1 files changed, 178 insertions, 0 deletions
diff --git a/src/bigint/powm_mnt.cpp b/src/bigint/powm_mnt.cpp
new file mode 100644
index 000000000..6091d467a
--- /dev/null
+++ b/src/bigint/powm_mnt.cpp
@@ -0,0 +1,178 @@
+/*************************************************
+* Montgomery Exponentiation Source File          *
+* (C) 1999-2007 Jack Lloyd                       *
+*************************************************/
+
+#include <botan/def_powm.h>
+#include <botan/numthry.h>
+#include <botan/mp_core.h>
+
+namespace Botan {
+
+namespace {
+
+/*************************************************
+* Try to choose a good window size               *
+*************************************************/
+u32bit choose_window_bits(u32bit exp_bits, u32bit,
+                          Power_Mod::Usage_Hints hints)
+   {
+   static const u32bit wsize[][2] = {
+      { 2048, 4 }, { 1024, 3 }, { 256, 2 }, { 128, 1 }, { 0, 0 }
+   };
+
+   u32bit window_bits = 1;
+
+   if(exp_bits)
+      {
+      for(u32bit j = 0; wsize[j][0]; ++j)
+         {
+         if(exp_bits >= wsize[j][0])
+            {
+            window_bits += wsize[j][1];
+            break;
+            }
+         }
+      }
+
+   if(hints & Power_Mod::BASE_IS_FIXED)
+      window_bits += 2;
+   if(hints & Power_Mod::EXP_IS_LARGE)
+      ++window_bits;
+
+   return window_bits;
+   }
+
+/*************************************************
+* Montgomery Reduction                           *
+*************************************************/
+inline void montgomery_reduce(BigInt& out, MemoryRegion<word>& z_buf,
+                              const BigInt& x_bn, u32bit x_size, word u)
+   {
+   const word* x = x_bn.data();
+   word* z = z_buf.begin();
+   u32bit z_size = z_buf.size();
+
+   bigint_monty_redc(z, z_size, x, x_size, u);
+
+   out.get_reg().set(z + x_size, x_size + 1);
+   }
+
+}
+
+/*************************************************
+* Set the exponent                               *
+*************************************************/
+void Montgomery_Exponentiator::set_exponent(const BigInt& exp)
+   {
+   this->exp = exp;
+   exp_bits = exp.bits();
+   }
+
+/*************************************************
+* Set the base                                   *
+*************************************************/
+void Montgomery_Exponentiator::set_base(const BigInt& base)
+   {
+   window_bits = choose_window_bits(exp.bits(), base.bits(), hints);
+
+   g.resize((1 << window_bits) - 1);
+
+   SecureVector<word> z(2 * (mod_words + 1));
+   SecureVector<word> workspace(z.size());
+
+   g[0] = (base >= modulus) ? (base % modulus) : base;
+   bigint_mul(z.begin(), z.size(), workspace,
+              g[0].data(), g[0].size(), g[0].sig_words(),
+              R2.data(), R2.size(), R2.sig_words());
+
+   montgomery_reduce(g[0], z, modulus, mod_words, mod_prime);
+
+   const BigInt& x = g[0];
+   const u32bit x_sig = x.sig_words();
+
+   for(u32bit j = 1; j != g.size(); ++j)
+      {
+      const BigInt& y = g[j-1];
+      const u32bit y_sig = y.sig_words();
+
+      z.clear();
+      bigint_mul(z.begin(), z.size(), workspace,
+                 x.data(), x.size(), x_sig,
+                 y.data(), y.size(), y_sig);
+
+      montgomery_reduce(g[j], z, modulus, mod_words, mod_prime);
+      }
+   }
+
+/*************************************************
+* Compute the result                             *
+*************************************************/
+BigInt Montgomery_Exponentiator::execute() const
+   {
+   const u32bit exp_nibbles = (exp_bits + window_bits - 1) / window_bits;
+
+   BigInt x = R_mod;
+   SecureVector<word> z(2 * (mod_words + 1));
+   SecureVector<word> workspace(2 * (mod_words + 1));
+
+   for(u32bit j = exp_nibbles; j > 0; --j)
+      {
+      for(u32bit k = 0; k != window_bits; ++k)
+         {
+         z.clear();
+         bigint_sqr(z.begin(), z.size(), workspace,
+                    x.data(), x.size(), x.sig_words());
+
+         montgomery_reduce(x, z, modulus, mod_words, mod_prime);
+         }
+
+      u32bit nibble = exp.get_substring(window_bits*(j-1), window_bits);
+      if(nibble)
+         {
+         const BigInt& y = g[nibble-1];
+
+         z.clear();
+         bigint_mul(z.begin(), z.size(), workspace,
+                    x.data(), x.size(), x.sig_words(),
+                    y.data(), y.size(), y.sig_words());
+
+         montgomery_reduce(x, z, modulus, mod_words, mod_prime);
+         }
+      }
+
+   z.clear();
+   z.copy(x.data(), x.size());
+
+   montgomery_reduce(x, z, modulus, mod_words, mod_prime);
+   return x;
+   }
+
+/*************************************************
+* Montgomery_Exponentiator Constructor           *
+*************************************************/
+Montgomery_Exponentiator::Montgomery_Exponentiator(const BigInt& mod,
+   Power_Mod::Usage_Hints hints)
+   {
+   if(!mod.is_positive())
+      throw Exception("Montgomery_Exponentiator: modulus must be positive");
+   if(mod.is_even())
+      throw Exception("Montgomery_Exponentiator: modulus must be odd");
+
+   window_bits = 0;
+   this->hints = hints;
+   modulus = mod;
+
+   mod_words = modulus.sig_words();
+
+   BigInt mod_prime_bn(BigInt::Power2, MP_WORD_BITS);
+   mod_prime = (mod_prime_bn - inverse_mod(modulus, mod_prime_bn)).word_at(0);
+
+   R_mod = BigInt(BigInt::Power2, MP_WORD_BITS * mod_words);
+   R_mod %= modulus;
+
+   R2 = BigInt(BigInt::Power2, 2 * MP_WORD_BITS * mod_words);
+   R2 %= modulus;
+   }
+
+}