Hi Giga,
for sure I have traced the issue. I get connection, send AUTH message, then the server waits indefinitelly for TSL handshake. I have tested many clients and I get the same. Maybe there is a filter on this port. The strange is that AUTH message passes! I'm trying to implement SFTP but B4A implementation is very poor. At present I'm unable to use it, the server states "Unknown protocol". I've tested the SFTP server using some commercial clients and it works very well.
Since I love to write original protocols. I'll create one just for added security.
Thank you to care about my problems! And have a nice week...
Mauro