Edit/2020-06-05/jovirkku: the given folders are no more correct on 3.4.0-devel.
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REPLY #2 TO A CUSTOMER
This is another way of tackling this issue - a late proposal from our R&D:
I did test that these instructions work e.g. I was able to set test root certificate to Xperia X device and show my test root cert in Root Certificate Manager application (installed from android play store).
1. The following link contains the instructions on how to convert PEM certificate to android format (*.0) and where to copy it.
(see the Android Phones & Tablets -> CAcert system trusted certificates -> creating + importing)
2. Login to the device with ssh (ssh firstname.lastname@example.org) and switch to root (devel-su)
3. Copy the certificate file in to folder
4. Check that file permissions are the same as other certificate files (*.0)
5. Open some android certificate manager application e.g Root Certificate Manager app and check that certificate is shown.
REPLY #1 TO A CUSTOMER
Your question was how to install the Let's Encrypt root certificate.
Unfortunately, we do not have a definite answer for you, however, the hints below can hopefully help you.
There is a way to add certificates to Sailfish OS but Synology is an Android app. Android apps (and Android Support of Sailfish OS) do not see the certificates installed on the Sailfish side.
One solution could be that you upload the certificate to the folder $HOME
/android_storage/my_certificates/ in your Xperia device. Android applications may be able to find the certificates from that folder.
The certificate can be uploaded to the device via SSH or via an SD card and copied to my_certificates folder using the File Browser application.
There are also Android applications for installing certificates to a system. One is Android Certificate Installer app, which can see /android_storage folder. That seems to use a web browser for installing the certificate.