1 files changed, 121 insertions, 0 deletions
diff --git a/CryptoPP/elgamal.h b/CryptoPP/elgamal.h
new file mode 100644
index 000000000..9afc30eee
--- /dev/null
+++ b/CryptoPP/elgamal.h
@@ -0,0 +1,121 @@
+#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