=0; j<17; j++) {
    if ((j==2)||(j==5)||(j==8)||(j=11)||(j==14)) {ff[j]=":"; // ff[i+kk]=":";
    l++; }
    //else ff[i+kk]=f[i];
    else ff[j]=f[j-l];
    printf(ff);
    printf("\n");
    printf("%i\n",l);
    }