B4J Tutorial STARTTLS - upgrading a non-tls socket to a tls socket.

Upgrading from a non-tls connection to a tls connection only makes sense when dealing with a protocol that specifically requires it. It is much simpler and also a bit more secure to start with a TLS connection, instead of upgrading it after the connection is made.

The most common case for this upgrade is SMTP in STARTTLS mode. This feature is already implemented in (i/j)Net libraries.

The steps to implements the TLS upgrade are:
1. Set a timeout to the network socket and set AsyncStreams.ContinueAfterTimeout = True.
This timeout is needed to allow us to later stop AsyncStreams from reading the streams without closing them.

Example (using AsyncStreamsText class):
Socket1.As(JavaObject).GetFieldJO("socket").RunMethod("setSoTimeout", Array(1000)) 'timeout of 1 second
AST.Initialize(Me, "AST", Socket1.InputStream, Socket1.OutputStream)
AST.astreams.ContinueAfterTimeout = True
The timeout cannot be too short as it will break the TLS handshake.

2. When the connection should be upgraded:
- Create a SSLSocket using JavaObject.
- Call AsyncStreams.StopWithoutClosingStreams
- Call Sleep with a duration larger than the socket timeout.
- Start the handshake.

3. Create a new AsyncStreams with the new streams.

In order to test it I've wrote a draft of a SMTP client. It is attached.

It depends on jRandomAccessFile v2.34+: https://www.b4x.com/android/forum/threads/updates-to-internal-libaries.48274/post-844245



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