Decrypting mod_session-created cookie

[Mikhail T.]

From PHP I need to be able to set and read the session cookies created by 
mod_session_crypto.

Suppose, I know the SessionCryptoCipher (aes256, the default) and the 
SessionCryptoPassphrase, how can I decrypt the session-cookie? Its value is 
available to PHP as _REQUEST['session']...

I have both openssl and mcrypt PHP-extensions available on the server, but the 
straightforward approach of

   $decrypted = openssl_decrypt($_REQUEST['session'], 'aes256', $password);

is failing... Thank you! Yours,

   -mi