elgamal.h
#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 &parameters) 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 &parameters) 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
ElGamal::Encryptor
PK_FinalTemplate< ElGamalObjectImpl< DL_EncryptorBase< Integer >, SchemeOptions, SchemeOptions::PublicKey > > Encryptor
implements PK_Encryptor interface
Definition: elgamal.h:111
DL_KeyAgreementAlgorithm_DH
DH key agreement algorithm.
Definition: pubkey.h:1434
DL_GroupParameters_GFP
GF(p) group parameters.
Definition: gfpcrypt.h:119
ElGamalObjectImpl
Definition: elgamal.h:78
DL_GroupParameters< Integer >
EnumToType
Definition: cryptlib.h:99
SecBlock< byte >
DL_GroupParameters_GFP_DefaultSafePrime
GF(p) group parameters that default to same primes.
Definition: gfpcrypt.h:141
DL_KeyDerivationAlgorithm
interface for key derivation algorithms used in DL cryptosystems
Definition: pubkey.h:926
RandomNumberGenerator
interface for random number generators
Definition: cryptlib.h:668
ElGamalKeys
Definition: elgamal.h:95
DL_CryptoSchemeOptions
_
Definition: pubkey.h:1272
ElGamal
ElGamal encryption scheme with non-standard padding.
Definition: elgamal.h:103
DecodingResult
used to return decoding results
Definition: cryptlib.h:197
DL_PublicKey_GFP_OldFormat
provided for backwards compatibility, this class uses the old non-standard Crypto++ key format ...
Definition: gfpcrypt.h:265
Integer
multiple precision integer and basic arithmetics
Definition: integer.h:26
DL_ObjectImplBase
_
Definition: pubkey.h:1281
dsa.h
RandomNumberGenerator::GenerateBlock
virtual void GenerateBlock(byte *output, size_t size)
generate random array of bytes
Integer::Encode
void Encode(byte *output, size_t outputLen, Signedness=UNSIGNED) const
encode in big-endian format
DL_PrivateKey_GFP_OldFormat
provided for backwards compatibility, this class uses the old non-standard Crypto++ key format ...
Definition: gfpcrypt.h:304
ElGamalBase
Definition: elgamal.h:9
DL_SymmetricEncryptionAlgorithm
interface for symmetric encryption algorithms used in DL cryptosystems
Definition: pubkey.h:934
ElGamal::Decryptor
PK_FinalTemplate< ElGamalObjectImpl< DL_DecryptorBase< Integer >, SchemeOptions, SchemeOptions::PrivateKey > > Decryptor
implements PK_Decryptor interface
Definition: elgamal.h:113
DL_KeyAgreementAlgorithm< Integer >
Integer::ByteCount
unsigned int ByteCount() const
number of significant bytes = ceiling(BitCount()/8)
NameValuePairs
interface for retrieving values given their names
Definition: cryptlib.h:224
PK_FinalTemplate
A template implementing constructors for public key algorithm classes.
Definition: pubkey.h:1488