elgamal.cpp

/*
* 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");

   const size_t k_bits = m_group.exponent_bits();
   const BigInt k(rng, k_bits);

   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);
   }

}