Kerberos/DCE, the Secure Shell, and Practical Internet Security
San Diego Supercomputer Center, San Diego, California, U.S.A.
October 10, 1996
Table of Contents
Continuing with work described at the Fairbanks (Fall 1995) CUG
conference, SDSC now has an operational Kerberos environment for
authentication/encryption within SDSC (Cray, Paragon, and
workstations), and we are integrating it with DCE systems (DCE security
daemon ("secd") and HPSS). However, instead of extending these
services to our national (and international) user community, we are
using more focused and concise software systems, the Secure Shell (SSH
and F-SSH) and Secure Network Key (SNK), to meet our immediate
Internet goals. This paper will discuss Kerberos/DCE, the Secure
Shell, and SNK, how we are using them, and our longer-term
Of necessity, computer security is a continuing SDSC focus. As Tom Perrine, manager of SDSC's
Security Technologies team, has noted
, cracking tools are now being
shared, attacks are becoming more sophisticated, and the use of
plain-text passwords is the most significant security problem in our
For SDSC, as with most Internet sites, plain-text passwords, exploited
via simple network "sniffing," is a key vulnerability.
SDSC is using DCE/Kerberos, the Secure Shell, and Secure Network Keys
(one-time passwords) to mitigate this problem. In this paper, we will
help introduce the Cray community to a valuable new tool, the Secure
Shell, describe SDSC's current and planned use of Kerberos/DCE and
other tools, briefly describe our HPSS plans (with particular emphasis
on secure authentication), and outline some related SDSC
The Secure Shell is a valuable new tool for improving Internet
security, either in conjunction with Kerberos/DCE or independently
deployed. In this author's opinion, it is the most practical
currently available solution for some of the more serious Internet
security problems. For sites without the resources for a full
Kerberos/DCE installation, the Secure Shell offers a relatively simple
and easily provided solution.
Both the Secure Shell (SSH) and Kerberos/DCE deal effectively with the
plain-text password problem. Each also has additional features,
with some overlapping general functionality.
The Secure Shell is like the PGP of interactive access. It is a
relatively simple, freely available solution that solves a difficult
problem today, while large organizations are still in the process of
creating more complex solutions. It was created mostly by one person,
<,email@example.com>, at Helsinki University of Technology, Finland.
Kerberos, for example, has been under development for many years and
is still in Beta (currently Kerberos version 5 Beta 7). DCE has also
been under development for many years and still relies on Kerberos
utilities for secure network authentication. Both are fairly large
and complex systems. The current Kerberos 5 Beta 7 and Beta 6 are much
cleaner and easier to install than previous Kerberos systems, but they are
still under development.
We currently have SSH running on our Cray C90, Paragon, IBM SP2, and
most workstations: Suns (SunOS and Solaris), SGIs (R4000s, R8000s, and
an R10000), DEC Alphas, IBM RS6000s, and PC's running NeXTStep and
Windows NT. SSH runs on a large number of Unix systems, and there is
a commercial version (F-SSH) available for PCs (and, soon,
Macintoshes) from Data
Fellows Ltd.  Data Fellows also markets and supports a commercial
We strongly recommend the use of SSH to the SDSC user community. It
is relatively easy to install, can be run independently
(i.e., individual workstations can run SSH without site-wide
coordination), provides strong password protection, encrypts X-window
sessions, and provides additional security and convenience
Source, documentation, and configure/make scripts are freely available
for Unix systems via the SSH home
page . Also see the SDSC remote
site SSH suggestions.
As described on the SSH home page, SSH (Secure Shell) is a program to
log into another computer over a network, to execute commands in a
remote machine, and to move files from one machine to another. It
provides strong authentication and secure communications over insecure
channels. Its features include the following:
SSH is intended as a complete replacement for rlogin, rsh, rcp, and
rdist. It can also replace telnet in many cases.
- Strong authentication. Closes several security holes (e.g., IP
routing, DNS spoofing, and listening for passwords from the
network). New authentication methods: .rhosts together with
RSA-based host authentication, and pure RSA authentication.
- All communications are automatically and transparently
encrypted. Encryption is also used to protect against spoofed
packets and hijacked connections.
