Asymmetric cryptography example in C#

Question

I need to send confidential data to a server over a TCP connection. I have done a lot of researching and I understand the theoretical part. Based on what I have researched I want to do the following:

Note there is a server and a client: (we assume that public keys of either the client or server can be obtain by anyone)

1. client creates his public and private key. He is able to encrypt with his private key and decrypt with his public key.

2. server creates his public and private keys. private key is used to decrypt messages and public key is used to encrypt messages. (note is the other way around as with the client)

3. the client get's the server's public key. client then will be able to encrypt messages with that key and the only one that will be able to decrypt that message would be the server's private key.

4. since the server needs to be certain that the message comes from that specific client then the client will encrypt his name (signature) with his private key.

5. so the client message will contain: data to be send, client's public key, client name encrypted with the client's private key.

6. the client will encrypt the message with the public key from the server. client will then send that message to the server.

7. the server will decrypt the message it just received with his private key.

8. once the message is decrypted it will contain the data (info), encrypted signature, public key from client.

9. finally, the server will decrypt the client signature with the public key that was contained on the message to verify that the message is from that client.

---

OK so this is how asymmetric cryptography works. I have also researched about the classes that enable you to create this key pairs with the .NET framework. The classes that I researched that enable you do create this public and private key pairs are:

System.Security.Cryptography.DES
System.Security.Cryptography.DSACryptoServiceProvider 
System.Security.Cryptography.ECDsa 
System.Security.Cryptography.ECDsaCng 
System.Security.Cryptography.ECDiffieHellman 
System.Security.Cryptography.ECDiffieHellmanCng 
System.Security.Cryptography.RSA 
System.Security.Cryptography.RSACryptoServiceProvider 

so now my problems comes on how do I use one of this classes to do it with C#? I understand how the theoretical part works but how do I do what I just described with code. I have researched for some examples but I am having a hard time understanding them.

here is one example that I found that I believe does what I described:

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace Example
{
    class Program
    {
        static CngKey aliceKey;
        static CngKey bobKey;
        static byte[] alicePubKeyBlob;
        static byte[] bobPubKeyBlob;

        static void Main()
        {
            CreateKeys();
            byte[] encrytpedData = AliceSendsData("secret message");
            BobReceivesData(encrytpedData);

            Console.Read();

        }

        private static void CreateKeys()
        {
            aliceKey = CngKey.Create(CngAlgorithm.ECDiffieHellmanP256);
            bobKey = CngKey.Create(CngAlgorithm.ECDiffieHellmanP256);
            alicePubKeyBlob = aliceKey.Export(CngKeyBlobFormat.EccPublicBlob);
            bobPubKeyBlob = bobKey.Export(CngKeyBlobFormat.EccPublicBlob);
        }

        private static byte[] AliceSendsData(string message)
        {
            Console.WriteLine("Alice sends message: {0}", message);
            byte[] rawData = Encoding.UTF8.GetBytes(message);
            byte[] encryptedData = null;

            using (var aliceAlgorithm = new ECDiffieHellmanCng(aliceKey))
            using (CngKey bobPubKey = CngKey.Import(bobPubKeyBlob,
                  CngKeyBlobFormat.EccPublicBlob))
            {
                byte[] symmKey = aliceAlgorithm.DeriveKeyMaterial(bobPubKey);
                Console.WriteLine("Alice creates this symmetric key with " +
                      "Bobs public key information: {0}",
                      Convert.ToBase64String(symmKey));

                using (var aes = new AesCryptoServiceProvider())
                {
                    aes.Key = symmKey;
                    aes.GenerateIV();
                    using (ICryptoTransform encryptor = aes.CreateEncryptor())
                    using (MemoryStream ms = new MemoryStream())
                    {
                        // create CryptoStream and encrypt data to send
                        var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write);

                        // write initialization vector not encrypted
                        ms.Write(aes.IV, 0, aes.IV.Length);
                        cs.Write(rawData, 0, rawData.Length);
                        cs.Close();
                        encryptedData = ms.ToArray();
                    }
                    aes.Clear();
                }
            }
            Console.WriteLine("Alice: message is encrypted: {0}",
                  Convert.ToBase64String(encryptedData)); ;
            Console.WriteLine();
            return encryptedData;
        }

        private static void BobReceivesData(byte[] encryptedData)
        {
            Console.WriteLine("Bob receives encrypted data");
            byte[] rawData = null;

            var aes = new AesCryptoServiceProvider();

