der_encode_length
firstbytes = 1 + der_encode_length(custom_data->nbytes1, NULL)
secondbytesinner = 1 + der_encode_length(custom_data->nbytes2, NULL)
secondbytes = 1 + der_encode_length(secondbytesinner, NULL) + secondbytesinner;
seqbytes = 1 + der_encode_length(sizeof(t_true) + firstbytes + secondbytes, NULL)
der_encode_length(sizeof(t_true) + firstbytes + secondbytes, &p);
der_encode_length(custom_data->nbytes1, &p);
der_encode_length(secondbytesinner, &p);
der_encode_length(custom_data->nbytes2, &p);