- X11 connection forwarding provides secure X11 sessions. This
is normally provided automatically for the user.
- Arbitrary TCP/IP ports can be redirected over the encrypted
channel in both directions.
- The client RSA-authenticates the server machine in the beginning of
every connection to prevent Trojan horses (by routing or DNS
spoofing) and man-in-the-middle attacks. The server
RSA-authenticates the client machine before accepting .rhosts or
/etc/hosts.equiv authentication (to prevent DNS, routing, or IP
- An authentication agent, running in the user's local workstation
or laptop, can be used to hold the user's RSA authentication
- Multiple convenience features fix annoying problems with rlogin
The basic operation is very simple, much like telnet. On a
workstation, a user enters ssh and the name of a host. For example,
will create a login session (as with telnet or rlogin), but the entire
session will be encrypted, including the password and any passwords
used with 'su'. For many users, that is all they need
SSH also provides a remote copy (scp) capability which is preferable to
rcp, as it provides stronger authentication and can encrypt the transmitted
Users who use SSH frequently can use alternative SSH authentication
mechanisms instead of passwords. By running 'ssh-keygen' they can
create a pair of RSA public/private keys. They then copy the
individual public key to the systems they want to log into, into the
file ~/.ssh/authorized_keys. At SDSC, since this file is NFS mounted
on most of our workstations, users will be able to SSH to most
workstations without a password. An example and details are available
from an SDSC
SSH example page (see the WWW version of this paper for the
When users run ssh-keygen, by default, the individual public and
private key files are stored in their home directory (in ~/.ssh).
One's private RSA key is like a password and should be protected,
so it shouldn't be stored in an NFS file system (the data would
traverse the network in the clear). Instead, users can store it on
the local workstation, for example, in /tmp/id, encrypting it with a
short password, and/or using the ssh-agent to hold the decrypted
key. SSH man pages explain how to use these facilities.
SDSC developed a number of extensions for the SSH daemon to run
properly on the Cray C90 under Unicos 8 and 9.0 and these are
available in the current SSH release. This includes PTY handling,
setting up the session account (via acctid), setting up the job (via
setjob), and setting limits and permissions (via setlimits). The
foundation routines, including encryption, had previously been ported
to the Cray, but SDSC was the first to completely install it (i.e.,
run it as root).
The Cray version of the SSH daemon does not, however, prompt for
account selection as Cray Research's login does, when users have
multiple accounts available on a particular user id. For the time
being, this is acceptable.
The SSH protocols and algorithms are fairly efficient, and so the
encryption, while not free, is not excessively costly either. An SSH
session to a Paragon node has significantly better interactivity
(quicker echoing of keystrokes) than an encrypted Kerberos session
(krlogin -x), at least with our port of the krlogin daemon (basically
5 beta 5 code).
Even with DCE/Kerberos available at SDSC, we are using SSH as an
option for users, both internally and remotely. Attempting to support
Kerberos for our nationally distributed user community on a wide
variety of platforms would have been prohibitively difficult. SSH,
although not trivial, can often be compiled and installed by users
(It should be noted that the current Kerberos release (5 Beta 7) is
much more solid than previous versions. On many workstations, it can
now be configured, compiled, and installed with few difficulties.
However, it is still substantially larger and more difficult to work
with than SSH.)
SSH uses RSA to exchange a private key for each session, and then uses
conventional encryption (including triple DES and IDEA) for the
session. Each host has an RSA public/private key set and the SSH
daemon (SSHD) maintains an RSA public/private key set for itself
(which it changes periodically). When an SSH client connects to the
SSH daemon, the SSH daemon sends its host and daemon public keys. The
SSH client randomly picks a session key, RSA encrypts it in both keys,
and then sends it to SSH daemon. Only the SSH daemon will be able to
decrypt the packet. The SSH client and server then have a shared
secret key for the session.
The licensing and legality issues for SSH are a little complicated due
to the use of RSA, IDEA, and encryption technology in general, but are
becoming less so since Data Fellows now has RSA and other licenses.
