Found here
Phew.. finally got this working. What a royal pain in the ass! Here is how I did it (based on @Cl3MM post above):
- Download R1 GlobalSign Root Certificate: https://secure.globalsign.net/cacert/Root-R1.crt
- Save it somewhere local, so you can easily access it from a command prompt. It should save with the name Root-R1.crt.
- Convert to a PEM file. Here is where the post above didn't work for me, but this did: openssl x509 -in Root-R1.crt -out mycert.pem -outform PEM -inform DEF
- Copy the new mycert.pem file that it creates into: D:\Ruby23-x64\lib\ruby\2.3.0\rubygems\ssl_certs <-- this path will obviously be different depending on where you've got Ruby installed!
I can now install gems properly without any more annoying SSL errors. This was on Windows 10 Anniversary edition with Ruby 2.3.1