using System;

using System.IO;

using System.Security.Cryptography;

using System.Text;

​

public class Program

{

    public static byte[] alicePublicKey;

​

    public static void Main(string[] args)

    {

        Participant alice = new Participant("Alice");

        Participant bob = new Participant("Bob");

        alice.Send(bob, "VERY SECRET MESSAGE");

    }

}

​

public class Participant 

{

    public readonly String name;

    protected readonly ECDiffieHellmanCng ecdh;

    public Participant(String name) 

    {

        Console.WriteLine("[" + name + "] Creating ECDH");

        ecdh = new ECDiffieHellmanCng();

        ecdh.KeyDerivationFunction = ECDiffieHellmanKeyDerivationFunction.Hash;

        ecdh.HashAlgorithm = CngAlgorithm.Sha256;

        this.name = name;

    }

    public byte[] GetPublicKey() 

    {

        byte[] pk = ecdh.PublicKey.ToByteArray();

        Console.WriteLine("[" + name + "] Returning my PK: '" + BitConverter.ToString(pk) + "'");

        return pk;

    }

    public void Send(Participant receiver, String message) 

    {

        Console.WriteLine("[" + name + "] Sending message '" + message + "'");

        byte[] secretKey = DeriveKey(receiver.GetPublicKey());

        Console.WriteLine("[" + name + "] Derived shared key from " + receiver.name + "'s PK and " + name + "'s SK: '" + BitConverter.ToString(secretKey) + "'");       

        Payload p = Encrypt(secretKey, message);

        Console.WriteLine("[" + name + "] Encrypted '" + message + "' into '" + BitConverter.ToString(p.encrypted) + "'");

        receiver.Receive(this, p);

    }

    public void Receive(Participant sender, Payload payload) 

    {

        Console.WriteLine("[" + name + "] Received encrypted message '" + BitConverter.ToString(payload.encrypted) + "'");

        byte[] secretKey = DeriveKey(sender.GetPublicKey());

        Console.WriteLine("[" + name + "] Derived shared key from " + sender.name + "'s PK and " + name + "'s SK: '" + BitConverter.ToString(secretKey) + "'");     

        String message = Decrypt(secretKey, payload);

        Console.WriteLine("[" + name + "]  Decrypted into: '" + message + "'");

    }

    private byte[] DeriveKey(byte[] otherPublicKey)

    {

        return ecdh.DeriveKeyMaterial(CngKey.Import(otherPublicKey, CngKeyBlobFormat.EccPublicBlob));

    }   

    private Payload Encrypt(byte[] key, string message)

    {

        using (Aes aes = new AesCryptoServiceProvider())

        {

            aes.Key = key;

            byte[] iv = aes.IV;

​

            // Encrypt the message

            using (MemoryStream ciphertext = new MemoryStream())

            using (CryptoStream cs = new CryptoStream(ciphertext, aes.CreateEncryptor(), CryptoStreamMode.Write))

            {

                byte[] plaintextMessage = Encoding.UTF8.GetBytes(message);

                cs.Write(plaintextMessage, 0, plaintextMessage.Length);

                cs.Close();

                byte[] encryptedMessage = ciphertext.ToArray();

                return new Payload(encryptedMessage, iv);

            }

        }

    }       

    private String Decrypt(byte[] secretKey, Payload payload)

    {

        using (Aes aes = new AesCryptoServiceProvider())

        {

            aes.Key = secretKey;

            aes.IV = payload.iv;

            // Decrypt the message

            using (MemoryStream plaintext = new MemoryStream())

            {

                using (CryptoStream cs = new CryptoStream(plaintext, aes.CreateDecryptor(), CryptoStreamMode.Write))

                {

                    cs.Write(payload.encrypted, 0, payload.encrypted.Length);

                    cs.Close();

                    return Encoding.UTF8.GetString(plaintext.ToArray());

                }

            }

        }   

    }   

}

​

public class Payload

{

    public readonly byte[] encrypted;

    public readonly byte[] iv;

    public Payload(byte[] encrypted, byte[] iv) 

    {

        this.encrypted = encrypted;

        this.iv = iv;

    }

}

​