Reportedly, SSH can be used legally anywhere (except in France and a
few other countries where all encryption is forbidden). The
commercial version can be used legally almost anywhere, since it
includes the required patent licenses. The non-commercial version can
also be used legally for non-commercial purposes, as the RSA patent is
not valid outside of the U.S., and non-commercial use of IDEA is
allowed without a license. Also, the U.S. government can use RSA
without a license because it was invented at MIT with partial
government funding. So it appears that the non-commercial version is
suitable for educational and non-commercial use (i.e., where use of
the SSH/RSA/etc package is not being sold). Additional information is
available with the distribution and on the web pages.
The following table summarizes the key features and attributes of
SSH and Kerberos.
Although SSH provides much, we are continuing to make use of
Kerberos/DCE security and expect to make greater use of it in the
future. DCE has strong support from many vendors, and may be growing
in popularity. Kerberos 5, after years of development and Beta
releases, is nearing final release and provides remote access
functionality in a DCE environment. HPSS (High Performance Storage System)
, the new archival storage system being developed by IBM Government
Systems and four DOE laboratories makes use of DCE. DCE authentication
may eventually make Single Sign-On a realistic goal.
DCE, the Distributed Computing Environment, is being developed by the
Open Group which was formed in early 1996 by the consolidation of two
open systems consortia, X/Open Company Ltd (X/Open) and the Open
Software Foundation (OSF). The Open Group includes a large number of
computer vendors including IBM, DEC, Microsoft, and SGI/CR (and, until
its merger with SGI, CRI).
DCE consists of multiple components that work closely together,
including the DCE Remote Procedure Call (RPC), Cell and Global
Directory Services (CDS and GDS), Security Service, DCE Threads,
Distributed Time Service (DTS), and Distributed File Service (DFS).
The Threads, RPC, CDS, Security, and DTS components are commonly
referred to as the "secure core" and are the required components of
any DCE installation.
As reported at the Fairbanks (Fall 1995) Cray User Group conference in
Installation and Development of Kerberos" , SDSC was in the
process of installing and porting MIT's Kerberos 5 Beta 4 network
security software to SDSC systems, including the C90, Intel Paragon,
SUNs, DEC Alphas, SGIs and RS6000s. Since then we have converted from
a test to a production system, and are now installing 5 Beta 7.
Converting to production required the regeneration and secure
distribution of the v5srvtab files (identification files) to each
host, and changing the realm name. We also ported and installed
Kerberized daemons on the Paragon, installed TCP wrappers on the
various Kerberos daemons, corrected some additional bugs, and
encouraged staff use.
We now consider this Kerberos system "production" in the sense that it
is available for general use and supported by systems staff. Using it
and SSH, we have significantly reduced the number of plain-text
passwords on our local network. We still provide less secure services,
though, such as telnet and rlogin, although we discourage their
Although we had originally hoped to distribute Kerberos clients to our
user community, both legal and technical issues prevented it. The
legal issues involve the distribution of encryption technologies, and
particularly a few months ago, it was not clear what protections an
organization must take to distribute such software. We also felt that
supporting a Kerberos distribution for a large number of remote sites,
on a large number of architectures, would be excessively problematic
We experimented with 5 Beta 5 version of the Kerberos server (KDC) and
its backwards compatibility with Kerberos Version 4, but found that
moving forward with the DCE security daemon ("secd") would offer us
much more. With secd, we can support a Kerberos 5 environment and DCE
for HPSS. Rather than supporting Kerberos 4, which is weaker than
Kerberos 5, we are finding other solutions for Macintoshes and PCs.
These include APOP for email, and soon SSH and/or Kerberos 5 for
Macintoshes, and SSH for PCs.
To deal with another major source of plain-text passwords on our local
network, the Macintosh Eudora email package, we are installing Eudora
with APOP support (see RFC 1939 Post Office
Protocol). Eudora periodically logs in to the email (Post Office
Protocol) server to access user email. Via APOP, this is handled via
a challenge/response authentication method.
