openssl command

pfx pem cer pub pri

pfx file:
  https://www.openssl.org/docs/apps/pkcs12.html
  to a pem file
  openssl pkcs12 -in citic_root.pfx -out citic_root.pem -nodes
  
cer file:
  https://www.openssl.org/docs/apps/x509.html
  to rsa pub
  openssl x509 -in citic_test_server.cer -pubkey -noout
  
pem file:
  https://www.openssl.org/docs/apps/rsa.html
  to rsa pub or pri
  openssl rsa -in private_key_name.pem -out new_private.pem -pubout
  
get server cer:
  openssl s_client -connect 202.108.57.21:443 -showcerts

so
*.crt == *.pem
*.cer == *.der