/*

* Public Key Work Factor Functions

* (C) 1999-2007,2012 Jack Lloyd

*

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

*/

#include <botan/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);

   }

}

size_t if_work_factor(size_t bits)

   {

   // 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)

   {

   /*

   This uses a slightly tweaked version of the standard work factor

   function above. It assumes k is 1 (thus overestimating the strength

   of the prime group by 5-6 bits), and always returns at least 128 bits

   (this only matters for very small primes).

   */

   const size_t min_workfactor = 64;

   const double log2_k = 0;

   return 2 * std::max<size_t>(min_workfactor, nfs_workfactor(bits, log2_k));

   }

}