[COLUG] Enterprise Two Factor Authentication: $j=hexdec(md5($j));

[Jim]

On Wed, 14 Mar 2007 09:45:06 +1100 Duane <duane at cacert.org> wrote:

> [binary output from md5()] only became an option in php5 
> and later and a lot of php servers out there still use php4 or 
> even 3 in some cases.

OK. 

Even so, since we know that length of the output of md5sum fits 
in a 32-bit int, I still see simplification: 

   function encode($j,$nBitsPerEncodedChar,$encoding)
   {
      if (strlen($encoding)!=(1<<$nBitsPerEncodedChar)) {
         echo stderr "strlen(\"$encoding\")!=(1<<$nBitsPerEncodedChar)\n"; 
         exit(EXIT_FAILURE);
      }

      $j=hexdec(md5($j));

      /* Each loop converts $nBitsPerEncodedChar least significant bits of $j
      *  to character from $encoding. */
      $s="";
      for ($i=32;$i>0;$i-=$nBitsPerEncodedChar) {
         $s=$encoding[$j & ((1<<$nBitsPerEncodedChar)-1)].$s;
         $j>>=$nBitsPerEncodedChar;
         $j&=((1<<$i)-1); /* to avoid issue of possible arithmetic shift right filling with 1s */
      }

      return $s;
   }