9. The ever-popular miscellaneous section

This section covers some miscellaneous things about Secure Shell.Click here for the contents of this section.

9.1. Should I turn encryption off, for performance reasons?

No; you should keep it turned on, for security reasons.

Today's CPUs are fast enough that performance losses (if any) only are noticable for local Ethernet speeds, or faster.

You might want to specify blowfish encryption instead of the default, IDEA for SSH1 and 3DES for SSH2, with -c blowfish, for faster operation.

Following are some measurements where the different encryption methods were applied between a P5/90 and a 486/100, both running Linux, for copying files with scp across a lightly loaded Ethernet.

The model chosen was t=a+x/b; a is the startup time in seconds, and b the sustainable transfer rate in kB/s. Also given are the 68.3% confidence intervals for the data, as determined by the Levenberg-Marquardt algorithm as implemented a pre-3.6 version of gnuplot.

 
Encryption      a[s]      da[s]    b[kB/s]      db[kB/s]
none            2.37       0.37     386.1         5.8
rc4             1.96       0.27     318.2         2.9
tss             2.33       0.37     298.5         3.5
des             2.07       0.19     218.8         1.0
idea            2.25       0.45     169.6         1.3
3des            1.92       0.11     118.2         0.2

Across a heavily loaded Ethernet, rc4 encryption together with compression may actually be faster than using rcp.

9.2. Known security bugs with SSH

All versions of ssh prior to 1.2.12 had a security flaw which allowed local users to get access to the secret host key. This is fixed in 1.2.13 and later.
If you run ssh 1.2.13 on Alpha OSF 1.3 or SCO in C2 security mode, local users can gain root access. This is fixed by applying ftp://ftp.cs.hut.fi/pub/ssh/ssh-osf1-c2-setluid.patch or by upgrading to 1.2.27.
Versions of ssh prior to 1.2.17 had problems with authentication agent handling on some machines. There is a chance (a race condition) that a malicious user could steal another user's credentials. This should be fixed in 1.2.17.
The arcfour cipher is used in a way which makes it susceptible in version 1 of the ssh protocol. Therefore, its use has been disabled in 1.2.18 and later.
In versions prior to 1.2.23 there was a CRC32 Compensation attack that allowed the possibility of executing arbitrary commands. For details of the attack and/or patches, see http://www.core-sdi.com/ssh.
With 2.0.12,an sshd connection would not die, even if a complete connection was never fully established. That is, when he came from server B to server A, the session on server A would hang when he exits. Upgrading to 2.0.13 should fix this problem.

9.3 I don't like the commercial aspects of ssh.

The good news is the protocols ssh uses are freely available. There are no restrictions if anybody wants to write a version that is available under different conditions and is interoperable with existing SSH installations.

Secure Shell 2 is also on the Internet Standards Track. This means that a second, independent implementation is required. The only other current SSH2 implementation is lsh. Look at www.lysator.liu.se/~nisse/archive/ for more information.

You will have to be aware of patents, like RSA and IDEA, and export control issues before writing a second implementation.

9.4 Making SSH more transparent to users

Maintainer's note: This will probably be moved to another section in the FAQ later on...

Here is a simple script by Ross Golder to make using ssh more transparent. When you first login (open an Xterm etc.), you'll be prompted for your passphrase. After that, it will automatically configure your environment to the ssh-agent it started.

When logging out of a multi-user machine, I just use a 'killall ssh-agent' (not as root!).

Install the following file as $HOME/.ssh/setup and set the execute permission (chmod 700 setup), and add a call to it in your $HOME/.bashrc.

#!/bin/sh

# Checks for current ssh agent, otherwise starts one.

# For bash and ksh users:
#
# include the following in your ~/.bashrc or ~/.kshrc or ~/.profile
#
#       . $HOME/.ssh/setup
#
# For csh and tcsh users:
#
# include the following in your ~/.cshrc or ~/.tcshrc
#
#       source $HOME/.ssh/setup
#

SSH_ENV=$HOME/.ssh/environment

function start_agent {
	echo "Initialising new SSH agent..."
	ssh-agent > ${SSH_ENV}
	chmod 600 ${SSH_ENV}
	. ${SSH_ENV} > /dev/null
	ssh-add
}

# Source SSH settings, if applicable
if [ -f "${SSH_ENV}" ]; then
	. ${SSH_ENV} > /dev/null
	ps ${SSH_AGENT_PID} > /dev/null || {
		start_agent;
	}
else
	start_agent;	
fi

9.5 Alternatives to SSH

There are several other secure connection or authentication bits of software about. You might want to check them out as well.

srp: http://srp.stanford.edu/srp/
stunnel: http://mike.daewoo.com.pl/computer/stunnel/
ssleay-related: http://www.psy.uq.edu.au:8080/~ftp/Crypto/
stone: http://hp.vector.co.jp/authors/VA000770/stone/

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