Add `Using Certificates` to Documentation

[ashtum]

We need to provide an explanation on using certificates and certificate authorities for client and server roles. Additionally, we should include a subsection that addresses common pitfalls and troubleshooting.

[Zen0x7]

I'll leave this as a suggested input resource:

In order to use SSL on TCP communications then you should use a SSL Context like the following code:

boost::asio::ssl::context ssl_context{boost::asio::ssl::context::tlsv12};
ssl_context.set_options(boost::asio::ssl::context::default_workarounds | boost::asio::ssl::context::single_dh_use);
ssl_context.use_certificate_chain_file("certificates/public.pem");
ssl_context.use_private_key_file("certificates/private.key", boost::asio::ssl::context::pem);
ssl_context.use_tmp_dh_file("certificates/dh-params.pem");

If you don't know how to how to get the public and private keys, you could use OpenSSL to generate them:

openssl genpkey -algorithm RSA -out private.key
openssl req -newkey rsa:2048 -key private.key -nodes -out server.csr -subj "/CN=*.company.com"
openssl x509 -req -in server.csr -signkey private.key -out public.pem -days 365000
openssl dhparam -out dh-params.pem 2048

Of course, those certificates will be not trusted by in-market browsers as they're not signed by a trusted authority.

If you're interested about how to use those certificates in the client, you could generate another public certificate using the third line of previous command, to get and use it on the clients context.

IMHO, when you're using those certificates in IoT implementations, use the highest quantity of days as possible, as expired certificates can raise handshake exceptions.

Be aware that using SSL will increase the CPU usage and network bandwidth in a directly proportional way to the encryption size (see this tool and compare the base64 result of "abc" being encrypted with 2048 bit key).