            int nBytes = aes.BlockSize >> 3;
            byte[] iv = new byte[nBytes];
            for (int i = 0; i < iv.Length; i++)
                iv[i] = encryptedData[i];

            using (var bobAlgorithm = new ECDiffieHellmanCng(bobKey))
            using (CngKey alicePubKey = CngKey.Import(alicePubKeyBlob,
                  CngKeyBlobFormat.EccPublicBlob))
            {
                byte[] symmKey = bobAlgorithm.DeriveKeyMaterial(alicePubKey);
                Console.WriteLine("Bob creates this symmetric key with " +
                      "Alices public key information: {0}",
                      Convert.ToBase64String(symmKey));

                aes.Key = symmKey;
                aes.IV = iv;

                using (ICryptoTransform decryptor = aes.CreateDecryptor())
                using (MemoryStream ms = new MemoryStream())
                {
                    var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Write);
                    cs.Write(encryptedData, nBytes, encryptedData.Length - nBytes);
                    cs.Close();

                    rawData = ms.ToArray();

                    Console.WriteLine("Bob decrypts message to: {0}",
                          Encoding.UTF8.GetString(rawData));
                }
                aes.Clear();
            }
        }
    }
}

In this program I believe the client is Alice and the server is Bob. I have to split this program into two parts. I am having a hard time understanding it and if I give it a try most likely I will make it work. Anyways how can I split this program into a server side code and client side code. I know how to send bytes between server and client. But I don't want to make it work without understanding what is going on. maybe you guys can show me an easier example.

---

EDIT

I managed to separate the code: here is the server code (the ip address of my computer happened to be 192.168.0.120) :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Net;
using System.Security.Cryptography;
using System.IO;


namespace ServerListener
{
    class Program
    {
        static TcpListener server;


        //static CngKey aliceKey;
        static CngKey bobKey;
        static byte[] alicePubKeyBlob;
        static byte[] bobPubKeyBlob;

        static void Main(string[] args)
        {

            CreateKeys();

            IPAddress ipAddress = IPAddress.Parse("192.168.0.120");
            server = new TcpListener(ipAddress, 54540);
            server.Start();
            var client = server.AcceptTcpClient();
            var stream = client.GetStream();

            alicePubKeyBlob = new byte[bobPubKeyBlob.Length];
            stream.Read(alicePubKeyBlob, 0, alicePubKeyBlob.Length);

            stream.Write(bobPubKeyBlob, 0, bobPubKeyBlob.Length);

            byte[] encrytpedData = new byte[32];

            stream.Read(encrytpedData, 0, encrytpedData.Length);

            BobReceivesData(encrytpedData);


        }

        private static void CreateKeys()
        {
            //aliceKey = CngKey.Create(CngAlgorithm.ECDiffieHellmanP256);
            bobKey = CngKey.Create(CngAlgorithm.ECDiffieHellmanP256);
            //alicePubKeyBlob = aliceKey.Export(CngKeyBlobFormat.EccPublicBlob);
            bobPubKeyBlob = bobKey.Export(CngKeyBlobFormat.EccPublicBlob);
        }


        private static void BobReceivesData(byte[] encryptedData)
        {
            Console.WriteLine("Bob receives encrypted data");
            byte[] rawData = null;

            var aes = new AesCryptoServiceProvider();

            int nBytes = aes.BlockSize >> 3;
            byte[] iv = new byte[nBytes];
            for (int i = 0; i < iv.Length; i++)
                iv[i] = encryptedData[i];

            using (var bobAlgorithm = new ECDiffieHellmanCng(bobKey))
            using (CngKey alicePubKey = CngKey.Import(alicePubKeyBlob,
                  CngKeyBlobFormat.EccPublicBlob))
            {
                byte[] symmKey = bobAlgorithm.DeriveKeyMaterial(alicePubKey);
                Console.WriteLine("Bob creates this symmetric key with " +
                      "Alices public key information: {0}",
                      Convert.ToBase64String(symmKey));

                aes.Key = symmKey;
                aes.IV = iv;

                using (ICryptoTransform decryptor = aes.CreateDecryptor())
                using (MemoryStream ms = new MemoryStream())
                {
                    var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Write);
                    cs.Write(encryptedData, nBytes, encryptedData.Length - nBytes);
                    cs.Close();

                    rawData = ms.ToArray();

