SSL/TLS Pentesting: What is a SSL certificate

SSL/TLS supports the use of authentication via X.509 certificates, which are often termed "SSL certificates" when used in this context. Server certificates enable the client to verify that it is connecting to the correct host. Though not usually used for HTTPS, SSL/TLS can also support mutual authentication in which the client proves its own identity through the provision of its own certificate.
Some of the main security properties which should be considered when setting up a certificate, include:
"Not Before" - This gives the start date of the certificate and should be a date in the past.
"Not After" - This gives the expiry date of the certificate after which is should not be trusted. It is therefore important to ensure that this is a date in the future. As the expiry date approaches, a new certificate should be issued to replace it.
"Signature Algorithm" - This is the algorithm used to ensure the certificate's integrity. MD5 has been shown to be inadequate for this, with collision attacks allowing fake, but valid, certificates to be generated. SHA1 is in the process of being phased out due to known weaknesses, with SHA2 hash functions being the preferred alternative.
"Public-Key" - The public key should be long enough to ensure that attacks are computationally infeasible. In the case of RSA, 2048 bit public keys are now considered a sensible minimum to protect against factoring attacks.
"Issuer" - This is the entity which has issued the certificate and should be a trusted party recognised by both the client and server. The issuer is typically a third-party certificate authority (such as DigiCert in the example above), though larger organisations often operate their own certificate authority to sign certificates for internal use. While it is possible to generate so-called "self-signed" certificates, these prevent the client from authenticating the server and open up the possibility of man-in-the-middle attacks in which an attacker dupes the client and/or server into communicating with the attacker rather than each other.

"Subject" and "Subject Alternative Name" - These should contain the DNS information necessary to tie the IP of the server running the SSL/TLS service. If these values are not valid domain names (or wildcard domains), then the client will be unable to determine whether or not the certificate is associated with the server in question and cannot therefore use it to authenticate the server.



to viewing the details of a servers certificate 
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command : openssl s_client -connect example.com:443 | openssl x509 -noout -text
you will get an output same like below

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Thanks 
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