-type alg() ::
          #jose_jwe_alg_pbes2{hmac :: undefined | sha256 | sha384 | sha512,
                              salt :: undefined | binary(),
                              iter :: undefined | pos_integer(),
                              wrap :: undefined | aes_gcm_kw | aes_kw | c20p_kw | xc20p_kw,
                              bits :: undefined | 128 | 192 | 256,
                              iv :: undefined | binary(),
                              tag :: undefined | binary()}.