The following table summarizes the SDSC Kerberos environment.
|Security Function or Attribute||Secure Shell ||Kerberos/DCE Security
|Encrypts/Conceals Interactive Passwords ||Yes
|Encrypts Entire Interactive Session ||Yes, default
|Secure Passwordless Logins ||Yes, user-managed, developing
Single Sign-On ||Yes, possible Single Sign-On||
|Authentication Forwardable ||Yes, currently difficult
|Centralized Password Administration ||No
|Each Computer Independent (i.e., no site-wide coordination
needed) ||Yes ||No|
|File Transfer (rcp-like, without plain-text passwords)
|Encrypts File Data ||Yes, default ||Yes,
|Encrypts/Conceals FTP Passwords ||Planned ||Yes, if
using Kerberized ftp and ftpd|
|Encrypts X-Windows Sessions ||Yes ||No|
|Compresses Terminal and/or X-Windows Data ||Yes, often 3-5
|Encrypts Arbitrary Sockets ||Yes ||No|
|Requires Synchronized Clocks ||No ||Yes,
|Application Programming Interface ||Planned
|Available For Many Unix Systems ||Yes ||Yes|
|Available For Macs ||Very soon ||V4, V5
|Available For PCs ||Yes, Prerelease/Commercial ||V4,
Good V5s are rare|
|Source code size (SSH 1.2.14, Kerberos 5B7) ||About 44,000
lines (203 *.c files) ||About 250,000 lines, 975 *.c
|Available for Crays ||Yes, as distributed ||Yes, Cray
DCE 1.1, or from SDSC, or Sandia|
|Can it be used legally in the US? ||Probably
|Can it be used legally outside the US? ||Yes (as
|Architecture||Operating System orFunction||Current||Planned
|Cray||Unicos 9.0 ||SDSC 5B4 ||Cray 5B5|
|Intel Paragon||OSF/1 ||SDSC 5B5 ||5B7|
|Sun||SunOS 4.1.x ||5B7 ||5B7|
|Sun||Solaris 2.5 ||5B7 ||5B7|
|SGI||Irix 5.3 ||5B4 ||5B7|
|SGI||Irix 6.2 ||5B7 ||5B7|
|Digital||Digital Unix 3.x ||5B4 ||5B7|
|IBM||AIX 4.1.4 ||5B6 ||5B7|
|IBM||AIX 3.2.5 ||5B4 ||5B7|
|Sun/IBM||Security Server||5B5 KDC ||DCE 1.1
We are now installing the current version of Kerberos on our
workstations. This version, 5 Beta 7 (and 5 Beta 6 last summer), is
significantly improved from 5 Beta 5 and earlier versions and can now
be configured, compiled, and installed with few difficulties. We have
compiled Kerberos Version 5 Beta 7 on SunOS 4.1.4, Solaris 2.5, IRIX
5.3 and 6.2, and Digital Unix OSF/1 2.0. KRB 5 Beta 7 was first
installed on Solaris (as we had no previous version of Kerberos
there), and on IRIX (as there are known bugs with the previous
Kerberos installation there).
We are also integrating our DCE (HPSS) and Kerberos environments.
With changes and additions contributed by a DOE working group, led by
ANL (Douglas Engert), Kerberos 5 Beta 6 (and 5 Beta 7) can be combined
with OSF/DCE by using the DCE security server (secd) instead of the
Kerberos KDC (Key Distribution Center). This allows for single
sign-on using the DCE userid and password and provides the best
features of both DCE and Kerberos, e.g.,
DCE applications such as DFS and kerberized clients
such as rlogin, telnet, or FTP. These kerberized clients can be run on
machines with or without DCE, and can be used to establish encrypted
We plan to run a DCE secd to support HPSS DCE authentication (both
internal to HPSS and for user authentication), and to support our
Kerberos environment. This will also provide support for additional
DCE services if needed. In the future, for example, we may want to
make use of the DCE Distributed File System.
Cray Research now supports Kerberos 5 Beta 5 utilities and daemons
(ktelnet, klogin, krsh, kcp) as part of their DCE 1.1 product. This
is a welcome and appreciated move. We plan to purchase their product to
replace our own port of 5 Beta 4. Our current 5 Beta 4 ktelnet daemon
is not functioning under Unicos 9.0, apparently due to Unicos 9.0
differences in the setsid and related system calls.
We are currently using Secure Network Key (SNK) for interactive logins
through a gateway, and for remote UniTree access.
