elgamal.h

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// elgamal.h - originally written and placed in the public domain by Wei Dai

/// \file elgamal.h
/// \brief Classes and functions for ElGamal key agreement and encryption schemes

#ifndef CRYPTOPP_ELGAMAL_H
#define CRYPTOPP_ELGAMAL_H

#include "cryptlib.h"
#include "modexppc.h"
#include "integer.h"
#include "gfpcrypt.h"
#include "pubkey.h"
#include "misc.h"
#include "oids.h"
#include "dsa.h"
#include "asn.h"

NAMESPACE_BEGIN(CryptoPP)

/// \brief ElGamal key agreement and encryption schemes base class
/// \since Crypto++ 1.0
class CRYPTOPP_NO_VTABLE ElGamalBase :
    public DL_KeyAgreementAlgorithm_DH<Integer, NoCofactorMultiplication>,
    public DL_KeyDerivationAlgorithm<Integer>,
    public DL_SymmetricEncryptionAlgorithm
{
public:
    virtual ~ElGamalBase() {}

    void Derive(const DL_GroupParameters<Integer> &groupParams, byte *derivedKey, size_t derivedLength, const Integer &agreedElement, const Integer &ephemeralPublicKey, const NameValuePairs &derivationParams) const
    {
        CRYPTOPP_UNUSED(groupParams); CRYPTOPP_UNUSED(ephemeralPublicKey);
        CRYPTOPP_UNUSED(derivationParams);
        agreedElement.Encode(derivedKey, derivedLength);
    }

    size_t GetSymmetricKeyLength(size_t plainTextLength) const
    {
        CRYPTOPP_UNUSED(plainTextLength);
        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();
        CRYPTOPP_ASSERT(len >= 3);

        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
    {
        CRYPTOPP_UNUSED(parameters);
        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
    {
        CRYPTOPP_UNUSED(parameters);
        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;
};

/// \brief ElGamal key agreement and encryption schemes default implementation
/// \tparam BASE Base class implementation
/// \tparam SCHEME_OPTIONS Scheme options
/// \tparam KEY ElGamal key classes
/// \since Crypto++ 1.0
template <class BASE, class SCHEME_OPTIONS, class KEY>
class ElGamalObjectImpl :
    public DL_ObjectImplBase<BASE, SCHEME_OPTIONS, KEY>,
    public ElGamalBase
{
public:
    virtual ~ElGamalObjectImpl() {}

    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;}
};

/// \brief ElGamal Public Key adapter
/// \tparam BASE PublicKey derived class
/// \details DL_PublicKey_ElGamal provides an override for GetAlgorithmID()
///  to utilize 1.3.14.7.2.1.1. Prior to DL_PublicKey_ElGamal, the ElGamal
///  keys [mistakenly] used the OID from DSA due to DL_GroupParmaters_GFP().
///  If you need to <tt>Load</tt> an ElGamal key with the wrong OID then
///  see <A HREF="https://www.cryptopp.com/wiki/ElGamal">ElGamal</A> on
///  the Crypto++ wiki.
/// \sa <A HREF="https://github.com/weidai11/cryptopp/issues/876">Issue 876</A>,
///  <A HREF="https://github.com/weidai11/cryptopp/issues/567">Issue 567</A>
/// \since Crypto++ 8.3
template <class BASE>
struct DL_PublicKey_ElGamal : public BASE
{
    virtual ~DL_PublicKey_ElGamal() {}

    /// \brief Retrieves the OID of the algorithm
    /// \return OID of the algorithm
    /// \details DL_PrivateKey_ElGamal provides an override for GetAlgorithmID()
    ///  to utilize 1.3.14.7.2.1.1. Prior to DL_PrivateKey_ElGamal, the ElGamal
    ///  keys [mistakenly] used the OID from DSA due to DL_GroupParmaters_GFP().
    ///  If you need to <tt>Load</tt> an ElGamal key with the wrong OID then
    ///  see <A HREF="https://www.cryptopp.com/wiki/ElGamal">ElGamal</A> on
    ///  the Crypto++ wiki.
    /// \sa <A HREF="https://github.com/weidai11/cryptopp/issues/876">Issue 876</A>,
    ///  <A HREF="https://github.com/weidai11/cryptopp/issues/567">Issue 567</A>
    virtual OID GetAlgorithmID() const {
        return ASN1::elGamal();
    }
};

