deslogin.1

一个使用des加密传输的unix下的login程序的服务器端和客户端

.\" @(#) $RCSfile$ $Revision: 1.3 $ $Date: 94/06/14 11:59:36 $
.TH DESLOGIN 1  14-Jun-94
.ds )H David A. Barrett
.ds ]W Rev. 1.1 June 1994
.SH NAME
deslogin \- DES authenticated and encrypted Network login
.SH SYNOPSIS
.B deslogin
[
.B \-dv
] [
.B \-g
.I gateway
] [ [ user\@]domain[:port] ]
.SH DESCRIPTION
.PP
.I Deslogin\^
is a network login program similar to telnet but more secure.  Unlike
telnet, it does not transmit passwords across the network.  Since passwords
(actually pass phrases) may be arbitrarily long, they don't fall prey to 
password guessing attacks like the eight-character ones of telnet and login do.
.PP
.I Deslogin
also encrypts both directions of the data stream of the login session, so
network snooping cannot reveal sensitive information accessed during the
session (such as using the su command to become root).  Encryption also 
protects the connection from being hijacked.
.PP
.I Deslogin
attempts to connect to the specified remote system as the specified 
user at the specified port.  If no system or user is given, 
.I deslogin
will prompt you.  If no port is specified, the default one
from /etc/services (usually 3005) is used.  If no user name is given, the
local user name from
.I (getlogin(3))
is used.
.PP
.I Deslogin
prompts you for a pass phrase.  This phrase, known only to
you and the remote machine, is used to authenticate your identity to
the remote host.  You may use an arbitrarily long sequence of characters
for the phrase.  I find that short sentences make excellent easy-to-use
choices.
.PP
If the authentication is successful, you will be logged into the remote
host with your standard shell, user-id, and environment.  All information
transmitted or received from the remote host is encrypted using the 
United States Data Encryption Standard in cipher-feedback mode.  The data
encryption key is unique for each
.I deslogin 
invocation.  This prevents playback attacks and drastically reduces 
the time the login passphrase is subject to attack.
If any part of authentication fails, 
.I deslogin
exits without comment.
.PP
Often (almost always), the remote host will place an 
inactivity timer on your login session.
This helps prevent the remote host from being compromised if you walk away
from your machine while a 
.I deslogin 
session is active.  Typically this timer
is kept short (default is 20 minutes) to reduce the window of opportunity for 
someone walking up to your machine while you are away.  Don't leave 
.I deslogin
sessions unattended.  If you don't have any input/output going to your
session for the inactivity period, the session is shut down immediately without
warning.  If the timeout is too short, talk to the installer of 
.I deslogind(3)
on the remote host.
.SS Options
.PP
.TP 11
.B  \-d
enable debug output (can't be used by attacker to compromise keys)
.TP
.B  \-v
Verbose mode.  Enable verbose output of messages.
.TP
.BI \-g\ gateway
Specifies a host:port to use as a gateway.  
.I Deslogin
sessions can be routed through a gateway process installed on a 
"firewall" machine.  The :port is optional.  If not specified the
default port from /etc/services (usually 3006) is used.  
Firewalls often exist at entrances to secure networks.
This option causes 
.I deslogin 
to prompt you for a gateway passphrase which is used to
authenticate yourself to the gateway.  
This passphrase is probably different than the passphrase for the
host you are logging into.
Once the gateway is satisfied of your
identity, it routes your login session to a remote machine, which will 
authenticate you and encrypts the session in the usual manner.  You have
no access to the gateway machine, and no user (even root) on the gateway
will be able to examine information flowing through the gateway.
.SH WARNINGS
.PP
Do not leave 
.I deslogin
sessions unattended.  Someone can walk right up to your keyboard and
do anything you can.
.PP
You should take great care to preserve the secrecy of your passphrase(s).
Don't let people watch the keyboard while you type in the passphrase; 
it's been done with telephone access cards with binoculars in airports!
Don't write down your passphrase; pick a sentence that is easy for you to
remember; any characters are allowed, including punction and spaces.
.PP
Always invoke 
.I deslogin 
on the local host.  Do not use
telnet (rlogin, xterm, etc.) to a remote machine, and then invoke 
.I deslogin
on 
that machine to get to another.  If you do, the keystrokes of your passphrase 
will appear on the network between the local and remote telnet hosts.
.PP
On machines served by X-windows, use the "Secure Keyboard" option from 
the xterm "main menu" usually invoked by the CTRL-<left mouse button>.  
This feature
is supposed to prevent X-windows clients authorized to connect to 
your server from
being able to intercept your keystrokes into that xterm window.  The window
should change color when this option is in effect.  
You only need to protect keystrokes
while you are entering the passphrase.  If you are concerned about your
session data being snooped, you still need to ensure that other X clients
cannot access your Xserver root and session windows.  Xwindows has other 
security holes such as unix-domain sockets in filesystem directories, frame 
buffer and keyboard device files.
.PP
Even then, you must make sure that the terminal pty (/dev/ttyp*) associated 
with your xterm 
window is not subject to hijacking or eavesdropping.  You can find out the
pty by using the 
.I tty(1) 
command.  Check the permissions and ownership of that
device file to ensure that it cannot be read by intruders on your local 
machine.  
.PP
The same applies to the device special files for your screen frame buffer and 
keyboard interface.
.PP
.I Deslogin
erases the memory storing your passphrase, and the resulting keys immediately 
after entry.  Only a hashed form of your session key remains buried
in the data segment of your 
.I deslogin 
process.  It is possible for a 
sufficiently determined attacker with root permission to your local machine
to read the memory of your 
.I deslogin
process and compromise your session key.
This won't help them determine your passphrase though.
.PP
In order to prevent idle traffic analysis by casual users, you should avoid
specifying any arguments to 
.I deslogin;
just respond to the prompts instead.
.PP
Beware of "trojan horses".  When you invoke 
.I deslogin,
make sure that you aren't invoking another program in a directory in your PATH environment variable which
has been placed there by a hostile user.  If you PATH includes directories
on remote machines, (especially including the current directory: '.'), be sure
that the remote machines are trusted by you.  Invoking 
.I deslogin 
by its full
path name on a local filessytem prevents this problem if the attacker
hasn't replaced the original.
.PP
It is still possible that someone on the network between the two 
.I deslogin
hosts can attempt to record or insert data (possibly from a previous
session) into your encrypted data stream.  If this happens, you will see
garbage characters appear on your screen since the inserted data will be from
a session using a different session key.
I've used this program for several years and have never seen this happen, but
it's possible.  
.PP
Summary: 
.I Deslogin
is designed to protect you from
unsecure networks traversed between your (secure) local host and a secure 
remote host.  It is best to use 
.I deslogin 
on a local machine which is not 
subject to a determined attack from hostile local users.  
This is quite commonly a valid assumption; just be aware
when you are making it.
If you have reason to
strongly distrust root capable users on the local machine, do not use 
.I deslogin
to access a highly secure machine.
.SH AUTHENTICATION PROTOCOL
.PP
.I Deslogin
uses a "challenge-response" protocol to authenticate users.  The
remote host encrypts a 64-bit unique unpredictable nonce and sends it as
the "challenge".
.I Deslogin
then asks you for your passphrase which is hashed into a 56-bit DES 
authentication key used to decrypt the challenge and send back the 64-bit 
"response".  The remote host (which shares knowledge of your passphrase) 
decrypts the response
and compares the result with the nonce used to generate the challenge.  
If they're equal, authentication succeeds and a unique session key
is generated by encrypting the challenge with your DES key.  The
authentication key is immediately destroyed and the session key 
is then used to encrypt all other data transferred.

