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ITL Bulletin for April 2006

ITL Bulletin for April 2006
ITL Bulletin for April 2006

Forwarded from: Elizabeth Lennon  



Shirley Radack, Editor
Computer Security Division
Information Technology Laboratory
National Institute of Standards and Technology
Technology Administration
U.S. Department of Commerce

The protection of sensitive information that is transmitted across
interconnected networks is critical to the overall security of an
organization's information and information systems. The Information
Technology Laboratory (ITL) at the National Institute of Standards and
Technology (NIST)  recently issued guidance to assist organizations in
strengthening their network security and in lessening the risks
associated with the transmission of sensitive information across
networks. The publication offers practical guidance on implementing
security services based on Internet Protocol Security (IPsec).

IPsec is a framework of open standards for ensuring private
communications over public networks. IPsec is frequently used to
achieve security controls in the layered protocols of network
communications, and to create a virtual private network (VPN). An
organization can build a VPN on top of existing physical networks to
create a secure communications mechanism for the data and control
information that is transmitted between networks. VPNs are used most
often to protect communications carried over public networks, such as
the Internet, which utilize Transmission Control Protocol/Internet
Protocol (TCP/IP)  network communications. When properly implemented,
VPNs can protect the confidentiality and integrity of data,
authenticate the origin of data, and provide data replay protection
and access control. However, VPNs cannot eliminate all risks since
flaws in algorithms or software, or insecure configuration settings
and values may still be exploited by hackers.

NIST Special Publication (SP) 800-77, Guide to IPsec VPNs

Written by Sheila Frankel of NIST, and Karen Kent, Ryan Lewkowski,
Angela D. Orebaugh, Ronald W. Ritchey, and Steven R. Sharma of Booz
Allen Hamilton, NIST SP 800-77 helps network architects, network
administrators, security staff, technical support staff, and computer
security program managers who are responsible for the technical
aspects of preparing, operating, and securing their organization's
networks. The information discussed is general in nature and can be
applied to many different hardware platforms, operating systems, and
applications.  Topics covered include the need for network layer
security services, the services that are available at the network
layer, and how IPsec can be implemented to provide these services. A
case-based approach illustrates how IPsec can be used to solve common
network security concerns. The guide explains IPsec planning and
implementation issues; it also discusses alternatives to IPsec and the
appropriate circumstances in which to deploy each alternative.

The appendices discuss the need for organizations to develop their
IPsec-related policies and present examples of common IPsec policy
issues that should be considered.  Also included in the appendices are
configuration files that are referenced by the case studies, a
glossary, an acronym list, and a compilation of print and online
resources that may be useful for IPsec planning and implementation.

The publication is available on NIST's web pages at: 

The Need for Network Security

Widely used throughout the world, Transmission Control
Protocol/Internet Protocol (TCP/IP) network communications are
composed of four layers of protocols that work together: application,
transport, network, and data link.  Security controls are available
for network communications at each of the four layers:

The application layer sends and receives data for an application.
Separate controls must be established for each application. While this
arrangement provides a high degree of control and flexibility for the
security of the application, it may cause the organization to devote
considerable resources to implement. The development of new
application layer security controls can also create new
vulnerabilities, and it may not be possible to develop the controls
for some applications.

The transport layer provides connection-oriented or connectionless
services to transport application layer services across networks.
Controls at this layer can protect data in a single communications
session between two hosts, and must be supported by both clients and

The network layer routes packets across networks. Controls at this
layer apply to all applications, rather than to specific applications.
Applications do not have to be modified to use the controls, but this
arrangement provides less control and flexibility for protecting
specific applications than the transport and application layer

The data link layer handles communications on the physical network
components.  Controls at this level protect a specific physical link.
Since each physical link must be secured separately, controls at this
level are not feasible for protecting connections that involve several
links, including most connections across the Internet.

As data is prepared for transport through the network, it is passed
from the highest to the lowest layer, with each layer adding more
information. Security controls at a higher layer cannot provide full
protection for the lower layers, because the lower layers add
information to the communications after the higher-layer security
controls have been applied. The lower-layer security controls are less
flexible and granular than higher-layer controls. As a result,
controls at the network layer are widely used to secure communications
and to provide a more balanced solution than can be achieved through
the application of the higher-layer and lower-layer security controls.

Internet Protocol Security (IPsec)

IPsec is the most commonly used network layer security control for
protecting communications. It was developed by the IPsec Working Group
of the Internet Engineering Task Force (IETF) as a framework of open
standards. Depending upon the implementation and configuration, IPsec
can provide the following types of protection:

* Ensuring the confidentiality of data through the application of a
cryptographic algorithm and a secret key, known only to the two
parties exchanging data. The data that is transmitted can be decrypted
only by someone who has the secret key.

* Assuring the integrity of data through the application of a message
authentication code (MAC), which is a cryptographic hash of the data.
The checksum is sent with the data.  The recipient can detect when the
data has been changed, either intentionally or unintentionally during
transit, if a new MAC is calculated on the received data and it does
not match the original MAC.

