You can use OpenSSL, which is commonly installed on Mac OS X and Linux systems and which is also available for other operating systems (the source code can be downloaded from OpenSSL Downloads and a Windows implementation is available at OpenSSL for Windows), to check the security certificate of a website using an openssl command in the form
openssl s_client -showcerts -connect fqdn:portwhere fqdn is the fully qualified domain name (FQDN) of the website and port is the port that the website is listening on for HTTPS connections, which is usually well-known port 443, though it may sometimes be another port, such as the registered port 8443. The
showcertsoption instructs openssl to show all certificates in the public key certificate chain.
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