Rewrite dl_work_factor using a lookup table with data from RFC 3526,

"More Modular Exponential (MODP) Diffie-Hellman groups for Internet
Key Exchange (IKE)", which removes Botan's dependency on standard math
library (which can be a big deal on embedded systems, and it seemed
silly to have just a single function cause us to pull in potentially
all of libm)

Also this makes the values Botan will pick for exponent sizes more
obvious; previously one would have to run through the computation or
call the function and observe the output.

1 files changed, 18 insertions, 16 deletions

diff --git a/src/util.cpp b/src/util.cpp
index e340ee7f4..dea0778a1 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -6,7 +6,6 @@
 #include <botan/util.h>
 #include <botan/bit_ops.h>
 #include <algorithm>
-#include <cmath>
 
 namespace Botan {
 
@@ -29,23 +28,26 @@ u32bit round_down(u32bit n, u32bit align_to)
    }
 
 /*************************************************
-* Return the work required for solving DL        *
+* Choose the exponent size for a DL group
 *************************************************/
-u32bit dl_work_factor(u32bit n_bits)
+u32bit dl_work_factor(u32bit bits)
    {
-   const u32bit MIN_ESTIMATE = 64;
-
-   if(n_bits < 32)
-      return 0;
-
-   const double log_x = n_bits / 1.44;
-
-   const double strength =
-      2.76 * std::pow(log_x, 1.0/3.0) * std::pow(std::log(log_x), 2.0/3.0);
-
-   if(strength > MIN_ESTIMATE)
-      return static_cast<u32bit>(strength);
-   return MIN_ESTIMATE;
+   /*
+   These values were taken from RFC 3526
+   */
+   if(bits <= 1536)
+      return 90;
+   else if(bits <= 2048)
+      return 110;
+   else if(bits <= 3072)
+      return 130;
+   else if(bits <= 4096)
+      return 150;
+   else if(bits <= 6144)
+      return 170;
+   else if(bits <= 8192)
+      return 190;
+   return 256;
    }
 
 /*************************************************