The purpose of our SNK gateway host is to provide secure interactive
access for staff and for a few general users. Staff can use this
while on travel so that the use of plain-text passwords can be avoided
(passwords used at conferences are frequently compromised). A user
telnet's or rlogin's to the SNK gateway host and then rlogin's to
another SDSC host.
Similarly, SNK support in UniTree protects user passwords during
remote FTP access.
The SNK system works as follows. When a user makes connection to a
SNK enabled server, a challenge number is displayed back. The user
enters his Personal ID, and then this challenge number on the SNK key
pad, and types in the response it provides. The server than compares
that response with the calculated appropriate response to verify
authenticity. The SNK card is about the size of a small calculator.
Thus it is a little cumbersome and inconvenient, but does provide very
secure access for a small number of people at a reasonable cost. Of
course, this wouldn't scale well to our national user community, where
easy access for thousands of users is a necessity.
The High Performance Storage System (HPSS) is a new archival storage
system being developed by IBM IBM Government Systems, LANL, LLNL, ORNL, and
Other collaboration partners and early deployment sites include the
Research Center, Maui High Performance
San Diego Supercomputer Center, Cornell Theory Center,
Fermi National Accelerator Laboratory, Caltech/Jet
Propulsion Laboratory, and the University of Washington.
The HPSS architecture is based on the IEEE Mass Storage Reference
Model: version 5 and is network-centered. The control network uses
the DCE's Remote Procedure Call technology. In implementation, the
control and data transfer networks may be physically separate or
An important feature of HPSS is its support for both parallel
and sequential input/output (I/O) and standard interfaces for
communication between processors (parallel or otherwise)
and storage devices. In typical use, clients direct a request for
data to an HPSS server. The HPSS server directs the
network-attached storage devices to transfer data directly,
sequentially, or in parallel to the client node(s) through the
high-speed data transfer network.
SDSC is running HPSS in test now on an IBM SP-2.
The SP-2 system includes 23 nodes (20 thin-nodes, 1 wide-node, and 2
high-nodes), 1 TB of SSA (Serial Storage Architecture) Disk, 60 GB of
HIPPI attached RAID, and two 3494 tape libraries. Some of this
equipment is partitioned for specific data intensive applications.
The HIPPI-attached disk will be used for IPI-3 third-party transfer
between HPSS and the Cray C90, Intel Paragon, and SGI hosts. Network
interfaces include HIPPI, FDDI, ATM, and Ethernet.
We plan to migrate from NSL UniTree to HPSS as our primary archival
storage system in early 1997. We will migrate metadata from our
existing archive system, as we convert from NSL UniTree to HPSS. We
are also using HPSS as the foundation for a number of major SDSC
data-intensive initiatives (see next section).
In addition to DCE RPC, HPSS makes use of DCE Encina as a transaction
processing layer. Encina provides journaling and recovery mechanisms,
and HPSS uses these to safeguard the Metadata (directory and index
The HPSS Parallel FTP (PFTP) utility is a high performance data
transfer user interface, similar to FTP but with extensions to allow
users to transfer to and from HPSS across parallel communication
Other user interface options will include the HSI utility which is
under development by SDSC (Mike Gleicher) and will be shared with the
HPSS community. HSI will include features of its predecessor, UTI - an NSL UniTree
Interface utility - with extensions for HPSS including parallel
I/O, class of service, multi-threading, and scheduling. HSI uses the
current HPSS API which is DCE RPC- and Encina-based and so limited to
DCE platforms for which DCE is available. Since SDSC is not currently
running DCE on our major platforms, including the Cray, we can not
make use of this immediately. The HPSS team (and/or SDSC) may develop
a non-DCE/Encina API, which would then be used by HSI.
For production and batch use, we are adding features from UFTP (our
current Unitree FTP user interface program) to PFTP. Not only will
this provide us with a simple, standard user interface, but will also
make the conversion process much simpler for our users, since the
basic user interface will be nearly identical. Our DataTree to
UniTree conversion, in contrast, included a completely new user
The UFTP features being incorporated into PFTP include reliable exit
status information, execute line stacking (multiple FTP commands on a
command line, separated by ";"), automatic retry of recoverable errors
(including distinguishing transitory and permanent errors), and
password-less secure authentication.