                    Console.WriteLine("Bob decrypts message to: {0}",
                          Encoding.UTF8.GetString(rawData));
                }
                aes.Clear();
            }
        }
    }
}

and here is the client code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Net;
using System.Security.Cryptography;
using System.IO;

namespace ClientAlice
{
    class Program
    {
        static CngKey aliceKey;
        //static CngKey bobKey;
        static byte[] alicePubKeyBlob;
        static byte[] bobPubKeyBlob;

        static void Main(string[] args)
        {

            CreateKeys();
            bobPubKeyBlob = new byte[alicePubKeyBlob.Length];

            TcpClient alice = new TcpClient("192.168.0.120", 54540);

            var stream = alice.GetStream();
            stream.Write(alicePubKeyBlob, 0, alicePubKeyBlob.Length);

            stream.Read(bobPubKeyBlob, 0, bobPubKeyBlob.Length);


            byte[] encrytpedData = AliceSendsData(":)");

            stream.Write(encrytpedData, 0, encrytpedData.Length);


        }


        private static void CreateKeys()
        {
            aliceKey = CngKey.Create(CngAlgorithm.ECDiffieHellmanP256);
            //bobKey = CngKey.Create(CngAlgorithm.ECDiffieHellmanP256);
            alicePubKeyBlob = aliceKey.Export(CngKeyBlobFormat.EccPublicBlob);
            //bobPubKeyBlob = bobKey.Export(CngKeyBlobFormat.EccPublicBlob);
        }

        private static byte[] AliceSendsData(string message)
        {
            Console.WriteLine("Alice sends message: {0}", message);
            byte[] rawData = Encoding.UTF8.GetBytes(message);
            byte[] encryptedData = null;

            using (var aliceAlgorithm = new ECDiffieHellmanCng(aliceKey))
            using (CngKey bobPubKey = CngKey.Import(bobPubKeyBlob,
                  CngKeyBlobFormat.EccPublicBlob))
            {
                byte[] symmKey = aliceAlgorithm.DeriveKeyMaterial(bobPubKey);
                Console.WriteLine("Alice creates this symmetric key with " +
                      "Bobs public key information: {0}",
                      Convert.ToBase64String(symmKey));

                using (var aes = new AesCryptoServiceProvider())
                {
                    aes.Key = symmKey;
                    aes.GenerateIV();
                    using (ICryptoTransform encryptor = aes.CreateEncryptor())
                    using (MemoryStream ms = new MemoryStream())
                    {
                        // create CryptoStream and encrypt data to send
                        var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write);

                        // write initialization vector not encrypted
                        ms.Write(aes.IV, 0, aes.IV.Length);
                        cs.Write(rawData, 0, rawData.Length);
                        cs.Close();
                        encryptedData = ms.ToArray();
                    }
                    aes.Clear();
                }
            }
            Console.WriteLine("Alice: message is encrypted: {0}",
                  Convert.ToBase64String(encryptedData)); ;
            Console.WriteLine();
            return encryptedData;
        }
    }
}

I thinks it is pretty secure. Every time it sends a different byte array although sending the same info!

Answer 1

Posting as an answer since it would be too long for a comment - it isn't specifically answering your question though.

As mentionned in the comment by driis, you should really rely on existing solutions which are regarded as being secure. That said, your protocol does have security issues:

• Communication is usually two-way, you however only seem to address one-way communication (client to server). This doesn't make much sense, since you say that you're going to use TCP, which is a two-way protocol in itself.

• Steps 4 and 5 are buggy: since you send the public key of the client inside the message, anyone could create a pair and encrypt the client identification using this pair. From your description the server has no forward knowledge of the client's keys, which makes this signature do nothing but ensure the integrity of the message - specifically is does not in any way make the client identification trustworthy.

For proper identification, you do have additional prerequisites; the server has to know the client's public key in advance or it has to be able to trust the client's claim to be himself by using a trusted 3rd party. This is what certificates and the certificate trust chains are about: if that client presents a certificate issued by the 3rd party X and the server trusts X, then he can assume that the client is who he pretends to be.

SSL basically supports two modes:

• Either only the server identity is verified and any client can communicate with it; the client's identity is not verified, only that (after the connection has been negotiated) it always is the same client which communicates to the server. This is the typical usage for online shopping etc. - you (as the client) trust the server and create a trusted connection, but the server does not know who you are.

• Two-way authentification can be done as well by using client certificates. The server has to know and trust either the client certificate directly or the issuer of the client certificate in order to negotiate the connection successfully. In this scenario, the server does indeed know who the client is, but the prerequisite as mentioned above has to be met.