1 files changed, 89 insertions, 0 deletions
diff --git a/src/core/elg_op.cpp b/src/core/elg_op.cpp
new file mode 100644
index 000000000..0d852d145
--- /dev/null
+++ b/src/core/elg_op.cpp
@@ -0,0 +1,89 @@
+/*************************************************
+* ElGamal Operations Source File                 *
+* (C) 1999-2007 Jack Lloyd                       *
+*************************************************/
+
+#include <botan/eng_def.h>
+#include <botan/pow_mod.h>
+#include <botan/numthry.h>
+#include <botan/reducer.h>
+
+namespace Botan {
+
+namespace {
+
+/*************************************************
+* Default ElGamal Operation                      *
+*************************************************/
+class Default_ELG_Op : public ELG_Operation
+   {
+   public:
+      SecureVector<byte> encrypt(const byte[], u32bit, const BigInt&) const;
+      BigInt decrypt(const BigInt&, const BigInt&) const;
+
+      ELG_Operation* clone() const { return new Default_ELG_Op(*this); }
+
+      Default_ELG_Op(const DL_Group&, const BigInt&, const BigInt&);
+   private:
+      const BigInt p;
+      Fixed_Base_Power_Mod powermod_g_p, powermod_y_p;
+      Fixed_Exponent_Power_Mod powermod_x_p;
+      Modular_Reducer mod_p;
+   };
+
+/*************************************************
+* Default_ELG_Op Constructor                     *
+*************************************************/
+Default_ELG_Op::Default_ELG_Op(const DL_Group& group, const BigInt& y,
+                               const BigInt& x) : p(group.get_p())
+   {
+   powermod_g_p = Fixed_Base_Power_Mod(group.get_g(), p);
+   powermod_y_p = Fixed_Base_Power_Mod(y, p);
+   mod_p = Modular_Reducer(p);
+
+   if(x != 0)
+      powermod_x_p = Fixed_Exponent_Power_Mod(x, p);
+   }
+
+/*************************************************
+* Default ElGamal Encrypt Operation              *
+*************************************************/
+SecureVector<byte> Default_ELG_Op::encrypt(const byte in[], u32bit length,
+                                           const BigInt& k) const
+   {
+   BigInt m(in, length);
+   if(m >= p)
+      throw Invalid_Argument("Default_ELG_Op::encrypt: Input is too large");
+
+   BigInt a = powermod_g_p(k);
+   BigInt b = mod_p.multiply(m, powermod_y_p(k));
+
+   SecureVector<byte> output(2*p.bytes());
+   a.binary_encode(output + (p.bytes() - a.bytes()));
+   b.binary_encode(output + output.size() / 2 + (p.bytes() - b.bytes()));
+   return output;
+   }
+
+/*************************************************
+* Default ElGamal Decrypt Operation              *
+*************************************************/
+BigInt Default_ELG_Op::decrypt(const BigInt& a, const BigInt& b) const
+   {
+   if(a >= p || b >= p)
+      throw Invalid_Argument("Default_ELG_Op: Invalid message");
+
+   return mod_p.multiply(b, inverse_mod(powermod_x_p(a), p));
+   }
+
+}
+
+/*************************************************
+* Acquire an ElGamal op                          *
+*************************************************/
+ELG_Operation* Default_Engine::elg_op(const DL_Group& group, const BigInt& y,
+                                      const BigInt& x) const
+   {
+   return new Default_ELG_Op(group, y, x);
+   }
+
+}