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elgamal.h

#ifndef CRYPTOPP_ELGAMAL_H
#define CRYPTOPP_ELGAMAL_H

#include "modexppc.h"

NAMESPACE_BEGIN(CryptoPP)

class ElGamalEncryptor : public PK_WithPrecomputation<PK_FixedLengthEncryptor>
{
public:
        ElGamalEncryptor(const Integer &p, const Integer &g, const Integer &y);
        ElGamalEncryptor(BufferedTransformation &bt);

        void DEREncode(BufferedTransformation &bt) const;

        void Precompute(unsigned int precomputationStorage=16);
        void LoadPrecomputation(BufferedTransformation &storedPrecomputation);
        void SavePrecomputation(BufferedTransformation &storedPrecomputation) const;

        void Encrypt(RandomNumberGenerator &rng, const byte *plainText, unsigned int plainTextLength, byte *cipherText);

        unsigned int MaxPlainTextLength() const {return STDMIN(255U, m_p.ByteCount()-3);}
        unsigned int CipherTextLength() const {return 2*m_p.ByteCount();}

        void RawEncrypt(const Integer &k, const Integer &m, Integer &a, Integer &b) const;

        const Integer & GetModulus() const {return m_p;}
        const Integer & GetGenerator() const {return m_g;}
        const Integer & GetPublicResidue() const {return m_y;}

        const ModExpPrecomputation & GetGPC() const {return m_gpc;}
        const ModExpPrecomputation & GetYPC() const {return m_ypc;}

protected:
        ElGamalEncryptor() {}
        unsigned int ExponentBitLength() const;

        Integer m_p, m_g, m_y;
        ModExpPrecomputation m_gpc, m_ypc;
};

class ElGamalDecryptor : public ElGamalEncryptor, public PK_FixedLengthDecryptor
{
public:
        ElGamalDecryptor(const Integer &p, const Integer &g, const Integer &y, const Integer &x);
        ElGamalDecryptor(RandomNumberGenerator &rng, unsigned int pbits);
        // generate a random private key, given p and g
        ElGamalDecryptor(RandomNumberGenerator &rng, const Integer &p, const Integer &g);

        ElGamalDecryptor(BufferedTransformation &bt);
        void DEREncode(BufferedTransformation &bt) const;

        unsigned int Decrypt(const byte *cipherText, byte *plainText);

        void RawDecrypt(const Integer &a, const Integer &b, Integer &m) const;

        const Integer & GetPrivateExponent() const {return m_x;}

protected:
        Integer m_x;
};

NAMESPACE_END

#endif

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