If you think about what "client-side" means, you'll understand why it wouldn't be possible. On the client side, all information is processed and all data is stored on the client's machine. Thus, in order to check passwords totally on the client side, you'd have to have the entire list of usernames passwords saved to every single computer that your site interacts with.
Keep in mind, also, that anything "client side" is inherently less secure than it would be server-side. So even if you could do this, it'd probably be a bad idea.
Now, that being said, here's how you could do it:
Come up with a password (note that you'll have to use a master password, not individual ones), hash the password, and store the hashed password in a variable, or something. It may sound insecure, but if you're hashing the pw, you could send everyone a personalized email with the password and it wouldn't make it any easier to hack. Hashing is a one-way operation. There is no way to reverse engineer it. Do make sure to make your stored password a constant, though, or someone could just change the value of their hashed password last minute to match your stored one. Give the css rule Display:none;
to your data with css so that it hides. Now prompt the user for a password. Then, take that password and run it through the same encryption as you did when you created the password. If that value and the one you stored match, bingo. Just revert your display:none;
and you're good to go.
Though I guess that still leaves the problem of somebody just firebugging your display:none
away. Guess you'd have to encrypt the data on the page too.