/// \brief ElGamal Private Key adapter
/// \tparam BASE PrivateKey derived class
/// \details DL_PrivateKey_ElGamal provides an override for GetAlgorithmID()
///  to utilize 1.3.14.7.2.1.1. Prior to DL_PrivateKey_ElGamal, the ElGamal
///  keys [mistakenly] used the OID from DSA due to DL_GroupParmaters_GFP().
///  If you need to <tt>Load</tt> an ElGamal key with the wrong OID then
///  see <A HREF="https://www.cryptopp.com/wiki/ElGamal">ElGamal</A> on
///  the Crypto++ wiki.
/// \sa <A HREF="https://github.com/weidai11/cryptopp/issues/876">Issue 876</A>,
///  <A HREF="https://github.com/weidai11/cryptopp/issues/567">Issue 567</A>
/// \since Crypto++ 8.3
template <class BASE>
struct DL_PrivateKey_ElGamal : public BASE
{
    virtual ~DL_PrivateKey_ElGamal() {}

    /// \brief Retrieves the OID of the algorithm
    /// \return OID of the algorithm
    /// \details DL_PrivateKey_ElGamal provides an override for GetAlgorithmID()
    ///  to utilize 1.3.14.7.2.1.1. Prior to DL_PrivateKey_ElGamal, the ElGamal
    ///  keys [mistakenly] used the OID from DSA due to DL_GroupParmaters_GFP().
    ///  If you need to <tt>Load</tt> an ElGamal key with the wrong OID then
    ///  see <A HREF="https://www.cryptopp.com/wiki/ElGamal">ElGamal</A> on
    ///  the Crypto++ wiki.
    /// \sa <A HREF="https://github.com/weidai11/cryptopp/issues/876">Issue 876</A>,
    ///  <A HREF="https://github.com/weidai11/cryptopp/issues/567">Issue 567</A>
    virtual OID GetAlgorithmID() const {
        return ASN1::elGamal();
    }

    /// \brief Check the key for errors
    /// \param rng RandomNumberGenerator for objects which use randomized testing
    /// \param level level of thoroughness
    /// \return true if the tests succeed, false otherwise
    /// \details There are four levels of thoroughness:
    ///   <ul>
    ///   <li>0 - using this object won't cause a crash or exception
    ///   <li>1 - this object will probably function, and encrypt, sign, other
    ///           operations correctly
    ///   <li>2 - ensure this object will function correctly, and perform
    ///           reasonable security checks
    ///   <li>3 - perform reasonable security checks, and do checks that may
    ///           take a long time
    ///   </ul>
    /// \details Level 0 does not require a RandomNumberGenerator. A NullRNG() can
    ///  be used for level 0. Level 1 may not check for weak keys and such.
    ///  Levels 2 and 3 are recommended.
    bool Validate(RandomNumberGenerator &rng, unsigned int level) const
    {
        // Validate() formerly used DL_PrivateKey_GFP implementation through
        // inheritance. However, it would reject keys from other libraries
        // like BouncyCastle. The failure was x < q. According to ElGamal's
        // paper and the HAC, the private key is selected in over [1,p-1],
        // Later Tsiounis and Yung showed the lower limit as [1,q-1] in
        // "On the Security of EIGamal Based Encryption". As such, Crypto++
        // will generate a key in the range [1,q-1], but accept a key
        // in [1,p-1]. Thanks to JPM for finding the reference. Also see
        // https://github.com/weidai11/cryptopp/commit/a5a684d92986.

        CRYPTOPP_ASSERT(this->GetAbstractGroupParameters().Validate(rng, level));
        bool pass = this->GetAbstractGroupParameters().Validate(rng, level);

        const Integer &p = this->GetGroupParameters().GetModulus();
        const Integer &q = this->GetAbstractGroupParameters().GetSubgroupOrder();
        const Integer &x = this->GetPrivateExponent();

        // Changed to x < p-1 based on ElGamal's paper and the HAC.
        CRYPTOPP_ASSERT(x.IsPositive());
        CRYPTOPP_ASSERT(x < p-1);
        pass = pass && x.IsPositive() && x < p-1;

        if (level >= 1)
        {
            // Minimum security level due to Tsiounis and Yung.
            CRYPTOPP_ASSERT(Integer::Gcd(x, q) == Integer::One());
            pass = pass && Integer::Gcd(x, q) == Integer::One();
        }
        return pass;
    }
};

