key_id, kem_id, kdf_id, aead_id, enc, ct = parse(enc_request)
info = concat(encode_str("message/bhttp request"),
              encode(1, 0),
              encode(1, key_id),
              encode(2, kem_id),
              encode(2, kdf_id),
              encode(2, aead_id))
rctxt = SetupBaseR(enc, skR, info)
request, error = rctxt.Open("", ct)