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elgamal.h

00001 #ifndef CRYPTOPP_ELGAMAL_H
00002 #define CRYPTOPP_ELGAMAL_H
00003 
00004 #include "modexppc.h"
00005 #include "dsa.h"
00006 
00007 NAMESPACE_BEGIN(CryptoPP)
00008 
00009 class CRYPTOPP_NO_VTABLE ElGamalBase : public DL_KeyAgreementAlgorithm_DH<Integer, NoCofactorMultiplication>, 
00010                     public DL_KeyDerivationAlgorithm<Integer>, 
00011                     public DL_SymmetricEncryptionAlgorithm
00012 {
00013 public:
00014     void Derive(const DL_GroupParameters<Integer> &groupParams, byte *derivedKey, size_t derivedLength, const Integer &agreedElement, const Integer &ephemeralPublicKey, const NameValuePairs &derivationParams) const
00015     {
00016         agreedElement.Encode(derivedKey, derivedLength);
00017     }
00018 
00019     size_t GetSymmetricKeyLength(size_t plainTextLength) const
00020     {
00021         return GetGroupParameters().GetModulus().ByteCount();
00022     }
00023 
00024     size_t GetSymmetricCiphertextLength(size_t plainTextLength) const
00025     {
00026         unsigned int len = GetGroupParameters().GetModulus().ByteCount();
00027         if (plainTextLength <= GetMaxSymmetricPlaintextLength(len))
00028             return len;
00029         else
00030             return 0;
00031     }
00032 
00033     size_t GetMaxSymmetricPlaintextLength(size_t cipherTextLength) const
00034     {
00035         unsigned int len = GetGroupParameters().GetModulus().ByteCount();
00036         if (cipherTextLength == len)
00037             return STDMIN(255U, len-3);
00038         else
00039             return 0;
00040     }
00041 
00042     void SymmetricEncrypt(RandomNumberGenerator &rng, const byte *key, const byte *plainText, size_t plainTextLength, byte *cipherText, const NameValuePairs &parameters) const
00043     {
00044         const Integer &p = GetGroupParameters().GetModulus();
00045         unsigned int modulusLen = p.ByteCount();
00046 
00047         SecByteBlock block(modulusLen-1);
00048         rng.GenerateBlock(block, modulusLen-2-plainTextLength);
00049         memcpy(block+modulusLen-2-plainTextLength, plainText, plainTextLength);
00050         block[modulusLen-2] = (byte)plainTextLength;
00051 
00052         a_times_b_mod_c(Integer(key, modulusLen), Integer(block, modulusLen-1), p).Encode(cipherText, modulusLen);
00053     }
00054 
00055     DecodingResult SymmetricDecrypt(const byte *key, const byte *cipherText, size_t cipherTextLength, byte *plainText, const NameValuePairs &parameters) const
00056     {
00057         const Integer &p = GetGroupParameters().GetModulus();
00058         unsigned int modulusLen = p.ByteCount();
00059 
00060         if (cipherTextLength != modulusLen)
00061             return DecodingResult();
00062 
00063         Integer m = a_times_b_mod_c(Integer(cipherText, modulusLen), Integer(key, modulusLen).InverseMod(p), p);
00064 
00065         m.Encode(plainText, 1);
00066         unsigned int plainTextLength = plainText[0];
00067         if (plainTextLength > GetMaxSymmetricPlaintextLength(modulusLen))
00068             return DecodingResult();
00069         m >>= 8;
00070         m.Encode(plainText, plainTextLength);
00071         return DecodingResult(plainTextLength);
00072     }
00073 
00074     virtual const DL_GroupParameters_GFP & GetGroupParameters() const =0;
00075 };
00076 
00077 template <class BASE, class SCHEME_OPTIONS, class KEY>
00078 class ElGamalObjectImpl : public DL_ObjectImplBase<BASE, SCHEME_OPTIONS, KEY>, public ElGamalBase
00079 {
00080 public:
00081     size_t FixedMaxPlaintextLength() const {return this->MaxPlaintextLength(FixedCiphertextLength());}
00082     size_t FixedCiphertextLength() const {return this->CiphertextLength(0);}
00083 
00084     const DL_GroupParameters_GFP & GetGroupParameters() const {return this->GetKey().GetGroupParameters();}
00085 
00086     DecodingResult FixedLengthDecrypt(RandomNumberGenerator &rng, const byte *cipherText, byte *plainText) const
00087         {return Decrypt(rng, cipherText, FixedCiphertextLength(), plainText);}
00088 
00089 protected:
00090     const DL_KeyAgreementAlgorithm<Integer> & GetKeyAgreementAlgorithm() const {return *this;}
00091     const DL_KeyDerivationAlgorithm<Integer> & GetKeyDerivationAlgorithm() const {return *this;}
00092     const DL_SymmetricEncryptionAlgorithm & GetSymmetricEncryptionAlgorithm() const {return *this;}
00093 };
00094 
00095 struct ElGamalKeys
00096 {
00097     typedef DL_CryptoKeys_GFP::GroupParameters GroupParameters;
00098     typedef DL_PrivateKey_GFP_OldFormat<DL_CryptoKeys_GFP::PrivateKey> PrivateKey;
00099     typedef DL_PublicKey_GFP_OldFormat<DL_CryptoKeys_GFP::PublicKey> PublicKey;
00100 };
00101 
00102 //! ElGamal encryption scheme with non-standard padding
00103 struct ElGamal
00104 {
00105     typedef DL_CryptoSchemeOptions<ElGamal, ElGamalKeys, int, int, int> SchemeOptions;
00106 
00107     static const char * StaticAlgorithmName() {return "ElgamalEnc/Crypto++Padding";}
00108 
00109     typedef SchemeOptions::GroupParameters GroupParameters;
00110     //! implements PK_Encryptor interface
00111     typedef PK_FinalTemplate<ElGamalObjectImpl<DL_EncryptorBase<Integer>, SchemeOptions, SchemeOptions::PublicKey> > Encryptor;
00112     //! implements PK_Decryptor interface
00113     typedef PK_FinalTemplate<ElGamalObjectImpl<DL_DecryptorBase<Integer>, SchemeOptions, SchemeOptions::PrivateKey> > Decryptor;
00114 };
00115 
00116 typedef ElGamal::Encryptor ElGamalEncryptor;
00117 typedef ElGamal::Decryptor ElGamalDecryptor;
00118 
00119 NAMESPACE_END
00120 
00121 #endif

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