edd: Entropy DDoS Detector

 * Author: comotion@users.sf.net
 * Idea by breno.silva@gmail.com
 http://lists.openinfosecfoundation.org/pipermail/oisf-wg-portscan/2009-October/000023.html
 * Thanks to ebf0 (http://www.gamelinux.org/) whose support code is good enough
 * License? If U have the code U have the GPL... >:-P

Entropy  H(P1) + H(P2) + ... + H(Pi) > H(P1P2..Pi) => DDOS


Pseudo code:
   # can segment into destport or src:port:dst:port
   for each packet
      count set bits
      count packet bits
      sum_entropy = Entropy(packet);
      track window of n last packets{
         increase set & total bit values
         if(H(window) > H(p1p2..pwin)
            => DDOS!
      }

/* calculate the simple entropy of a packet, ie
 * assume all bits are equiprobable and randomly distributed
 *
 * needs work: do this with pure, positive ints?
 * tresholds? markov chains? averaging?
 * SIMD bitcounts?
 * 
 * check this with our friend the kolmogorov
 */

Special: 
 - uses simplistic approximation of Entropy: bits set
 - uses very fast implementation of bitcount (lookup table or parallel)
 - counts Entropy for all packets, does not segment on port