AUSCERT External Security Bulletin Redistribution

           ESB-98.086 -- CERT Vendor-Initiated Bulletin VB-98.05
        Cisco PIX Private Link Key Processing and Cryptography Issues
                                05 June 1998


The CERT Coordination Center has released the following advisory 
concerning an error in the parsing of configuration file commands in the 
PIX Private Link feature (optionally installed on Cisco PIX firewalls) 
which reduces the effective key length for the PIX Private Link DES 
encryption to 48 bits from the nominal 56 bits.  

This reduction of the effective key length reduces the work involved
in a brute-force attack on the encryption by a factor of 256. That is,
knowledgeable attackers can, on the average, find the right key 256 times
faster than they would be able to find it with a true 56-bit key.

The following security bulletin is provided as a service to AusCERT's
members.  As AusCERT did not write this document, AusCERT has had no
control over its content.  As such, the decision to use any or all of this
information is the responsibility of each user or organisation, and should
be done so in accordance with site policies and procedures.

NOTE: This is only the original release of the security bulletin.  It will
not be updated when the original bulletin is.  If downloading at a later
date, it is recommended that the bulletin is retrieved from the original
authors to ensure that the information is still current.

Contact information for Cisco Systems, Inc. is included in the Security 
Bulletin below.  If you have any questions or need further information, 
please contact them directly.

Previous advisories and external security bulletins can be retrieved from:


If you believe that your system has been compromised, contact AusCERT or
your representative in FIRST (Forum of Incident Response and Security

Internet Email: auscert@auscert.org.au
Facsimile:	(07) 3365 7031
Telephone:	(07) 3365 4417 (International: +61 7 3365 4417)
		AusCERT personnel answer during Queensland business hours
		which are GMT+10:00 (AEST).
		On call after hours for emergencies.

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CERT* Vendor-Initiated Bulletin VB-98.05
June 03, 1998

Topic:  PIX Private Link Key Processing and Cryptography Issues
Source: Cisco

To aid in the wide distribution of essential security information, the CERT
Coordination Center is forwarding the following information from Cisco.
Cisco urges you to act on this information as soon as possible. Cisco contact
information is included in the forwarded text below; please contact them if
you have any questions or need further information.

=======================FORWARDED TEXT STARTS HERE============================


Field Notice:
PIX Private Link Key Processing and Cryptography Issues

June 3, 1998

PIX Private Link is an optional feature that can be installed in Cisco PIX
firewalls. PIX Private Link creates IP virtual private networks over
untrusted networks, such as the Internet, using tunnels encrypted with Data
Encryption Standard (DES) in ECB ("electronic codebook") mode.

An error in parsing of configuration file commands reduces the effective key
length for the PIX Private Link DES encryption to 48 bits from the nominal
56 bits.

Who Is Affected
All users of the PIX Private Link encryption product with PIX software
versions earlier than the date of this notice are affected. This includes
all PIX Private Link software through version 4.1.6.

If attackers know the details of the key-parsing error in the PIX Private
Link software, they will know 8 bits of the key ahead of time. This reduces
the effective key length from the attacker's point of view from 56 to 48
bits. This reduction of the effective key length reduces the work involved
in a brute-force attack on the encryption by a factor of 256. That is,
knowledgeable attackers can, on the average, find the right key 256 times
faster than they would be able to find it with a true 56-bit key.

In addition to this key-length issue, some customers have expressed concern
over the use of DES ECB mode for PIX Private Link encryption. Although the
use of ECB mode is intentional, ECB is not generally considered to be the
best mode in which to employ DES, because it tends to simplify certain forms
of cryptanalysis and may permit certain replay attacks. Technical details of
the relative merits of various encryption modes are beyond the scope of this
document. Interested readers should refer to a cryptography text for more
information, such as Bruce Schneier's Applied Cryptography.

This vulnerability has been assigned Cisco bug ID CSCdk11848.

Affected Software Versions
- - - ------------------------
This vulnerability affects all released versions of PIX Private Link
software with version numbers up to and including 4.1.6, and all
beta/interim software released earlier than the date of this notice.

Planned Software Fixes
- - - --------------------
The first regular release containing a fix for this problem will be version
4.2.1, which is tentatively scheduled for release in late June 1998. This
schedule is subject to change. Fixes for the 4.1 software release have not
yet been scheduled.

This fix extends the effective DES key length to a full 56 bits; ECB mode is
still used.

Customers who need to upgrade immediately may contact Cisco's Technical
Assistance Center (TAC) to obtain interim software. Interim software has not
been subjected to full testing; it has a greater chance of containing
serious bugs than would regular released software.

Interim releases are available only by special request from the Cisco TAC,
not via the regular download channels. Cisco advises customers to install
interim releases only if absolutely necessary. Customers who choose to
install interim releases should plan to upgrade to the regular released
software when it becomes available.

