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/*************************************************
* 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);
}
}
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