White page while adding Twitter account

Hi.
I've tried everything suggested on this post: http://drupal.org/node/404470.
- uninstalled and reinstalled different version of the module (with db update and cache clearing)
- using HMAC-SHA1 and RSA-SHA1 OAuth options, etc.
Unfortunately, without success.

The result is still the same, while clicking on the 'Add Account' button on the page http://www.sinyayadynya.com/en/user/1/edit/twitter, i'm getting a white page!

On my error log file, there is the following messages:
PHP Fatal error: Call to undefined function hash_hmac() in /usr/local/www/vhosts/dmetille/web.bodhi3.com/htdocs/sites/all/modules/oauth/oauth.lib.php on line 88, referer: http://www.sinyayadynya.com/en/user/1/edit/twitter

Is this a bug or i'm missing something?

Thanks for help.