Tales of a Code Monkey

I wanted to prototype encrypted communication channel between a client and a server. Now of course there are HTTPS and other TLS channels that work quite well, but what I have in mind is supposed to be used to transfer rather sensitive data. So how can I establish a secure channel through an HTTP/HTTPS channel?

1. Have the server generate an RSA key pair and send the public key to the client.
2. Have the client generate an AES key, encrypt it with the received public key, and send the encrypted key to the server.
3. Let the server decrypt the AES key.
4. Both the client and the server are now in possession of the same AES key and can therefore communicate securely.

Of course, the generated AES key should only be used for the communication with the one client which sent it, so some sort of secure key management on the server (also regarding the RSA key pair) is vital. Also, the AES key could periodically be updated (i.e. a new key generated). At the very least, every message sent back and forth encrypted with AES will have to use a separate IV — but naturally that IV could be part of the transmitted message. So let’s get a very basic REST API-based implementation going.

Generate RSA key-pair on the Server

[...]

public sealed class SessionKey
{
    public Guid Id;
    public byte[] SymmetricKey;
    public RSAParameters PublicKey;
    public RSAParameters PrivateKey;
}

[...]

private Dictionary<Guid, SessionKey> sessionKeys;

[...]

public RSAParameters Generate(Guid sessionId)
{
    // NOTE: Make the key size configurable.
    using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(2048))
    {
        SessionKey s = new SessionKey()
        {
            Id = sessionId,
            PublicKey = rsa.ExportParameters(false /* no private key info */),
            PrivateKey = rsa.ExportParameters(true /* with private key info */),
            SymmetricKey = null, // To be generated by the client.
        };

        sessionKeys.Add(id, s);

        return s.PublicKey;
    }
}

[...]

This key generation can then be used to generate a new RSA key pair whenever a new client connects and requests secure communication. Of course, make sure you send the public key back to the client, and not the private key — else there’s no point in encrypting in the first place.

Generate an AES key on the Client

[...]

// Get the Public Key from the Server
RSAParameters publicKey = GetFromServer(...);

// Holds the current session's key.
byte[] MySessionKey;

// Send encrypted session key to Server.
SendToServer(GenerateAndEncryptSessionKey(publicKey));

[...]

private byte[] GenerateAndEncryptSessionKey(RSAParameters publicKey)
{
    using (Aes aes = Aes.Create())
    {
        aes.KeySize = aes.LegalKeySizes[0].MaxSize;
        // Setting the KeySize generates a new key, but if you're paranoid, you can call aes.GenerateKey() again.

        MySessionKey = aes.Key;
    }

    using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
    {
        rsa.ImportParameters(publicKey);

        return rsa.Encrypt(MySessionKey, true /* use OAEP padding */);
    }
}

[...]

As you can see, we just take the public key we got from the server to set up the RSA provider and then encrypt the generated AES key using that public key. Once the client sends the encrypted key to the server, they both share the same secret and can securely communicate with each other.

Decrypt AES Key on the Server

[...]

public void SetSymmetricKey(Guid id, byte[] encryptedKey)
{
    SessionKey session = sessionKeys[id];

    using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider())
    {
        rsa.ImportParameters(session.PrivateKey);

        session.SymmetricKey = rsa.Decrypt(encryptedKey, true /* use use OAEP padding */);
    }
}

[...]

Since we already have the private key for this session, we can just use it to decrypt the AES key we got from the client. Again, making sure that the stored symmetric key is safe, is key to security.

Encrypt / Decrypt

Encrypting and decrypting can now be done the same way on both sides (since we’re using a symmetric-key algorithm). So here’s what that looks like.

[...]

public byte[] EncryptData(byte[] key, string data)
{
    using (Aes aes = Aes.Create())
    {
        byte[] result;

        aes.Key = key;
        aes.GenerateIV();

        using (ICryptoTransform encryptor = aes.CreateEncryptor())
        {
            using (MemoryStream ms = new MemoryStream())
            {
                using (CryptoStream cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter writer = new StreamWriter(cs))
                    {
                        writer.Write(data);
                    }
                }

                byte[] encrypted = ms.ToArray();
                result = new byte[aes.BlockSize / 8 + encrypted.Length];

                // Result is built as: IV (plain text) + Encrypted(data)
                Array.Copy(aes.IV, result, aes.BlockSize / 8);
                Array.Copy(encrypted, 0, result, aes.BlockSize / 8, encrypted.Length);

                return result;
            }
        }
    }
}

public string Decrypt(byte[] key, byte[] data)
{
    using (Aes aes = Aes.Create())
    {
        aes.Key = key;

        // Extract the IV from the data first.
        byte[] iv = new byte[aes.BlockSize / 8];
        Array.Copy(data, iv, iv.Length);
        aes.IV = iv;

        // The remainder of the data is the encrypted data we care about.
        byte[] encryptedData = new byte[data.Length - iv.Length];
        Array.Copy(data, iv.Length, encryptedData, 0, encryptedData.Length);

        using (ICryptoTransform decryptor = aes.CreateDecryptor())
        {
            using (MemoryStream ms = new MemoryStream(encryptedData))
            {
                using (CryptoStream cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader reader = new StreamReader(cs))
                    {
                        return reader.ReadToEnd();
                    }
                }
            }
        }
    }
}

[...]

As you can see, each time we encrypt something we generate a new IV, which we send at the beginning of the data to the other side. The other side then extracts the IV first and uses it to initialize AES.

REST APIs?

Using all this through REST APIs is trivial: All you really need to make sure is that the client sends the session GUID (or whatever you use to identify a session) with every encrypted message, either through the URL, parameters or headers. Of course it is vital to guarantee that a client cannot get access to another client’s session (e.g. to provide a new session key), but through ordinary (secure) authentication that should easily be doable.

Next Steps

As far as encryption is concerned, this should already do the trick. You may want to add signatures to the encrypted messages too, to make sure that the encrypted blocks have not been tampered with. In addition, the AES key exchange could be repeated periodically (maybe even after every exchanged message).