GetSharedSecret.cs

private static byte[] GetSharedSecret()
{
    String clientPrivateKeyPem = PlayerPrefs.GetString(_PlayerPrefs.ClientPrivateKey);
    String serverPublicKeyDer = PlayerPrefs.GetString(_PlayerPrefs.ServerPublicKey);
    if(String.IsNullOrEmpty(clientPrivateKeyPem))
    {
        throw new PlayerPrefsItemNotFoundException(_PlayerPrefs.ClientPrivateKey);
    }
    if(String.IsNullOrEmpty(serverPublicKeyDer))
    {
        throw new PlayerPrefsItemNotFoundException(_PlayerPrefs.ServerPublicKey);
    }
    byte[] serverPublicKeyDerByte = Conversion.StringToByteArray(serverPublicKeyDer);
    PemReader pemReader = new PemReader(new StringReader(clientPrivateKeyPem));
    AsymmetricCipherKeyPair keyPair = (AsymmetricCipherKeyPair)pemReader.ReadObject();
    ECPrivateKeyParameters ecPrivKey = (ECPrivateKeyParameters)keyPair.Private;
    ECPublicKeyParameters ecPubKey = (ECPublicKeyParameters)PublicKeyFactory.CreateKey(serverPublicKeyDerByte);
    IBasicAgreement aKeyAgree = AgreementUtilities.GetBasicAgreement("ECDH");
    aKeyAgree.Init(ecPrivKey);
    /* Leading zero statement on ECDH shared secret -> https://github.com/cose-wg/cose-issues/issues/3
    * BoucyCastle's BigInteger will convert the shared secret to BigInt and sometime, they remove leading zero
    * But Python's Cryptography ECDH shared secret will return byte and keep the leading zero
    * Hence, the server and the client is incompatible on this matter
    */
    BigInteger sharedSecret = aKeyAgree.CalculateAgreement(ecPubKey);
    // so, prepend 0x00 if the shared secret is not equal to 132/2 bytes
    byte[] sharedSecretByteArray = sharedSecret.ToByteArray();
    if(sharedSecretByteArray.Length != 66)
    {
        int diff = 66 - sharedSecretByteArray.Length;
        byte[] prependedSharedSecret = new byte[sharedSecretByteArray.Length + diff];
        for(int i = 0; i < diff; i++)
        {
            prependedSharedSecret[i] = 0x00;
        }
        Array.Copy(sharedSecretByteArray, 0, prependedSharedSecret, 1, sharedSecretByteArray.Length);
        return prependedSharedSecret;
    }
    return sharedSecretByteArray;
}