Disable protect content in index.php

Hi, I want some contents in my wordpress are protected by a password, but not in the index, where it appear an excerpt of the post visible for everybody, but when the user want to see the complete post have to put a concrete password. If I protect the post with a password when I write the post, I can´t see the except in the index, but the text to put the password. Someone can help me? many thanks!