> It appears there is a clear lack here : no correct sftp server and no
> correct sftp client.

...??  What's "incorrect" about the existing ones?

> [...], because there are more than one socket stream involved (and
> it's completely useless to protect the "command" socket without
> protecting the "data" socket).

I disagree.  There are plenty of cases where you don't care about
protecting the data being transferred but you're not about to send a
password (as is required for traditional non-anonymous FTP) in the
clear.  I do such transfers regularly (though not through either ftp or
sftp; I usually do them with an interactive ssh login and plus netcat).

But you're right if you mean that a general-purpose replacement needs
to protect the data stream as well as the command stream.

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