'Authenticate the cooke using the signature, verify that it
has not expired; and return the cookie\'s content'

def auth(self, cookie):

    decode = base64.decodestring(cookie.replace('_', '/').replace('~', '='))
    signature = decode[:_signature_size]
    expires = decode[_signature_size:_header_size]
    content = decode[_header_size:]
    if (signature == hmac.new(self.secret, content, sha1).digest()):
        if (int(expires) > int(make_time(time.time()))):
            return content
        else:
            pass
    else:
        pass
