elgamal.h
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#ifndef CRYPTOPP_ELGAMAL_H
#define CRYPTOPP_ELGAMAL_H
#include "modexppc.h"
#include "dsa.h"
NAMESPACE_BEGIN(CryptoPP)
class CRYPTOPP_NO_VTABLE ElGamalBase : public DL_KeyAgreementAlgorithm_DH<Integer, NoCofactorMultiplication>,
public DL_KeyDerivationAlgorithm<Integer>,
public DL_SymmetricEncryptionAlgorithm
{
public:
void Derive(const DL_GroupParameters<Integer> &groupParams, byte *derivedKey, size_t derivedLength, const Integer &agreedElement, const Integer &ephemeralPublicKey, const NameValuePairs &derivationParams) const
{
agreedElement.Encode(derivedKey, derivedLength);
}
size_t GetSymmetricKeyLength(size_t plainTextLength) const
{
return GetGroupParameters().GetModulus().ByteCount();
}
size_t GetSymmetricCiphertextLength(size_t plainTextLength) const
{
unsigned int len = GetGroupParameters().GetModulus().ByteCount();
if (plainTextLength <= GetMaxSymmetricPlaintextLength(len))
return len;
else
return 0;
}
size_t GetMaxSymmetricPlaintextLength(size_t cipherTextLength) const
{
unsigned int len = GetGroupParameters().GetModulus().ByteCount();
if (cipherTextLength == len)
return STDMIN(255U, len-3);
else
return 0;
}
void SymmetricEncrypt(RandomNumberGenerator &rng, const byte *key, const byte *plainText, size_t plainTextLength, byte *cipherText, const NameValuePairs ¶meters) const
{
const Integer &p = GetGroupParameters().GetModulus();
unsigned int modulusLen = p.ByteCount();
SecByteBlock block(modulusLen-1);
rng.GenerateBlock(block, modulusLen-2-plainTextLength);
memcpy(block+modulusLen-2-plainTextLength, plainText, plainTextLength);
block[modulusLen-2] = (byte)plainTextLength;
a_times_b_mod_c(Integer(key, modulusLen), Integer(block, modulusLen-1), p).Encode(cipherText, modulusLen);
}
DecodingResult SymmetricDecrypt(const byte *key, const byte *cipherText, size_t cipherTextLength, byte *plainText, const NameValuePairs ¶meters) const
{
const Integer &p = GetGroupParameters().GetModulus();
unsigned int modulusLen = p.ByteCount();
if (cipherTextLength != modulusLen)
return DecodingResult();
Integer m = a_times_b_mod_c(Integer(cipherText, modulusLen), Integer(key, modulusLen).InverseMod(p), p);
m.Encode(plainText, 1);
unsigned int plainTextLength = plainText[0];
if (plainTextLength > GetMaxSymmetricPlaintextLength(modulusLen))
return DecodingResult();
m >>= 8;
m.Encode(plainText, plainTextLength);
return DecodingResult(plainTextLength);
}
virtual const DL_GroupParameters_GFP & GetGroupParameters() const =0;
};
template <class BASE, class SCHEME_OPTIONS, class KEY>
class ElGamalObjectImpl : public DL_ObjectImplBase<BASE, SCHEME_OPTIONS, KEY>, public ElGamalBase
{
public:
size_t FixedMaxPlaintextLength() const {return this->MaxPlaintextLength(FixedCiphertextLength());}
size_t FixedCiphertextLength() const {return this->CiphertextLength(0);}
const DL_GroupParameters_GFP & GetGroupParameters() const {return this->GetKey().GetGroupParameters();}
DecodingResult FixedLengthDecrypt(RandomNumberGenerator &rng, const byte *cipherText, byte *plainText) const
{return Decrypt(rng, cipherText, FixedCiphertextLength(), plainText);}
protected:
const DL_KeyAgreementAlgorithm<Integer> & GetKeyAgreementAlgorithm() const {return *this;}
const DL_KeyDerivationAlgorithm<Integer> & GetKeyDerivationAlgorithm() const {return *this;}
const DL_SymmetricEncryptionAlgorithm & GetSymmetricEncryptionAlgorithm() const {return *this;}
};
struct ElGamalKeys
{
typedef DL_CryptoKeys_GFP::GroupParameters GroupParameters;
typedef DL_PrivateKey_GFP_OldFormat<DL_CryptoKeys_GFP::PrivateKey> PrivateKey;
typedef DL_PublicKey_GFP_OldFormat<DL_CryptoKeys_GFP::PublicKey> PublicKey;
};
//! ElGamal encryption scheme with non-standard padding
struct ElGamal
{
typedef DL_CryptoSchemeOptions<ElGamal, ElGamalKeys, int, int, int> SchemeOptions;
static const char * StaticAlgorithmName() {return "ElgamalEnc/Crypto++Padding";}
typedef SchemeOptions::GroupParameters GroupParameters;
//! implements PK_Encryptor interface
typedef PK_FinalTemplate<ElGamalObjectImpl<DL_EncryptorBase<Integer>, SchemeOptions, SchemeOptions::PublicKey> > Encryptor;
//! implements PK_Decryptor interface
typedef PK_FinalTemplate<ElGamalObjectImpl<DL_DecryptorBase<Integer>, SchemeOptions, SchemeOptions::PrivateKey> > Decryptor;
};
typedef ElGamal::Encryptor ElGamalEncryptor;
typedef ElGamal::Decryptor ElGamalDecryptor;
NAMESPACE_END
#endif