/// \brief ElGamal key agreement and encryption schemes keys
/// \details ElGamalKeys provide the algorithm implementation ElGamal key
///  agreement and encryption schemes.
/// \details The ElGamalKeys class used <tt>DL_PrivateKey_GFP_OldFormat</tt>
///  and <tt>DL_PublicKey_GFP_OldFormat</tt> for the <tt>PrivateKey</tt> and
///  <tt>PublicKey</tt> from about Crypto++ 1.0 through Crypto++ 5.6.5. At
///  Crypto++ 6.0 the serialization format was cutover to standard PKCS8 and
///  X509 encodings.
/// \details The ElGamalKeys class [mistakenly] used the OID for DSA from
///  about Crypto++ 1.0 through Crypto++ 8.2. At Crypto++ 8.3 the OID was
///  fixed and now uses ElGamal encryption, which is 1.3.14.7.2.1.1.
///  If you need to <tt>Load</tt> an ElGamal key with the wrong OID then
///  see <A HREF="https://www.cryptopp.com/wiki/ElGamal">ElGamal</A> on
///  the Crypto++ wiki.
/// \sa <A HREF="https://github.com/weidai11/cryptopp/issues/876">Issue 876</A>,
///  <A HREF="https://github.com/weidai11/cryptopp/issues/567">Issue 567</A>
/// \since Crypto++ 1.0
struct ElGamalKeys
{
    /// \brief Implements DL_GroupParameters interface
    typedef DL_CryptoKeys_GFP::GroupParameters GroupParameters;
    /// \brief Implements DL_PrivateKey interface
    typedef DL_PrivateKey_ElGamal<DL_CryptoKeys_GFP::PrivateKey> PrivateKey;
    /// \brief Implements DL_PublicKey interface
    typedef DL_PublicKey_ElGamal<DL_CryptoKeys_GFP::PublicKey> PublicKey;
};

/// \brief ElGamal encryption scheme with non-standard padding
/// \details ElGamal provide the algorithm implementation ElGamal key
///  agreement and encryption schemes.
/// \details The ElGamal class [mistakenly] used the OID for DSA from about
///  Crypto++ 1.0 through Crypto++ 8.2. At Crypto++ 8.3 the OID was fixed
///  and now uses ElGamal encryption, which is 1.3.14.7.2.1.1.
///  If you need to <tt>Load</tt> an ElGamal key with the wrong OID then
///  see <A HREF="https://www.cryptopp.com/wiki/ElGamal">ElGamal</A> on
///  the Crypto++ wiki.
/// \sa <A HREF="https://github.com/weidai11/cryptopp/issues/876">Issue 876</A>,
///  <A HREF="https://github.com/weidai11/cryptopp/issues/567">Issue 567</A>
/// \since Crypto++ 1.0
struct ElGamal
{
    typedef DL_CryptoSchemeOptions<ElGamal, ElGamalKeys, int, int, int> SchemeOptions;
    typedef SchemeOptions::PrivateKey PrivateKey;
    typedef SchemeOptions::PublicKey PublicKey;

    /// \brief The algorithm name
    /// \return the algorithm name
    /// \details StaticAlgorithmName returns the algorithm's name as a static
    ///  member function.
    CRYPTOPP_STATIC_CONSTEXPR const char* StaticAlgorithmName() {return "ElgamalEnc/Crypto++Padding";}

    /// \brief Implements DL_GroupParameters interface
    typedef SchemeOptions::GroupParameters GroupParameters;
    /// \brief Implements PK_Encryptor interface
    typedef PK_FinalTemplate<ElGamalObjectImpl<DL_EncryptorBase<Integer>, SchemeOptions, SchemeOptions::PublicKey> > Encryptor;
    /// \brief Implements PK_Encryptor interface
    typedef PK_FinalTemplate<ElGamalObjectImpl<DL_DecryptorBase<Integer>, SchemeOptions, SchemeOptions::PrivateKey> > Decryptor;
};

typedef ElGamal::Encryptor ElGamalEncryptor;
typedef ElGamal::Decryptor ElGamalDecryptor;

NAMESPACE_END

#endif