I2P: End-to-End encrypted and anonymous Internet https://i2pd.website/
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#ifndef EL_GAMAL_H__
#define EL_GAMAL_H__
#include <inttypes.h>
#include <cryptopp/integer.h>
#include <cryptopp/osrng.h>
#include <cryptopp/sha.h>
#include "CryptoConst.h"
#include "Log.h"
namespace i2p
{
namespace crypto
{
class ElGamalEncryption
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{
public:
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ElGamalEncryption (const uint8_t * key):
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)
{
// 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::AutoSeededRandomPool rnd;
CryptoPP::Integer y, k, a, b1;
};
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inline bool ElGamalDecrypt (const uint8_t * key, const uint8_t * encrypted,
uint8_t * data, bool zeroPadding = false)
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{
CryptoPP::Integer x(key, 256), a(zeroPadding? encrypted +1 : encrypted, 256),
b(zeroPadding? encrypted + 258 :encrypted + 256, 256);
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uint8_t m[255], hash[32];
a_times_b_mod_c (b, a_exp_b_mod_c (a, elgp - x - 1, elgp), elgp).Encode (m, 255);
CryptoPP::SHA256().CalculateDigest(hash, m+33, 222);
for (int i = 0; i < 32; i++)
if (hash[i] != m[i+1])
{
LogPrint ("ElGamal decrypt hash doesn't match");
return false;
}
memcpy (data, m + 33, 222);
return true;
}
}
}
#endif