/*
* ElGamal Operations
* (C) 1999-2007 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/

#include <botan/elg_op.h>

namespace Botan {

/*
* 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));
   }

}