Client Server Authentication

Question

I am creating a client server communication based on Asynchronous Sockets, my client will send the username and password to the server then the server will replay whether the account is valid, so i want to secure this steps so no one could record the conversation and keep sending it to my client to achieve illegal entry to the secret data

[The Question {Simplified}] How to securely authenticate the client to the server ... ?

[NOTE] I know SSL but i cant afford paying for a certificate so i need a free alternative to provide secure communication between my client and server.

Answer 1

As always, the most secure password is the one, that the server doesn't know, and that is never transmitted. So what you could do is:

• On the server, store the username, a random salt ("account salt") and a secure hash of the salted password ("server shared secret").
• On login, in a first step let the client transmit only the username (not secret)
• The server should reply with the account salt (not secret) and a randomly generated session salt (not secret). It is important, that the server generates the session salt.
• On the client, salt the password with the account salt and hash it (keep this as " client shared secret"), then salt the result with the session salt and hash it again. Transmit this as an authentication token (not secret)

• On the server, take the salted hash from your DB, salt it with the session salt and hash it - if this matches the authentication token, the connection is authenticated. (Client is authenticated to server)

• if you want to additionaly authenticate the server to the client, you repeat the procedure: Client generates a salt, server creates token from it by salting/hashing the stored secret.

• If you want to authenticate the single requests (not only the connection), salt them with the shared secret and hash them, send this as a per-request authentication field. Since in a valid login server shared secret and client shared secret are identical,both sides should come to the same result, thus verifying the authentication field.