* Providing peer authentication to ensure that network traffic and
data are sent from the expected host. The receiving IPsec endpoint can
confirm the identity of the sending IPsec endpoint.

* Providing replay protection to assure that the same data is not
delivered multiple times and that the data is delivered in an
acceptable order. IPsec cannot, however, ensure that the data has been
received in the exact order that it was sent.

* Providing traffic analysis protection by obscuring the identities of
the endpoints and the size of the data. Those who are monitoring
network traffic may not know which parties are communicating, how
often communications occur, or how much data is being exchanged.

* Providing access control by assuring that only authorized users can
access particular network resources. IPsec endpoints can also allow or
block certain types of network traffic, such as allowing web server
access but denying file sharing.

Components of IPsec

The IPsec network layer security protocol provides protection through
the following components, which are used in various combinations:

Authentication Header (AH) and Encapsulating Security Payload (ESP)
security protocols. ESP provides encryption and integrity protection
for packets, but it cannot directly protect the outermost IP header.
(It can protect it indirectly, if Internet Key Exchange (IKE) is used
to negotiate the IPsec protections.) AH provides integrity protection
for packet headers and data but without encryption. AH can also
protect some header information that ESP cannot protect. ESP is used
more frequently than AH because of its encryption capabilities and
other operational advantages.

Internet Key Exchange (IKE) protocol. IKE negotiates the cryptographic
algorithms and related security parameters and controls that comprise
the security associations (SAs)  that are applied to IPsec-protected
connections. Other protections provided by this protocol include
mutual authentication of endpoints; negotiation of secret keys;  and
management, update, and deletion of IPsec-protected communication
channels. An updated, streamlined IKE (version 2) has been
standardized, but most current implementations adhere to the original
IKE, version 1.

IP Payload Compression Protocol (IPComp). IPsec uses this protocol to
compress packet payloads before encrypting them.

For the IPsec-applied encryption and integrity-protection processes,
federal agencies are required to use cryptographic algorithms that are
specified in Federal Information Processing Standards (FIPS) or in
NIST Recommendations that are issued in NIST Special Publication 800
series. The FIPS-approved algorithms must be contained in
cryptographic modules that have been validated for conformance to FIPS
140-2, Security Requirements for Cryptographic Modules, through the
Cryptographic Module Validation Program (CMVP).  Algorithms that are
FIPS-approved include FIPS 197, Advanced Encryption Algorithm (AES),
the strongest approved algorithm and the preferred algorithm for
federal agency use. Also approved is the Triple Data Encryption
Algorithm (TDEA), which is specified in American National Standard
(ANSI) X9.52-1998 and validated using the tests that are contained in
NIST SP 800-20, Modes of Operation Validation System for the Triple
Data Encryption Algorithm. In addition, the FIPS-approved algorithm
for message authentication is FIPS 198, Keyed-Hash Message
Authentication Code. This algorithm is used to construct a Keyed-Hash
Message Authentication Code (HMAC) using secure hash algorithms that
are specified in FIPS 180-2, Secure Hash Standard.

Virtual Private Networks (VPNs)

The VPN is a virtual network, which is built on top of existing
physical networks, and which provides a secure communications
mechanism for data and IP information exchanged between public
networks. This method of networking can be less expensive for an
organization than utilizing dedicated private telecommunications lines
to provide communications between the organization's home and branch
offices, and between remote telecommuters and the main servers. There
are three models for VPNs:

The gateway-to-gateway model protects communications between two
specific networks, such as an organization's main office network and a
branch office network, or between two business partners' networks.

The host-to-gateway model protects communications between one or more
individual hosts and an organization's specific network, allowing
hosts on unsecured networks, such as traveling employees and
telecommuters, to have access to the organization's internal services.

The host-to-host model protects communications between two specific
computers and is most often used when a small number of users need
access to a remote system.

IPsec implementations can be used to support VPN services.  SP 800-77
establishes the following requirements for the configuration of IPsec

* The VPN must provide confidentiality protection through encryption
for any information that will traverse a VPN and that should not be
seen by non-VPN users.

* A VPN must use a FIPS-approved encryption algorithm. The Advanced
Encryption Algorithm in Cipher Block Chaining mode (AES-CBC) is highly
recommended. Triple DES in Cipher Block Chaining mode (3DES-CBC) is
acceptable as well.

* A VPN must always provide integrity protection.

* A VPN must use a FIPS-approved algorithm to provide for integrity
protection.  HMAC-SHA-1 is highly recommended and is based on FIPS
198, Keyed-Hash Message Authentication Code (HMAC), and FIPS 180-2,
Secure Hash Signature Standard.

* A VPN should provide replay protection.

* IKE security associations (SAs) for applications of IKE version 1
should have a lifetime no greater than 24 hours, and IPsec SAs should
have a lifetime no greater than 8 hours. For IKE version 2, IKE SAs
for the original packets should be re-keyed at least every 24 hours,
and SAs for encapsulated packets associated with the original packets
should be re-keyed after 8 hours at most.

