/*

* Public Key Work Factor Functions

* (C) 1999-2007,2012 Jack Lloyd

*

* Botan is released under the Simplified BSD License (see license.txt)

*/

#include <botan/internal/workfactor.h>

#include <algorithm>

#include <cmath>

namespace Botan {

size_t ecp_work_factor(size_t bits) {

   return bits / 2;

}

namespace {

size_t nfs_workfactor(size_t bits, double log2_k) {

   // approximates natural logarithm of an integer of given bitsize

   const double log2_e = 1.44269504088896340736;

   const double log_p = bits / log2_e;

   const double log_log_p = std::log(log_p);

   // RFC 3766: k * e^((1.92 + o(1)) * cubrt(ln(n) * (ln(ln(n)))^2))

   const double est = 1.92 * std::pow(log_p * log_log_p * log_log_p, 1.0 / 3.0);

   // return log2 of the workfactor

   return static_cast<size_t>(log2_k + log2_e * est);

}

}  // namespace

size_t if_work_factor(size_t bits) {

   if(bits < 512) {

      return 0;

   }

   // RFC 3766 estimates k at .02 and o(1) to be effectively zero for sizes of interest

   const double log2_k = -5.6438;  // log2(.02)

   return nfs_workfactor(bits, log2_k);

}

size_t dl_work_factor(size_t bits) {

   // Lacking better estimates...

   return if_work_factor(bits);

}

size_t dl_exponent_size(size_t bits) {

   if(bits == 0) {

      return 0;

   }

   if(bits <= 256) {

      return bits - 1;

   }

   if(bits <= 1024) {

      return 192;

   }

   if(bits <= 1536) {

      return 224;

   }

   if(bits <= 2048) {

      return 256;

   }

   if(bits <= 4096) {

      return 384;

   }

   return 512;

}

}  // namespace Botan