The authentication system for this PFTP/UFTP utility will allow its
use from batch jobs on the Cray without either placing passwords into
scripts or transmitting plain-text passwords across the network.
There are two main choices, a method similar to our current Unitree
UFTP mechanism or Kerberos/DCE.
Our current UFTP mechanism uses a 'verify' daemon and support software
from LLNL. The UniTree FTP daemon is able to connect to the verify daemon
on the Cray and, via a three-way handshake that includes the use of
Unix file permissions, confirm the identity of the user. This is
similar to the 'ident' protocol (see RFC 1413 Identification
Protocol), and we could perhaps make use of it, either directly or
with some modification.
If we use this approach, we would have to DCE authenticate on behalf
of the user on the HPSS server. To do this, we may use a program
developed by Cornell Theory Center (Jeff Deutsch) and Sandia (Bill
Rahe) to access DCE credentials from a keytab file.
Alternatively, we could use Kerberos or DCE authentication on the
Cray. This would be more in-line with the design of HPSS but would
require some support for long-duration and cron jobs, since
authentication credentials are normally set to expire after a day or
so. Unicos includes a mechanism that "borrows" the restarting
processes credentials. After that, the tickets (in the NQE case) are
refreshed on a periodic basis before they expire. We expect that this
mechanism would be workable for our HPSS interface.
We are currently evaluating both approaches.
Meta-computing, the use of a set of computers as an integrated whole,
is a major goal of current and near future SDSC projects. Key components
of this include security and a Single Sign-On system.
Single Sign-On (SSO) is a system where users could authenticate (log
in) once and repeatedly access the multiple hosts of the meta-computing
environment. If or when the DCE/Kerberos environment becomes
ubiquitous, Single Sign-On will become a realistic goal.
Data-intensive computing is a major research initiative of SDSC,
and we are using HPSS as a foundation for that development. Our goal
is to provide the software infrastructure to support the immense
datasets of biology, geophysics, and astronomy, managing data through
object-relational databases integrated with archival storage systems.
SDSC is leading a number of major collaborative projects in this field,
including the Massive Data Analysis Project
(MDAS) and Distributed Object
Computation Testbed (DOCT).
The Massive Data Analysis Project involves defining and prototyping an
architecture for accessing very large data archives. The proposed
architecture will support data-intensive applications that manipulate
very large data sets by building upon object-relational database
technology and archival storage technology. We will develop
interfaces to integrate these two technologies. Another goal is to
research the software intrastructure needed to replace the Unix file
system paradigm with a data analysis and Application Programming
Interface (API) system capable of accessing terabyte data sets.
The Distributed Object Computation Testbed (DOCT) will create an
environment for handling complex documents on geographically
distributed data archives and computing platforms. A persistent object
representation based on the Legion computation environment will be
used to integrate text search systems, document handling systems, and
intelligent agents that support electronic submission of
documents. The documents will be stored on distributed databases
systems that are integrated with archival storage systems. Research
activities include development of a distributed scheduling system, and
development of the requirements for supporting electronic filing of
Security is a key component of modern computing, particularly as we
move to more network-centric and meta-computing systems. Two key
technologies are emerging as important components of this.
Today, the Secure Shell (SSH) can be used to easily and efficiently
provide secure authentication and privacy on a wide variety of
computer systems. It is a practical interim solution and also a
useful supplement to Kerberos and DCE security. This author highly
recommends SSH to the Cray community and hopes to see it become widely
deployed in the HPC and general computing communities.
Kerberos/DCE is continuing development, and is becoming a more
practical solution. Kerberos 5 Beta 7 can now be configured, compiled,
and installed with few difficulties on many workstations, and Cray
Research now provides Kerberos 5 Beta 5 as part of its DCE 1.1
product. We expect that Kerberos/DCE will become a widespread and
important technology, especially as Kerberos 5 moves into the final
release stage (Version 5 Release 1.0), which is expected in the next
This work was funded in part by National Science Foundation Cooperative
All brand and product names are trademarks or registered trademarks of
their respective holders.