There is a very similar question here that but as far as I can tell it deals with one input PEM file whereas I have three, one of which is a chain file.
I have these three files generated by a LetsEncrypt helper program (win-acme).
foo.bar.co.uk-chain.pem
foo.bar.co.uk-crt.pem
foo.bar.co.uk-key.pem
What commands should I run in openssl (or otherwise) to generate two files in the form of foo.bar.co.uk.key and foo.bar.co.uk.crt?
The extensions in filenames do not matter.
foo.bar.co.uk-key.pem is foo.bar.co.uk.key and foo.bar.co.uk.crt is either just foo.bar.co.uk-crt.pem or the concatenation of foo.bar.co.uk-crt.pem and foo.bar.co.uk-chain.pem depending on where/how it is used.
PEM is just the name of the format to encode either the certificate or the key.
But again (since it is a very widespread false assumption): the filenames, including their extensions, do not matter at all. You can call your file foo.bar and it will work the same way, as long as the content is right.
