High cost encryption but less cost decryption

Question

I want that user/attacker to encrypt the data and send to server. Now I want an algorithm that is completely opposite to standard algorithms (fast to use, hard to decrypt), i.e it is very hard to encrypt the data like passwords using a key send by the server, to protect against random attacks, but very easy to decrypt so server consumes very less time in validating the user, but it becomes very hard for attacker, to every time encrypt new trial password with the key send by server.

Once again I am not talking about SSL.

Answer 1

It sounds as if you're looking for a proof-of-work scheme -- one of the applications for such a scheme is just what you describe: To force a client to do a certain amount of work and so prevent it from flooding a server with requests.