How to switch to SSL during an open TCP connection

[FalconMuc]

Hi everybody,
in several situations it is necessary to switch from un unencrypted connection “TCP” to an encrypted connection “SSL”. Two examples for this are the communication with a SMTP server (command STARTTLS) or with a FTP server (-> FTPS command AUTH TLS).

I already tested direct SSL connections e.g. with “implicit” SSL connections successfully. This is possible if the server provides a dedicated port to directly connect via SSL to (e.g. implicit SSL connection to a SMTP server, port 465). This works really good e.g for sending encrypted emails.

In several situations the server does not offer such a dedication port for SSL for implicit SSL, but offers “explicit” SSL (SMTP or FTP(S)). In several documents “implicit” SSL is called the “old” way. In this situation of “explicit SSL” the connection is started without encryption and during the communication with the server a special command e.g. “STARTTLS (SMTP)” or “AUTH TLS (FTPS)” is used to switch to an encrypted connection.

Now to the ESP-AT. “Implicit” SSL works good so far, here the encrypted connection is established from the very beginning. But how can I switch to an encrypted connection while a normal „TCP“ connection is „running“. I must not disconnect from the server first to start a “SSL” connection because I have to initiate the encrypted connection via a command.

Your answers are highly appreciated.
Thomas

[esp-at]

same to https://github.com/espressif/esp-at/issues/833