Arduino Can't Download library_index.json: How to add an intermediate certificate to a MacOS Java application

gistfile1.txt

# I will use the Arduino application as an example, as I encountered this error while doing some projects at home.
# I think that this method will work with most Java based apps, but YMMV
# I assume you have access to a Unix terminal and basic knowledge of how to use it.

# Step 1: get the certificates from the website you have trouble with

openssl s_client -showcerts -verify 5 -connect downloads.arduino.cc:443 < /dev/null

# This will output a verbose list of a series of certificates (a chain), which is used to establish trust.
# Each certificate starts with a BEGIN CERTIFICATE line and ends with an END CERTIFICATE line. These are part of the certificate.
# Create a file for each certificate, and name it something reasonable.  I created three text files called
# cisco_root.crt, cisco_primary.crt and cisco_secondary.crt with the output from the openssl command.

# Step 2: find the cacert file for the Java runtime bundled with you application.  Usually this is part of the application bundle
# installed in the /Applications folder.  E.g.:
# /Applications/Arduino.app/Contents/PlugIns/jre8u252-b09.jre/Contents/Home/lib/security

# create a backup of the cacert file

cp cacert cacert.bak

# Step 3: add the certificates using the keytool

# the cacert default password is: changeit
# all lowercase

keytool -import -trustcacerts -alias cisco_root -file ~/Documents/CiscoCrt/cisco_umbrella_root.crt -keystore cacerts
keytool -import -trustcacerts -alias cisco_primary -file ~/Documents/CiscoCrt/cisco_umbrella_primary.crt -keystore cacerts
keytool -import -trustcacerts -alias cisco_secondary -file ~/Documents/CiscoCrt/cisco_umbrella_secondary.crt -keystore cacerts

# Step 4: Restart the arduino app, and you should now be able to load the library file!