restricting function using ! GET secure?

[eluser]gloosemo[/eluser]
i need to restrict access to a function. i can't do it using private because i need the function available to ajax call, but i don't want user to access it by posting directly into browser.

if i make ajax call using GET, then place this at top of function:

Code:

$getData = $this->input->get();
if ( ! $getData ) die ("You cannot access this page. Please hit back on your browser");

this seems to work, but could the user access it by manually adding get variables to their url string? this makes me think i should only use POST, but isnt that slower than get.

Thanks, G