authentication method
Can't you just use standard SSL sockets to secure the connection, validate the user with a password, and then send the message to be published? If there won't be many clients, you can even use a self-signed certificate and put it in a KeyStore in the client app, that way you won't need to buy a certificate from Verisign or anyone else.
If you are storing the user's password hash on the server (as I think you should otherwise how are you going to validate the user's password), then you can first reproduce that hash on the client side (let's assume you hash user+pass) and then append that to the message and hash the result. You send the user id, hash and message to the server.
On the server side you receive a request with a user id, you retrieve the user's password (the server only has stored the hash of user+pass), you append that to the message and hash it again, and compare it against the hash in the request. If it matches, the user can publish the message.
Oh and you should use HTTP POST, instead of sending data on the URL. The server should not accept data on the URL for the publish request.
I am writing a server-client application to receive user message and publish it.
Thinking about authentication method.

Asymmetric encryption,  probably RSA.
Hash (salt+password+'msg'+'userid'), SHA256
HMAC, SHA256. seems to be more secured than the method 2. Also involve hashing the password and msg data.
Symmetric Encryption of the 'msg' with static password stored on both sides, probably AES.

There is no need for encryption as the msg would be publish online anyway. So I am more lean to the HMAC or PKI method.
I'm using java to send the request (including whatever authentication method to be implemented)
www.mysite.com/foo?userid=12345&msg=hello&token=abc1234
python on the server side to receive request and make sure the request is from the valid user.
The problems are in integration and make sure both sides understand each other's authentication token. It is a big factor for me. And the availability of the libraries available on both language are taken into account as well.
Or I can choose to rewrite the server side into java. (Why not python? The client side application need to be in java)
What do you think?

EDIT: I would not treat this as a web application. The application does not serve any webpage. Most of the operations are not related to the web. Just the client-server communication go through internet. And sending POST/GET via HTTP is what I thought of as the simpler albeit insecure way to do it. Feel free to suggest me any other alternative. 
SSL is a secure way for encryption. But it does not prevent attacker from sending message to the server impose as the user. Anyone can initiate a HTTPS connection. And the content of the communication does not need to be encrypted. It will be published online by the server anyway. What I need is a way to authenticate the message and sender.
For now I am lean to HMAC algorithm. RSA would be more secure but it comes with more developer efforts as well. Will see how it goes.
Thank you.