Creating an AsymmetricCipherKeyPair with just the

Question

I'm having a bit of an issue. I'm trying to write a client-server application (more a POC really), with the client requesting the public key from the server, the server then generates this key AsymmetricCipherKeyPair with the following

    private static AsymmetricCipherKeyPair GenerateKeyPair() {
      RsaKeyPairGenerator g = new RsaKeyPairGenerator();
      g.Init(new KeyGenerationParameters(new SecureRandom(), 1024));
      var pair = g.GenerateKeyPair();
      return pair;
}

This generates correctly and can encrypt and encrypt correctly(on the server side, but that defeats the purpose)

So I then send a serialized public key to the client

    private static byte[] SerialisePublic(AsymmetricCipherKeyPair keypair) {
    SubjectPublicKeyInfo publicKeyInfo = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(keypair.Public);
    byte[] serializedPublicBytes = publicKeyInfo.ToAsn1Object().GetDerEncoded();
    return serializedPublicBytes;
}
    private static void SendBytes(TcpClient client, byte[] send) {
    NetworkStream stream = client.GetStream();
    stream.Write(send, 0, send.Length);
    stream.Flush();
}

However now, when I try to initialise the keypair on the otherside, I need a private key (which also defeats the purpose)

I tried the following

 static string EncryptString(string key, string message) {
        string res = "";
        using (var rsa = new RSACryptoServiceProvider(128)) {
            try {
                rsa.FromXmlString(string.Format("<RSAKeyValue>{0}</RSAKeyValue>", key));
                byte[] b = Encoding.ASCII.GetBytes(message);
                var encryptedData = rsa.Encrypt(b, true);
                var base64Encrypted = Convert.ToBase64String(encryptedData);
                res = base64Encrypted;
            }
            catch (Exception ex) {
            }
            finally {
                rsa.PersistKeyInCsp = false;
            }
            return res;
        }
    }

(In the absence of a keypair, I thought I could hack it)

however as this does not contain nearly half the information required for encryption, it fails, hard.

I'd like to keep to using Bouncy Castle if possible, however I am open to suggestions.

Thanks in advance.

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UPDATE

Both client and server are being written in C#

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Update 2 (thanks Andrew)

This will initialise the RsaEngine with the public key provide by the server(or at least is should) (This didn't work, don't use it)

    static byte[] EncryptBytes(byte[] key, byte[] message) {
        byte[] res = null;
        AsymmetricKeyParameter ic = (AsymmetricKeyParameter)PublicKeyFactory.CreateKey(key);
        RsaEngine e = new RsaEngine();
        e.Init(true, ic);
        res = e.ProcessBlock(message, 0, message.Length);
        return res;
    }

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Update 3

So that didn't work, the PublicKeyFactory doesn't seem to like the returned key throwing: Unknown object in GetInstance: Org.BouncyCastle.Asn1.DerApplicationSpecific

I think it could be an issue in the Serialisation on server side and then the deserialisation on the client side.

So maybe a CryptoStream for the transmission of a public key and additional authentication (passphrase, secret knock or a joke) when a client connects to a server.

stay tuned

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