Extract Certificate Information with OpenSSL

Fetch the X.509 Public Key Certificate File 

 openssl s_client -connect google.com:443 -showcerts </dev/null | openssl x509 -outform pem > googlecert.pem 

 Decode the Entire Certificate 

 openssl x509 -in googlecert.pem -noout -text 

 Extract Specific Information from the Certificate 

 Extract the Subject 

 openssl x509 -in googlecert.pem -noout -subject subject=CN = *.google.com 

 Extract the Issuer 

 $ openssl x509 -in googlecert.pem -noout -issuer issuer=C = US, O = Google Trust Services LLC, CN = GTS CA 1C3 

 Extracting the Extension Fields 

 $ openssl x509 -in googlecert.pem -noout -ext subjectAltName X509v3 Subject Alternative Name: DNS:*.google.com, DNS:*.appengine.google.com, DNS:*.bdn.dev, DNS:*.cloud.google.com, DNS:*.crowdsource.google.com, DNS:*.datacompute.google.com, ...(truncated)