1 files changed, 63 insertions, 52 deletions

diff --git a/src/lib/pubkey/mce/gf2m_small_m.cpp b/src/lib/pubkey/mce/gf2m_small_m.cpp
index 3de748939..11da30962 100644
--- a/src/lib/pubkey/mce/gf2m_small_m.cpp
+++ b/src/lib/pubkey/mce/gf2m_small_m.cpp
@@ -9,14 +9,11 @@
 */
 
 #include <botan/gf2m_small_m.h>
-#include <botan/code_based_util.h>
 #include <string>
 #include <stdexcept>
 
 namespace Botan {
 
-namespace gf2m_small_m {
-
 #define MAX_EXT_DEG 16
 
 namespace {
@@ -41,78 +38,94 @@ unsigned int prim_poly[MAX_EXT_DEG + 1] = {
    0210013		/* extension degree 16 */
 };
 
-}
-
-u32bit encode_gf2m(gf2m to_enc, byte* mem)
+std::vector<gf2m> gf_exp_table(size_t deg, gf2m prime_poly)
    {
-   mem[0] = to_enc >> 8;
-   mem[1] = to_enc & 0xFF;
-   return sizeof(to_enc);
+   // construct the table gf_exp[i]=alpha^i
+
+   std::vector<gf2m> tab((1 << deg) + 1);
+
+   tab[0] = 1;
+   for(size_t i = 1; i < tab.size(); ++i)
+      {
+      const bool overflow = tab[i - 1] >> (deg - 1);
+      tab[i] = (tab[i-1] << 1) ^ (overflow ? prime_poly : 0);
+      }
+
+   return tab;
    }
 
-gf2m decode_gf2m(const byte* mem)
+const std::vector<gf2m>& exp_table(size_t deg)
    {
-   gf2m result;
-   result = mem[0] << 8;
-   result |= mem[1];
-   return result;
+   static std::vector<gf2m> tabs[MAX_EXT_DEG + 1];
+
+   if(deg < 2 || deg > MAX_EXT_DEG)
+      throw std::runtime_error("GF2m_Field does not support degree " + std::to_string(deg));
+
+   if(tabs[deg].empty())
+      tabs[deg] = gf_exp_table(deg, prim_poly[deg]);
+
+   return tabs[deg];
    }
 
-// construct the table gf_exp[i]=alpha^i
-void Gf2m_Field::init_exp()
+std::vector<gf2m> gf_log_table(size_t deg, const std::vector<gf2m>& exp)
    {
-   m_gf_exp_table.resize(1 << get_extension_degree());
+   std::vector<gf2m> tab(1 << deg);
 
-   m_gf_exp_table[0] = 1;
-   for(size_t i = 1; i < gf_ord(); ++i)
+   tab[0] = (1 << deg) - 1; // log of 0 is the order by convention
+   for (size_t i = 0; i < tab.size(); ++i)
       {
-      m_gf_exp_table[i] = m_gf_exp_table[i - 1] << 1;
-      if (m_gf_exp_table[i - 1] & (1 << (get_extension_degree()-1)))
-         {
-         m_gf_exp_table[i] ^= prim_poly[get_extension_degree()];
-         }
+      tab[exp[i]] = i;
       }
-
-   // hack for the multiplication
-   m_gf_exp_table[gf_ord()] = 1;
+   return tab;
    }
 
-// construct the table gf_log[alpha^i]=i
-void Gf2m_Field::init_log()
+const std::vector<gf2m>& log_table(size_t deg)
    {
-   m_gf_log_table.resize(1 << get_extension_degree());
+   static std::vector<gf2m> tabs[MAX_EXT_DEG + 1];
 
-   m_gf_log_table[0] = gf_ord(); // log of 0 par convention
-   for (size_t i = 0; i < gf_ord() ; ++i)
-      {
-      m_gf_log_table[m_gf_exp_table[i]] = i;
-      }
+   if(deg < 2 || deg > MAX_EXT_DEG)
+      throw std::runtime_error("GF2m_Field does not support degree " + std::to_string(deg));
+
+   if(tabs[deg].empty())
+      tabs[deg] = gf_log_table(deg, exp_table(deg));
+
+   return tabs[deg];
    }
 
+}
 
-Gf2m_Field::Gf2m_Field(size_t extdeg)
+u32bit encode_gf2m(gf2m to_enc, byte* mem)
    {
-   if(extdeg < 2 || extdeg > MAX_EXT_DEG)
-      throw std::runtime_error("Gf2m_Field does not support degree " + std::to_string(extdeg));
+   mem[0] = to_enc >> 8;
+   mem[1] = to_enc & 0xFF;
+   return sizeof(to_enc);
+   }
 
-   m_gf_extension_degree = extdeg;
-   m_gf_cardinality = 1 << extdeg;
-   m_gf_multiplicative_order = m_gf_cardinality - 1;
+gf2m decode_gf2m(const byte* mem)
+   {
+   gf2m result;
+   result = mem[0] << 8;
+   result |= mem[1];
+   return result;
+   }
 
-   init_exp();
-   init_log();
+GF2m_Field::GF2m_Field(size_t extdeg) : m_gf_extension_degree(extdeg),
+                                        m_gf_multiplicative_order((1 << extdeg) - 1),
+                                        m_gf_log_table(log_table(m_gf_extension_degree)),
+                                        m_gf_exp_table(exp_table(m_gf_extension_degree))
+   {
    }
 
-gf2m Gf2m_Field::gf_div(gf2m x, gf2m y)
+gf2m GF2m_Field::gf_div(gf2m x, gf2m y) const
    {
-   s32bit sub_res = static_cast<s32bit>(m_gf_log_table[x]) - static_cast<s32bit>( m_gf_log_table[y]);
-   s32bit modq_res = static_cast<s32bit>(_gf_modq_1(sub_res));
-   s32bit div_res = static_cast<s32bit>(x) ? static_cast<s32bit>(m_gf_exp_table[modq_res]) : 0;
+   const s32bit sub_res = static_cast<s32bit>(gf_log(x) - static_cast<s32bit>(gf_log(y)));
+   const s32bit modq_res = static_cast<s32bit>(_gf_modq_1(sub_res));
+   const s32bit div_res = static_cast<s32bit>(x) ? static_cast<s32bit>(gf_exp(modq_res)) : 0;
    return static_cast<gf2m>(div_res);
    }
 
 // we suppose i >= 0. Par convention 0^0 = 1
-gf2m Gf2m_Field::gf_pow(gf2m x, int i)
+gf2m GF2m_Field::gf_pow(gf2m x, int i) const
    {
    if (i == 0)
       return 1;
@@ -123,13 +136,11 @@ gf2m Gf2m_Field::gf_pow(gf2m x, int i)
       // i mod (q-1)
       while (i >> get_extension_degree())
          i = (i & (gf_ord())) + (i >> get_extension_degree());
-      i *= m_gf_log_table[x];
+      i *= gf_log(x);
       while (i >> get_extension_degree())
          i = (i & (gf_ord())) + (i >> get_extension_degree());
-      return m_gf_exp_table[i];
+      return gf_exp(i);
       }
    }
 
 }
-
-}