* The Diffie-Hellman (DH) group of values is used to specify the
encryption generator type and key length to be used for generating
shared secrets. The value used to establish the secret keying material
for IKE and IPsec should be consistent with current security
requirements.  Specific DH groups are defined for use with IKE. DH
group 2 should be used for Triple DES and for AES with a 128-bit key.
For greater security, DH group 5 or DH group 14 may be used for AES.
IPsec implementations include DH group 2;  most include DH group 5;
very few include DH group 14. Use of the larger DH groups results in
increased processing time.

IPsec Planning and Implementation

NIST advises that agencies apply the principles of the System
Development Life Cycle and carry out a risk-based and phased approach
in planning for and implementing IPsec in their networked systems.
This approach enables agencies to determine appropriate priorities for
protecting their systems, to apply appropriate technologies, including
the use of IPsec and VPNs, and to incorporate new technology when
needed to meet changing requirements.

Organizations should identify their needs to protect their networked
communications and determine which computers, networks, and data are
part of the networked communications. They should determine how their
needs can best be met, and where and how security technology should be

The next phase of the risk-based approach is to design the solution
that meets the needs, taking into account four major issues: The
architectural design includes consideration of host and gateway
placement, IPsec client software selection, and host address space
management. An authentication method, such as pre-shared key or
digital signature, should be selected. The algorithms for encryption
and integrity protection, and the key strength for algorithms that
support multiple key lengths, should be selected. The packet filter
should be determined to control the types of traffic to be permitted
and denied, and to apply appropriate protection and compression
measures to each type of permitted traffic, and packet filters. The
decisions made regarding authentication, cryptography, and packet
filters should be documented in the organization's IPsec policy.

Organizations should then implement and test a prototype of the
designed solution in a laboratory or test environment.  The primary
goals of the testing are to evaluate the functionality, performance,
scalability, and security of the solution, and to identify any issues,
such as compatibility and interoperability of the IPsec components.  
The security of the implementation is a special concern, since no
protocol can be totally secure.  Special attention should be paid to
the security of stored keys, the traffic that passes through the
packet filters, and the use of patches that are developed as new
vulnerabilities are found.

When the testing has been completed and all issues have been resolved,
organizations should deploy the solution by migrating gradually to the
use of IPsec throughout the enterprise. The gradual approach enables
managers to replace the existing network infrastructure and
applications, to train users, to evaluate the impact of the IPsec
solution, and to resolve issues. Most of the issues that can occur
during IPsec deployment are the same types of issues that occur during
any large IT deployment.  Service to users, the performance of the
network, and client software may be affected.

The last phase of the planning and implementation cycle is to manage
the solution throughout its life cycle. In this phase, the IPsec
architecture, policies, software, and other components of the deployed
solution are maintained.  Patches to IPsec software should be tested
and applied as appropriate. The management phase also involves
providing support for new sites and users, monitoring performance,
testing the system periodically, and adapting new policies as
requirements change. The life cycle process is repeated when
enhancements or significant changes need to be incorporated into the

More Information

The IPsec protocols were developed within the IPsec Working Group of
the Internet Engineering Task Force (IETF). They are defined in two
types of documents: Request for Comment (RFC), which are accepted
standards, and Internet-Drafts, which are working documents that may
become RFCs. A list of IPsec documents can be found at 

Federal agencies must use FIPS-approved encryption algorithms
contained in validated cryptographic modules.  The Cryptographic
Module Validation Program (CMVP) is a joint effort of NIST and the
Communications Security Establishment (CSE) of the Government of
Canada. The CMVP coordinates FIPS 140-2 testing and has issued
validation certificates for more than 600 cryptographic modules. The
CMVP website is located at 

FIPS 140-2, Security Requirements for Cryptographic Modules, is
available at See for information on FIPS-approved 
symmetric key algorithms. FIPS-approved algorithms must also be used
for digital signatures. See 

The National Vulnerability Database (NVD) is a comprehensive database
of cyber security vulnerabilities in information technology (IT)
products. It was developed by NIST with the support of the National
Cyber Security Division (NCSD) of the U.S. Department of Homeland
Security. The NVD integrates all publicly available U.S.  government
vulnerability resources and includes references to industry resources.

NIST publications can help you in planning and implementing a
comprehensive approach to IT security. For information about NIST
publications and standards that are referenced in the IPsec guide, as
well as other security-related publications, see 


Any mention of commercial products or reference to commercial
organizations is for information only; it does not imply
recommendation or endorsement by NIST nor does it imply that the
products mentioned are necessarily the best available for the purpose.

Elizabeth B. Lennon
Information Technology Laboratory
National Institute of Standards and Technology
100 Bureau Drive, Stop 8900
Gaithersburg, MD 20899-8900
Telephone (301) 975-2832
Fax (301) 975-2378

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