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/*
* ElGamal
* (C) 1999-2007,2018,2019 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/elgamal.h>
#include <botan/internal/pk_ops_impl.h>
#include <botan/internal/monty_exp.h>
#include <botan/keypair.h>
#include <botan/blinding.h>
namespace Botan {
/*
* ElGamal_PublicKey Constructor
*/
ElGamal_PublicKey::ElGamal_PublicKey(const DL_Group& group, const BigInt& y) :
DL_Scheme_PublicKey(group, y)
{
}
/*
* ElGamal_PrivateKey Constructor
*/
ElGamal_PrivateKey::ElGamal_PrivateKey(RandomNumberGenerator& rng,
const DL_Group& group,
const BigInt& x)
{
m_x = x;
m_group = group;
if(m_x.is_zero())
{
const size_t exp_bits = m_group.exponent_bits();
m_x.randomize(rng, exp_bits);
m_y = m_group.power_g_p(m_x, exp_bits);
}
else
{
m_y = m_group.power_g_p(m_x, m_group.p_bits());
}
}
ElGamal_PrivateKey::ElGamal_PrivateKey(const AlgorithmIdentifier& alg_id,
const secure_vector<uint8_t>& key_bits) :
DL_Scheme_PrivateKey(alg_id, key_bits, DL_Group::ANSI_X9_42)
{
m_y = m_group.power_g_p(m_x, m_group.p_bits());
}
/*
* 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, "OAEP(SHA-256)");
}
namespace {
/**
* ElGamal encryption operation
*/
class ElGamal_Encryption_Operation final : public PK_Ops::Encryption_with_EME
{
public:
size_t ciphertext_length(size_t) const override { return 2*m_group.p_bytes(); }
size_t max_raw_input_bits() const override { return m_group.p_bits() - 1; }
ElGamal_Encryption_Operation(const ElGamal_PublicKey& key, const std::string& eme);
secure_vector<uint8_t> raw_encrypt(const uint8_t msg[], size_t msg_len,
RandomNumberGenerator& rng) override;
private:
const DL_Group m_group;
std::shared_ptr<const Montgomery_Exponentation_State> m_monty_y_p;
};
ElGamal_Encryption_Operation::ElGamal_Encryption_Operation(const ElGamal_PublicKey& key,
const std::string& eme) :
PK_Ops::Encryption_with_EME(eme),
m_group(key.get_group())
{
const size_t powm_window = 4;
m_monty_y_p = monty_precompute(key.get_group().monty_params_p(),
key.get_y(),
powm_window);
}
secure_vector<uint8_t>
ElGamal_Encryption_Operation::raw_encrypt(const uint8_t msg[], size_t msg_len,
RandomNumberGenerator& rng)
{
BigInt m(msg, msg_len);
if(m >= m_group.get_p())
throw Invalid_Argument("ElGamal encryption: Input is too large");
/*
Some ElGamal implementations generate keys where using short exponents
is unsafe. Always use full length exponents to avoid this.
See https://eprint.iacr.org/2021/923 for details.
*/
const size_t k_bits = m_group.p_bits() - 1;
const BigInt k(rng, k_bits, false);
const BigInt a = m_group.power_g_p(k, k_bits);
const BigInt b = m_group.multiply_mod_p(m, monty_execute(*m_monty_y_p, k, k_bits));
return BigInt::encode_fixed_length_int_pair(a, b, m_group.p_bytes());
}
/**
* ElGamal decryption operation
*/
class ElGamal_Decryption_Operation final : public PK_Ops::Decryption_with_EME
{
public:
ElGamal_Decryption_Operation(const ElGamal_PrivateKey& key,
const std::string& eme,
RandomNumberGenerator& rng);
size_t plaintext_length(size_t) const override { return m_group.p_bytes(); }
secure_vector<uint8_t> raw_decrypt(const uint8_t msg[], size_t msg_len) override;
private:
BigInt powermod_x_p(const BigInt& v) const
{
const size_t powm_window = 4;
auto powm_v_p = monty_precompute(m_monty_p, v, powm_window);
return monty_execute(*powm_v_p, m_x, m_x_bits);
}
const DL_Group m_group;
const BigInt& m_x;
const size_t m_x_bits;
std::shared_ptr<const Montgomery_Params> m_monty_p;
Blinder m_blinder;
};
ElGamal_Decryption_Operation::ElGamal_Decryption_Operation(const ElGamal_PrivateKey& key,
const std::string& eme,
RandomNumberGenerator& rng) :
PK_Ops::Decryption_with_EME(eme),
m_group(key.get_group()),
m_x(key.get_x()),
m_x_bits(m_x.bits()),
m_monty_p(key.get_group().monty_params_p()),
m_blinder(m_group.get_p(),
rng,
[](const BigInt& k) { return k; },
[this](const BigInt& k) { return powermod_x_p(k); })
{
}
secure_vector<uint8_t>
ElGamal_Decryption_Operation::raw_decrypt(const uint8_t msg[], size_t msg_len)
{
const size_t p_bytes = m_group.p_bytes();
if(msg_len != 2 * p_bytes)
throw Invalid_Argument("ElGamal decryption: Invalid message");
BigInt a(msg, p_bytes);
const BigInt b(msg + p_bytes, p_bytes);
if(a >= m_group.get_p() || b >= m_group.get_p())
throw Invalid_Argument("ElGamal decryption: Invalid message");
a = m_blinder.blind(a);
const BigInt r = m_group.multiply_mod_p(m_group.inverse_mod_p(powermod_x_p(a)), b);
return BigInt::encode_1363(m_blinder.unblind(r), p_bytes);
}
}
std::unique_ptr<PK_Ops::Encryption>
ElGamal_PublicKey::create_encryption_op(RandomNumberGenerator& /*rng*/,
const std::string& params,
const std::string& provider) const
{
if(provider == "base" || provider.empty())
return std::unique_ptr<PK_Ops::Encryption>(new ElGamal_Encryption_Operation(*this, params));
throw Provider_Not_Found(algo_name(), provider);
}
std::unique_ptr<PK_Ops::Decryption>
ElGamal_PrivateKey::create_decryption_op(RandomNumberGenerator& rng,
const std::string& params,
const std::string& provider) const
{
if(provider == "base" || provider.empty())
return std::unique_ptr<PK_Ops::Decryption>(new ElGamal_Decryption_Operation(*this, params, rng));
throw Provider_Not_Found(algo_name(), provider);
}
}
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