/*************************************************
* DSA 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 DSA Operation                          *
*************************************************/
class Default_DSA_Op : public DSA_Operation
   {
   public:
      bool verify(const byte[], u32bit, const byte[], u32bit) const;
      SecureVector<byte> sign(const byte[], u32bit, const BigInt&) const;

      DSA_Operation* clone() const { return new Default_DSA_Op(*this); }

      Default_DSA_Op(const DL_Group&, const BigInt&, const BigInt&);
   private:
      const BigInt x, y;
      const DL_Group group;
      Fixed_Base_Power_Mod powermod_g_p, powermod_y_p;
      Modular_Reducer mod_p, mod_q;
   };

/*************************************************
* Default_DSA_Op Constructor                     *
*************************************************/
Default_DSA_Op::Default_DSA_Op(const DL_Group& grp, const BigInt& y1,
                               const BigInt& x1) : x(x1), y(y1), group(grp)
   {
   powermod_g_p = Fixed_Base_Power_Mod(group.get_g(), group.get_p());
   powermod_y_p = Fixed_Base_Power_Mod(y, group.get_p());
   mod_p = Modular_Reducer(group.get_p());
   mod_q = Modular_Reducer(group.get_q());
   }

/*************************************************
* Default DSA Verify Operation                   *
*************************************************/
bool Default_DSA_Op::verify(const byte msg[], u32bit msg_len,
                            const byte sig[], u32bit sig_len) const
   {
   const BigInt& q = group.get_q();

   if(sig_len != 2*q.bytes() || msg_len > q.bytes())
      return false;

   BigInt r(sig, q.bytes());
   BigInt s(sig + q.bytes(), q.bytes());
   BigInt i(msg, msg_len);

   if(r <= 0 || r >= q || s <= 0 || s >= q)
      return false;

   s = inverse_mod(s, q);
   s = mod_p.multiply(powermod_g_p(mod_q.multiply(s, i)),
                      powermod_y_p(mod_q.multiply(s, r)));

   return (mod_q.reduce(s) == r);
   }

/*************************************************
* Default DSA Sign Operation                     *
*************************************************/
SecureVector<byte> Default_DSA_Op::sign(const byte in[], u32bit length,
                                        const BigInt& k) const
   {
   if(x == 0)
      throw Internal_Error("Default_DSA_Op::sign: No private key");

   const BigInt& q = group.get_q();
   BigInt i(in, length);

   BigInt r = mod_q.reduce(powermod_g_p(k));
   BigInt s = mod_q.multiply(inverse_mod(k, q), mul_add(x, r, i));

   if(r.is_zero() || s.is_zero())
      throw Internal_Error("Default_DSA_Op::sign: r or s was zero");

   SecureVector<byte> output(2*q.bytes());
   r.binary_encode(output + (output.size() / 2 - r.bytes()));
   s.binary_encode(output + (output.size() - s.bytes()));
   return output;
   }

}

/*************************************************
* Acquire a DSA op                               *
*************************************************/
DSA_Operation* Default_Engine::dsa_op(const DL_Group& group, const BigInt& y,
                                      const BigInt& x) const
   {
   return new Default_DSA_Op(group, y, x);
   }

}