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/*
* ElGamal
* (C) 1999-2007 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
#include <botan/elgamal.h>
#include <botan/numthry.h>
#include <botan/keypair.h>
#include <botan/workfactor.h>
namespace Botan {
/*
* ElGamal_PublicKey Constructor
*/
ElGamal_PublicKey::ElGamal_PublicKey(const DL_Group& grp, const BigInt& y1)
{
group = grp;
y = y1;
}
/*
* ElGamal_PrivateKey Constructor
*/
ElGamal_PrivateKey::ElGamal_PrivateKey(RandomNumberGenerator& rng,
const DL_Group& grp,
const BigInt& x_arg)
{
group = grp;
x = x_arg;
if(x == 0)
x.randomize(rng, 2 * dl_work_factor(group_p().bits()));
y = power_mod(group_g(), x, group_p());
if(x_arg == 0)
gen_check(rng);
else
load_check(rng);
}
ElGamal_PrivateKey::ElGamal_PrivateKey(const AlgorithmIdentifier& alg_id,
const secure_vector<byte>& key_bits,
RandomNumberGenerator& rng) :
DL_Scheme_PrivateKey(alg_id, key_bits, DL_Group::ANSI_X9_42)
{
y = power_mod(group_g(), x, group_p());
load_check(rng);
}
/*
* Check Private ElGamal Parameters
*/
bool ElGamal_PrivateKey::check_key(RandomNumberGenerator& rng,
bool strong) const
{
if(!DL_Scheme_PrivateKey::check_key(rng, strong))
return false;
if(!strong)
return true;
return KeyPair::encryption_consistency_check(rng, *this, "EME1(SHA-1)");
}
ElGamal_Encryption_Operation::ElGamal_Encryption_Operation(const ElGamal_PublicKey& key)
{
const BigInt& p = key.group_p();
powermod_g_p = Fixed_Base_Power_Mod(key.group_g(), p);
powermod_y_p = Fixed_Base_Power_Mod(key.get_y(), p);
mod_p = Modular_Reducer(p);
}
secure_vector<byte>
ElGamal_Encryption_Operation::encrypt(const byte msg[], size_t msg_len,
RandomNumberGenerator& rng)
{
const BigInt& p = mod_p.get_modulus();
BigInt m(msg, msg_len);
if(m >= p)
throw Invalid_Argument("ElGamal encryption: Input is too large");
BigInt k(rng, 2 * dl_work_factor(p.bits()));
BigInt a = powermod_g_p(k);
BigInt b = mod_p.multiply(m, powermod_y_p(k));
secure_vector<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;
}
ElGamal_Decryption_Operation::ElGamal_Decryption_Operation(const ElGamal_PrivateKey& key,
RandomNumberGenerator& rng)
{
const BigInt& p = key.group_p();
powermod_x_p = Fixed_Exponent_Power_Mod(key.get_x(), p);
mod_p = Modular_Reducer(p);
BigInt k(rng, p.bits() - 1);
blinder = Blinder(k, powermod_x_p(k), p);
}
secure_vector<byte>
ElGamal_Decryption_Operation::decrypt(const byte msg[], size_t msg_len)
{
const BigInt& p = mod_p.get_modulus();
const size_t p_bytes = p.bytes();
if(msg_len != 2 * p_bytes)
throw Invalid_Argument("ElGamal decryption: Invalid message");
BigInt a(msg, p_bytes);
BigInt b(msg + p_bytes, p_bytes);
if(a >= p || b >= p)
throw Invalid_Argument("ElGamal decryption: Invalid message");
a = blinder.blind(a);
BigInt r = mod_p.multiply(b, inverse_mod(powermod_x_p(a), p));
return BigInt::encode_locked(blinder.unblind(r));
}
}
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