def hash(cls, secret, user, realm, encoding=None):
        # NOTE: this was deliberately written so that raw bytes are passed through
        # unchanged, the encoding kwd is only used to handle unicode values.
        if not encoding:
            encoding = cls.default_encoding
        uh.validate_secret(secret)
        if isinstance(secret, unicode):
            secret = secret.encode(encoding)
        user = to_bytes(user, encoding, "user")
        realm = to_bytes(realm, encoding, "realm")
        data = render_bytes("%s:%s:%s", user, realm, secret)
        return hashlib.md5(data).hexdigest()