Support encryptionless sftp using sftp-over-tcp

[HoverHell]

For performance reasons, it might be preferable to use unencrypted (and unauthenticated) sshfs, e.g. when running over localhost or a trusted local network.

For this, sshfs supports a directport=… option which skips the 'ssh' part and directly uses sftp protocol (for reference, the server-part of this can be started using socat TCP4-LISTEN:12345,bind=192.168.0.1 EXEC:/usr/lib/sftp-server,nofork)

(not sure whether this is a duplicate; I could not find anything relevant)

[daira]

I'm skeptical that encryption and authentication has a significant performance impact. HoverHell?, can you measure that?

[HoverHell]

Apparently I can't answer that because of the spambayes.

[zooko]

HoverHell: sorry about that. ☹ Attempting to fix it. Here's the comment that you tried to post that spambayes ate:

significant performance impact relative to other factors

I think, measuring it relative to other factors would require implementing it. I haven't gotten into the code yet, and I'm mostly suggesting it as a potentially-simple addition. Though it is possible I will get to experiment with this later.

[daira]

Technically it would require implementing it, but the implementation could be an easier hack such as commenting out code in twisted.conch (that wouldn't be suitable for a "real" implementation), for instance.

[exarkun]

I think that null cipher SSH is poorly supported by SSH implementations. My recollection is that OpenSSH initially supported it for quite some time and at some point decided to drop it. Twisted Conch supported it at some point. I don't remember if that support went away or not.