This is its description:

Prevent users from logging into your WordPress installation for a certain timeframe or permanently. Works also great with the multisite user management area. The locked users are easily manageable within the users overview page. The settings are within the user edit pages. No user can lock himself. No user with a lower role can lock a higher user and administrators in a network can not lock super administrators. If a user is locked, he is instantly logged out of any session until the lock expires or is removed.

No useless overhead, no ads. Just a tiny, but very effective plugin to keep your website secure.

• you do not want to delete a user, but you want to make sure he can not access the site (at least for a certain timeframe)
• an employee leaves your company and access should be removed, but you want to keep his user as an author in the system
• you want to punish a certain user which did bad things for a certain timeframe
• you want only few persons to have access to your WordPress site in order to minimize risk of incidents
•  … or maybe some other use case

Would be great to hear whatever feedback or improvement ideas 😉 

1. Learning the Basics of Network communication

• Networking Basics: Short explanation of networking basics like TCP, UDP and ports.
• Java IO Tutorial: It’s absolutely necessary to be familiar with I/O and streams to do sockets operations!
• Java TCP Sockets Tutorial: A very useful introduction to (TCP) sockets programming.
• Java Datagrams Tutorial: Provides all the information you need to send and receive datagram packets using UDP.
• Java Concurrency Tutorial: A tutorial about concurrency that covers threads, thread-pools and synchronization.
• Java Programmierhandbuch und Referenz – Section 13.2.3: German tutorial for using datagram sockets. (optional)

Important Java-API Classes:

• IO: IO Package API
• Concurrency: Thread API, Runnable API, ExecutorService API, Executors API
• Java TCP Sockets: ServerSocket API, Socket API
• Java Datagrams: DatagramSocket API, DatagramPacket API

2. Learning RMI

• Java RMI Tutorial: A short introduction into RMI.
• JGuru RMI Tutorial: A more detailed tutorial about RMI.
• JavaInsel RMI Tutorial: German introduction into RMI programming. (optional)

Important Java-API Classes:

• RMI: Remote API, UnicastRemoteObject API, Registry API, LocateRegistry API
• Properties: Properties API
• IO: IO Package API

3. Security

• Chapter 9 – Security from the book Distributed Systems: Principles and Paradigms (2nd edition)
• Java Cryptography Architecture (JCA) Reference Guide: Tutorial about the Java Cryptography Architecture (JCA)
• Handbook of Applied Cryptography (free version) (optional)

Important Java-API Classes:

• Java SE 6: Cipher, SecureRandom, and Signature classes
• Bouncy Castle: PEMReader and Base64