How can I get the public key of a webpage like verisign , etc. using HTTPS protocol?
This command will show you the certificate (use -showcerts as an extra parameter if you want to see the full chain):

openssl s_client -connect the.host.name:443


This will get the certificate and print out the public key:

openssl s_client -connect the.host.name:443 | openssl x509 -pubkey -noout


If you want to dig further, this question might be of interest.