When the fix is installed, it will be necessary to upgrade both ends of each
Private Link tunnel at the same time. This is because key the modified key
parsing algorithm will lead old and new versions to derive different
encryption keys from the same configuration file.

Software upgrades to correct this key-length problem will be offered free of
charge to all PIX Private Link customers, regardless of their service
contract status. Customers under contract may obtain upgrades through their
usual procedures. Customers not under contract should call the Cisco TAC.
Contact information for the TAC is in the "Cisco Security Procedures"
section at the end of this message, and is available on Cisco's Worldwide
Web site at http://www.cisco.com/.

The use of ECB mode was a deliberate design decision for the PIX Private
Link product, and will not be changed. However, future IPSEC/IKE products
for the PIX platforms will use other encryption modes.

- - - ---------
There is no configuration workaround.

Exploitation and Public Announcements
Cisco has had no reports of malicious exploitation of this vulnerability.

Cisco knows of no public announcements of this vulnerability before the date
of this notice. This vulnerability was discovered by an engineering analysis
conducted by a Cisco customer at a security incident response organization.

Status of This Notice
This is a final field notice. Although Cisco cannot guarantee the accuracy
of all statements in this notice, all the facts have been checked to the
best of our ability. Cisco does not anticipate issuing updated versions of
this notice unless there is some material change in the facts. Should there
be a significant change in the facts, Cisco may update this notice.

- - - ----------
This notice will be posted on Cisco's Worldwide Web site at
http://www.cisco.com/warp/public/770/pixkey-pub.shtml. In addition to
Worldwide Web posting, the initial version of this notice is being sent to
the following e-mail and Usenet news recipients:

   * cust-security-announce@cisco.com
   * firewalls@lists.gnac.net
   * comp.security.firewalls
   * bugtraq@netspace.org
   * first-teams@first.org (includes CERT/CC)
   * Various internal Cisco mailing lists

Future updates of this notice, if any, will be placed on Cisco's Worldwide
Web server, but may or may not be actively announced on mailing lists or
newsgroups. Users concerned about this problem are encouraged to check the
URL given above for any updates.

Revision History
- - - --------------
 Revision 1.0,        Initial released version
 08:00 AM

Cisco Security Procedures
Please report security issues with Cisco products, and/or sensitive security
intrusion emergencies involving Cisco products, to security-alert@cisco.com.
Reports may be encrypted using PGP; public RSA and DSS keys for
"security-alert@cisco.com" are on the public PGP keyservers.

The alias "security-alert@cisco.com" is used only for reports incoming to
Cisco. Mail sent to the list goes only to a very small group of users within
Cisco. Neither outside users nor unauthorized Cisco employees may subscribe
to "security-alert@cisco.com".

Please do not use "security-alert@cisco.com" for configuration questions,
for security intrusions that you do not consider to be sensitive
emergencies, or for general, non-security-related support requests. We do
not have the capacity to handle such requests through this channel, and will
refer them to the TAC, delaying response to your questions. We advise
contacting the TAC directly with these requests. TAC contact numbers are as

   * +1 800 553 2447 (toll-free from within North America)
   * +1 408 526 7209 (toll call from anywhere in the world)
   * e-mail: tac@cisco.com

All formal public security notices generated by Cisco are sent to the public
mailing list "cust-security-announce@cisco.com". For information on
subscribing to this mailing list, send a message containing the single line
"info cust-security-announce" to "majordomo@cisco.com". An analogous list,
"cust-security-discuss@cisco.com" is available for public discussion of the
notices and of other Cisco security issues.

This notice is copyright 1998 by Cisco Systems, Inc. This notice may be
redistributed freely after the release date given at the top of the notice,
provided that redistributed copies are complete and unmodified, including
all date and version information.

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If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (FIRST). See http://www.first.org/team-info/.

We strongly urge you to encrypt any sensitive information you send by email.
The CERT Coordination Center can support a shared DES key and PGP. Contact
the CERT staff for more information.

Location of CERT PGP key

CERT Contact Information
- - ------------------------
Email    cert@cert.org

Phone    +1 412-268-7090 (24-hour hotline)
                CERT personnel answer 8:30-5:00 p.m. EST
                (GMT-5)/EDT(GMT-4), and are on call for
                emergencies during other hours.

Fax      +1 412-268-6989

Postal address
        CERT Coordination Center
        Software Engineering Institute
        Carnegie Mellon University
        Pittsburgh PA 15213-3890

CERT publications, information about FIRST representatives, and other
security-related information are available from

CERT advisories and bulletins are also posted on the USENET newsgroup

To be added to our mailing list for CERT advisories and bulletins, send your
email address to
In the subject line, type
        SUBSCRIBE  your-email-address

* Registered U.S. Patent and Trademark Office.

The CERT Coordination Center is part of the Software Engineering
Institute (SEI). The SEI is sponsored by the U. S. Department of Defense.

This file: ftp://ftp.cert.org/pub/cert_bulletins/VB-98.05.cisco

Version: 2.6.2


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