i2pd/crypto/ElGamal.h

#ifndef EL_GAMAL_H__
#define EL_GAMAL_H__

#include <inttypes.h>
#include <cryptopp/integer.h>
#include <cryptopp/osrng.h>
#include <cryptopp/dh.h>
#include <cryptopp/sha.h>
#include "CryptoConst.h"
#include "Log.h"

namespace i2p
{
namespace crypto
{

    class ElGamalEncryption
    {
        public:

            ElGamalEncryption (const uint8_t * key)
            {
                CryptoPP::AutoSeededRandomPool rnd; 
                CryptoPP::Integer y (key, 256), k (rnd, CryptoPP::Integer::One(), elgp-1);
                a = a_exp_b_mod_c (elgg, k, elgp);
                b1 = a_exp_b_mod_c (y, k, elgp);
            }

            void Encrypt (const uint8_t * data, int len, uint8_t * encrypted, bool zeroPadding = false) const
            {
                // calculate b = b1*m mod p
                uint8_t m[255];
                m[0] = 0xFF;
                memcpy (m+33, data, len);
                CryptoPP::SHA256().CalculateDigest(m+1, m+33, 222);
                CryptoPP::Integer b (a_times_b_mod_c (b1, CryptoPP::Integer (m, 255), elgp));

                // copy a and b
                if (zeroPadding)
                {
                    encrypted[0] = 0;
                    a.Encode (encrypted + 1, 256);
                    encrypted[257] = 0;
                    b.Encode (encrypted + 258, 256);
                }   
                else
                {
                    a.Encode (encrypted, 256);  
                    b.Encode (encrypted + 256, 256);
                }   
            }

        private:

            CryptoPP::Integer a, b1;    
    };

    inline bool ElGamalDecrypt (const uint8_t * key, const uint8_t * encrypted, 
        uint8_t * data, bool zeroPadding = false)
    {
        CryptoPP::Integer x(key, 256), a(zeroPadding? encrypted +1 : encrypted, 256), 
            b(zeroPadding? encrypted + 258 :encrypted + 256, 256);
        uint8_t m[255];
        a_times_b_mod_c (b, a_exp_b_mod_c (a, elgp - x - 1, elgp), elgp).Encode (m, 255);
        if (!CryptoPP::SHA256().VerifyDigest (m + 1, m + 33, 222))
        {
            LogPrint ("ElGamal decrypt hash doesn't match");
            return false;
        }
        memcpy (data, m + 33, 222);
        return true;
    }   

    inline void GenerateElGamalKeyPair (CryptoPP::RandomNumberGenerator& rnd, uint8_t * priv, uint8_t * pub)
    {
#if defined(__x86_64__) || defined(__i386__) || defined(_MSC_VER)   
        rnd.GenerateBlock (priv, 256);
        a_exp_b_mod_c (elgg, CryptoPP::Integer (priv, 256), elgp).Encode (pub, 256);
#else
        CryptoPP::DH dh (elgp, elgg);
        dh.GenerateKeyPair(rnd, priv, pub); 
#endif      
    }   
}
}   

#endif