When you create an instance of this class, pass in a secret
phrase. The phrase is hashed with sha1 to make it the correct
length and form for an auth key. That allows you to use a long
string as the secret phrase, which means you can make it very
difficult to guess.
Each {@link Auth_OpenID_DumbStore} instance that is created for use by
your consumer site needs to use the same $secret_phrase.