.SH ENVIRONMENT
.PP
See the 
.I deslogin(1)
manual page under the 
.I ENVIRONMENT 
section for useful environment variables such as 
.B TERM
and 
.B RHOSTNAME.

.SH MISSING FEATURES
.PP
A more secure system would allow you to authenticate the identity of the
remote host.  In practice this is not likely to be a serious problem since
someone attempting to "spoof" a remote host can only trick you into 
"signing" the authentication challenge packet by encrypting it with the
DES key corresponding to your passphrase.  The conjecture is that DES has
properties that make this computationally infeasible. Some disagree.
.PP
Some method for the local netlogin to authenticate itself to you should be
devised to prevent trojan horses from masquerading as netlogin and
tricking you into revealing your passphrase.
.PP
There needs to be a way for users to change their passphrases.  Currently,
the administrator on the remote machine must set the passphrase.  This
could be construed as a "feature" since it allows the administrator to
verify the passphrase difficult to guess.
.SH ACKNOWLEDGEMENTS
.PP
The author has made every attempt to produce a secure and portable 
authenticated login program.  I havn't had time to integrate this with 
telnet and the myriads of telnet protocol extensions, which would have been 
the "right" way to implement this program.  Comments and improvements
will be greatly appreciated and should be directed to the author:
.PP
David A. Barrett ([e-mail])
.PP
Copyright 1994 by David. A. Barrett.
.PP
This program is not to be distributed for profit or included in such
software without written permission from the author.
No permission is required for non-profit use.

.SH FILES
.ta 1i
/dev/tty	to output prompt and read passphrase 
.DT
.SH SEE ALSO
cipher(1),
telnet(1),
rlogin(1),
deslogind(1),
deslogingw(1),
tty(1),
getlogin(3).
.\" index	\fIdeslogin\fR \- DES authenticated encrypted login \s-2DESLOGIN\s+1(1)\s+1
.\"
.\" toc	\s-2DESLOGIN\s+1(1)\s+1:\0\0\fIdeslogin\fR 	 DES authenticated/encrypted login