PKCS 12
Contents
Full Title
PKCS #12: Personal Information Exchange Syntax", PKCS Version 1.1, December 2012.
Context
The full PKCS #12 standard is very complex. It enables buckets of complex objects such as PKCS #8 structures, nested deeply. But in practice it is normally used to store just one private key and its associated certificate chain.
Problem
You need .pfx file to install https on website for Microsoft's or many other web servers.
If you have two separate files: certificate (.cer or pem) and private key (.crt) they need to be converted to P12 or PFX format.
Solution
You will need to use openssl.
openssl pkcs12 -export -out domain.name.pfx -inkey domain.name.key -in domain.name.crt
The key file is just a text file with your private key in it.
If you have a root CA and intermediate certs, then include them as well using multiple -in params
openssl pkcs12 -export -out domain.name.pfx -inkey domain.name.key -in domain.name.crt -in intermediate.crt -in rootca.crt
You can install openssl from here: openssl
References
This standard is now maintained as RFC 7292