Mini-Messaging-Service

How to Run

• To compile the code run javac *.java
• To run the client run java Client Username SERVER_IP_ADDRESS PORT_NO MODE

  MODE is 0 for unencrypted messages, 1 is for encrypted messages without signature, 2 is for encrypted message with signature.

• To run the server run java Server PORT_NO MODE

Parameter Validation

If the mode of the client/server is negative integer or > 2, error message is printed and client/server is exited completely.

Sending and Receiving a Message

Suppose Saurav is sending a message to Nikhil

• Saurav sends the message as @Nikhil [MESSAGE]
• Saurav receives the message Succesfully sent upon succesful delivery or any other appropriate message.
• Nikhil sees the message as @Saurav: [MESSAGE] (SAFE)

  Safe indicates the signature is verified and is only available in Mode 2 If the message is tampered the sender gets the message Message Authenticaition Failed

Registration and Deregistration

• The initial username is taken from the parameters. If this username is taken or invalid the client is prompted to enter a valid username till the username is verified by the server.
• If the mode of the client does not match with that of the server, error message is printed and the client is exited completely.
• The client can deregister by typing in DEREGISTER.
• On pressing Ctrl-C the client is deregistered and a message is printed on the server that it exited abruptly.
• When Ctrl-C is pressed on the server, all clients are closed after printing a message that the server closed.

Socket and Port Handling

• While initialising the server if the port is occupied by some other process or its permission is denied the error message is printed and the server exits completely
• While initialising the client if the port or ip address does not match with that of the server the error is printed and the client exits completely