Preface
Older Android devices such as my HTC One M8 running stock Android 6 Marshmallow may not trust newer SSL certificate authorities triggering invalid certificate issues. While in some cases you can bypass it, in others it may prevent you to perform your actions. Like the built-in Android download will just fail unless it finds a valid certificate.Install new certificate authority
To fix it you will need to install new certificate authorities. You can notably download letsencrypt.org certificate in der format. Rename it to.crt. Then on HTC One M8 I needed to go to Settings > Security > Install from storage — name it and select: VPN and apps. You will need to have set a screen lock option for this to work.All done, sites using this authority should not give you any warnings in the various apps you are using such as Fulguris Web Browser and the built-in Android download manager.
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