1 files changed, 97 insertions, 0 deletions
diff --git a/botan/src/math/numbertheory/reducer.cpp b/botan/src/math/numbertheory/reducer.cpp
new file mode 100644
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+++ b/botan/src/math/numbertheory/reducer.cpp
@@ -0,0 +1,97 @@
+/*
+* Modular Reducer
+* (C) 1999-2007 Jack Lloyd
+*
+* Distributed under the terms of the Botan license
+*/
+
+#include <botan/reducer.h>
+#include <botan/numthry.h>
+#include <botan/mp_core.h>
+
+namespace Botan {
+
+/*
+* Modular_Reducer Constructor
+*/
+Modular_Reducer::Modular_Reducer(const BigInt& mod)
+   {
+   if(mod <= 0)
+      throw Invalid_Argument("Modular_Reducer: modulus must be positive");
+
+   modulus = mod;
+   mod_words = modulus.sig_words();
+
+   modulus_2 = Botan::square(modulus);
+   mod2_words = modulus_2.sig_words();
+
+   mu = BigInt(BigInt::Power2, 2 * MP_WORD_BITS * mod_words) / modulus;
+   mu_words = mu.sig_words();
+   }
+
+/*
+* Barrett Reduction
+*/
+BigInt Modular_Reducer::reduce(const BigInt& x) const
+   {
+   if(mod_words == 0)
+      throw Invalid_State("Modular_Reducer: Never initalized");
+
+   BigInt t1 = x;
+   t1.set_sign(BigInt::Positive);
+
+   if(t1 < modulus)
+      {
+      if(x.is_negative() && t1.is_nonzero())
+         return modulus - t1;
+      return x;
+      }
+
+   if(t1 >= modulus_2)
+      return (x % modulus);
+
+   t1 >>= (MP_WORD_BITS * (mod_words - 1));
+   t1 *= mu;
+   t1 >>= (MP_WORD_BITS * (mod_words + 1));
+
+   t1 *= modulus;
+   t1.mask_bits(MP_WORD_BITS * (mod_words+1));
+
+   BigInt t2 = x;
+   t2.set_sign(BigInt::Positive);
+   t2.mask_bits(MP_WORD_BITS * (mod_words+1));
+
+   t1 = t2 - t1;
+
+   if(t1.is_negative())
+      {
+      BigInt b_to_k1(BigInt::Power2, MP_WORD_BITS * (mod_words+1));
+      t1 += b_to_k1;
+      }
+
+   while(t1 >= modulus)
+      t1 -= modulus;
+
+   if(x.is_negative() && t1.is_nonzero())
+      t1 = modulus - t1;
+
+   return t1;
+   }
+
+/*
+* Multiply, followed by a reduction
+*/
+BigInt Modular_Reducer::multiply(const BigInt& x, const BigInt& y) const
+   {
+   return reduce(x * y);
+   }
+
+/*
+* Square, followed by a reduction
+*/
+BigInt Modular_Reducer::square(const BigInt& x) const
+   {
+   return reduce(Botan::square(x));
+   }
+
+}