def pseudonymize_user_name(self, user_name: UserName) -> PseudoUserName:
    hasher = hashlib.sha256()
    hasher.update(user_name.encode('utf-8'))

    # salt
    hasher.update(b'\0')
    hasher.update(self.salt)

    pseudonymized = base64.b64encode(hasher.digest()).decode('utf-8')
    return PseudoUserName(pseudonymized)