#include <stdio.h>
#include <ctype.h>

unsigned char *foo(unsigned char *dst, const unsigned char *src)
{
   static const unsigned char nibble[] = 
   {
      '0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'
   };
   unsigned char *start = dst;
   while ( *src )
   {
      *dst++ = nibble [ *src >> 4    ];
      *dst++ = nibble [ *src++ & 0xF ];
   }
   *dst = '\0';
   return start;
}

unsigned char *bar(unsigned char *dst, const unsigned char *src)
{
   unsigned char *start = dst;
   while ( *src )
   {
      if ( isdigit(*src) )
      {
         *dst = *src++ - '0';
      }
      else
      {
         *dst = *src++ - 'A' + 10;
      }
      *dst <<= 4;
      if ( isdigit(*src) )
      {
         *dst++ += *src++ - '0';
      }
      else
      {
         *dst++ += *src++ - 'A' + 10;
      }
   }
   *dst = '\0';
   return start;
}

int main(void)
{
   const unsigned char abc[] = "ABCMNO";
   unsigned char over [ sizeof abc * 2  - 1 ], back [ sizeof abc ];
   printf("\"%s\" -> \"%s\"\n", abc,  foo(over, abc));
   printf("\"%s\" -> \"%s\"\n", over, bar(back, over));
   return 0;
}

/* my output
"ABCMNO" -> "4142434D4E4F"
"4142434D4E4F" -> "ABCMNO"
*/